Chemistry (A_ 29_34)
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The Spectrophotometric Determination of the Effect of UV-C Light on Free Chlorine Concentration What is the effect of UV-C light exposure over time on free chlorine concentration of local pool water? An Extended Essay in Chemistry 3951 words
II Table of Contents 1. Introduction……………………………………………………………………….. 1.1 Background Information 1.1.1 Use of Chlorine In Pools 1.1.2 Use of UV-C Light 1.1.3 DPD and Free Chlorine Reaction 1.1.4 Beer-Lambert Law 1.2 Research Question 1.3 Hypothesis 2. Experimental Design……………………………………………………………... 2.1 Variables 2.2 Method and Procedure 3. Analysis……………………………………………………………………………. 3.1 Qualitative Observations 3.2 Pre-Experiment Calculated Data 3.3 Raw Quantitative Data 3.4 Processed Quantitative Data 4. Discussion…………………………………………………………………………. 4.1 Summary of Results 4.2 Comparison to Accepted Scientific Context 4.3 Extensions and Further Investigations 1 1 1 3 4 4 5 5 6
6 7 9 9 9 11 13 18 18 18 20
III 4.3.1 Impacts of Chlorine Photolysis 4.3.2 Preventing Chlorine Photodecomposition 5. Evaluation…………………………………………………………………………. 5.1 Strengths 5.1.1 Frequent Readings 5.1.2 Purity of Water 5.2 Weaknesses 5.2.1 Uneven UV Exposure 5.2.2 Temperature 5.2.3 pH 6. Conclusion………………………………………………………………………
....
7. Bibliography………………………………………………………………………. 8. Appendix…..………………………………………………………………………. 8.1 Apparatus 8.2 Averaging 20 23 26 26 26 26 27 27 27 28 30 31 34 34 34
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1 1. Introduction 1.1 Background Information 1.1.1 Use of Chlorine In Pools Chlorine has been used as a disinfecting and sterilizing agent in swimming pools and drinking water since the early 1900s (Olsen, 2007, p. 132). Normally, water is chlorinated through the addition of either chlorine gas (Cl 2 ) or the hypochlorite ion (OCl - ) as an aqueous compound. If hypochlorite is added to the water, often as sodium hypochlorite (NaOCl), it reacts with water, forming hypochlorous acid (HOCl) following the equation: NaOCl (aq) + H 2 O (l) → HOCl (aq) + Na + (aq) + OH - (aq) If gaseous chlorine is added, it reacts with water, forming hypochlorous acid following the equation: Cl 2 (g) + H 2 O (l) → HOCl (aq) + H + (aq) + Cl - (aq) Although hypochlorous acid is weak in nature, with a pKa of approximately 7.5 (Remucal & Manley, 2016, p. 567), it can dissociate into its conjugate base, the hypochlorite ion, and a hydrogen ion, following the equation (Kleijnen et al., 2011, p. 13): HOCl (aq) ⇌
H + (aq) + OCl - (aq) The dominant species is primarily dependent on the pH of the water. As visible in Figure 1
,
OCl - would be dominant at higher pH, whereas at lower pH, HOCl would make up most of the available chlorine in the pool water. HOCl is approximately 10 times more reactive than OCl - and thus a more effective sanitizer (Kleijnen et al., 2011, p. 13), but the pH must be kept within government-set limits for safety reasons.
2 Figure 1: Relationship Between HOCl and OCl¯ at Various pH Values, Retrieved from “Basic Chemistry of Chlorination” (Hydro Instruments, n.d.)
Chlorine found in water as either OCl - or HOCl make up what is referred to as ‘free available chlorine’. When added to water, chlorine will first oxidize certain surrounding ions to form precipitates. Such precipitates may include ferric hydroxide and manganese dioxide and can be filtered out due to their insoluble nature (Minnesota Rural Water Association, 1994, p. 4). Next, added chlorine will react with nitrogenous compounds, notably ammonia (NH 3 ), to form chloramines: monochloramine (NH 2 Cl), dichloramine (NHCl 2 ), and nitrogen trichloride (NCl 3 ). These chloramines make up what is called ‘combined residual chlorine’ and are less efficient as short-term disinfecting agents, but are often kept in water for long-term disinfection purposes. Whatever is left of the added chlorine remains in the water as free available chlorine and is used primarily to target organic matter. ‘Total residual chlorine’ includes both free chlorine concentration as well as combined residual chlorine concentration (Victorin, Hellström, & Rylander, 1972, p. 313). Free chlorine primarily acts as a disinfectant for organic matter by attacking the cell wall where it proceeds to break down lipids and react with enzymes and proteins within the cell. As a
3 result, the cell is no longer able to reproduce and is thus no longer considered viable (Kleijnen et al., 2011, p. 13).
1.1.2 Use of UV-C Light The UV light spectrum ranges from 200 to 390 nanometers in wavelength. As with chlorine, UV-C light is primarily used as a disinfectant and sterilizing agent, with optimal sterilizing functionality at 254 nanometers. It is used to disinfect a number of everyday objects from laboratory goggles to drinking water (Wyckomar Inc. UV Purification Systems, 2012, p.1). Figure 2
portrays the photometric data of the UV-C light source used in this study. Figure 2: Photometric Data for Philips TUV 25W 1SL/25 Lamp Used as UV-C Light Source, Retrieved from TUV T8 (Philips Lighting, 2018, p.2) Like chlorine, UV-C light acts as a sterilizing agent through the penetration of the cell wall. It causes the DNA of the cell to be rearranged such that the cell can no longer reproduce (Wyckomar Inc. UV Purification Systems, 2012, p. 1).
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4 1.1.3 DPD and Free Chlorine Reaction Hach DPD packets each contain a buffer to stabilize pH, as the reaction between DPD and chlorine varies with pH, as well as the main reagent, DPD ( N,N-diethyl-p-phenylenediamine ). As visible in Figure 3 , free chlorine oxidizes DPD, turning the colourless solution into a shade of magenta called ‘Würster Dye’ (Hach Company, p. 1). Experiment 33 in the “Chemistry with Vernier” textbook states that “the intensity of the sample’s color is directly proportional to its chlorine concentration” (Holmquist, Randall, Volz, 2007). Thus, absorbance can be measured and concentration calculated thereafter. Figure 3: Reaction between DPD and Chlorine, Retrieved from the Hach Company (n.d., p.2) 1.1.4 Beer-Lambert Law The Beer-Lambert law states that absorbance and concentration are directly proportional and gives this equation where A is absorbance, ε is molar absorptivity, b is the light’s path length, and c is concentration: A = εbc
5 Since the absorbance being measured is always that of the same chemical, Würster Dye, molar absorptivity will remain constant throughout all of the differently concentrated solutions. Similarly, since the same type of cuvette is being used for each measurement, the length which the light travels will be the same. Thus, absorbance can be said to be equal to concentration multiplied by a constant (k). A = (k)(c) 1.2 Research Question The question that this paper aims to answer is “what is the effect of UV-C exposure over time on free chlorine concentration of local pool water?” This question stemmed from the initial interest of studying a concept that is frequently encountered by the general population, but whose specific phenomena are seldom explored. This was something that could be explored locally but can be applied globally. 1.3 Hypothesis The free chlorine will likely demonstrate a decrease in concentration over time in samples that are not exposed to UV-C light, as water must often be re-chlorinated due to breakdown as a result of it acting as a disinfectant. However, it is expected that UV-C light exposure will cause an increase in the rate of decay of chlorine, thus decreasing the concentration in the samples by a greater amount over a given period of time.
6 2. Experimental Design 2.1 Variables Type of Variable Variable Description Independent Time (t) Total time elapsed, in minutes, since the first recording was taken. Dependent Concentration of Free Chlorine (c) The calculated concentration, in parts per million, of chlorine in a sample at each measured point in time. Controlled Purity of water Tap water contains ions such as sodium and chloride ions. To mitigate the effect of this variable on the results, distilled water was used as it contains fewer (theoretically none) of these ions. Additionally, the distilled water was taken from the same source to maintain the level of purity. UV-C light source The UV-C light source, a Philips TUV 25W 1SL/25 lamp, was not changed for the duration of the experiment. Different UV lamps emit light at different intensities, which may either increase or decrease the resulting conclusion. Pool water sample All of the pool water used in the experiment was taken from the same 2-litre bottle which was filled at the local pool the morning of the experiment to maintain the same level of purity. The contaminant concentration and composition of the water would likely affect the decay rates calculated.
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7 2.2 Method and Procedure The method and procedure of this experiment were adapted from Experiment 33 of the “Chemistry with Vernier” textbook, “Determining the Free Chlorine Content of Swimming Pool Water” (Holmquist, Randall, Volz, 2007). This textbook experiment provided the initial idea of working with pool chlorination. It also suggested the underlying method, including the use of DPD packets as well as the determination of concentrations through the preparation of standard solutions and the use of the Beer-Lambert law. Several modifications were made to the experiment method and procedure with the aim of answering the research question as well as increasing data accuracy and precision, all while working with apparatus that was accessible. First, using a 1 mL pipette, 1.00 ± 0.01 mL of 8.25% bleach was transferred into a 1000 ± 0.6 mL volumetric flask in which it was diluted with distilled water to create a stock solution. Next, the original procedure limited the concentrations of the standard solutions to a maximum of 5 ppm. After researching and learning that the regulations require for the concentration of free chlorine in public pools to be between 1 and 10 ppm, solutions with concentrations of 1 through 10 ppm were prepared so that data could be interpolated. These solutions were prepared by first calculating the volume of stock solution required to create a solution of each desired concentration, then using the appropriate pipette(s) to transfer the amount necessary into a 100.00 ± 0.08 mL flask and diluting it using distilled water. The suggested method for determining the concentration of pool water samples was plotting a graph and using the line of best fit to determine concentration using a given absorbance value, as shown in Figure 4
.
8 Figure 4: Graph Portraying Suggested Method and Beer’s Law from “Determining the Free Chlorine Content of Swimming Pool Water” (Holmquist, Randall, Volz, 2007) For greater accuracy, and so that uncertainty values could easily be transferred, the data points from the standard solutions were used to develop a constant (k) which was later used to calculate the concentration of free chlorine in each sample. The introduction of UV-C light exposure over time as well as the control group was not included in the design of the original experiment. Twelve beakers of pool water were placed in a dark cupboard, forming the control group. Another 12 beakers were placed in a goggle sanitizing cabinet which uses a Phillips TUV 25W 1SL/25 lamp for sanitation. A final beaker of water was used to measure the initial concentration of free chlorine in the pool water. Every 15 minutes, a beaker was taken out of each environment, and a packet of DPD was poured and mixed into each sample. The DPD was mixed in until dissolved using a stirring rod. Afterwards, as instructed by the Vernier experiment, a cuvette was filled with the DPD-pool water solution to approximately the ¾ mark and was then inserted into the pre-calibrated spectrophotometer. The absorbance value at a wavelength of 560.20 ± 7 nanometers was recorded. This was repeated for all of the samples.
9 3. Analysis 3.1 Qualitative Observations Qualitative Data Descriptor Intensity of Solutions with DPD As time progressed, the shade of the magenta solution that resulted from the mixture of DPD into the samples of chlorine became visibly lighter. The initial sample produced a very dark shade of magenta, whereas the solution that was tested after 180 minutes of UV light exposure produced a light pink colour. The lightening of the colour produced by samples in the control group over time when mixed with DPD was also noticeable, though less drastic. Speed of DPD - Chlorine Reaction In the initial samples, the reaction between the DPD powder and the chlorine in the water samples was almost instantaneous: as soon as the powder touched the surface of the water, it began to turn magenta. In later samples, this did not seem to be the case; the change would be less dramatic and quick. 3.2 Pre-Experiment Calculated Data
Figure 5: Calculation for the Conversion from Percent Weight by Volume (%W/V) of Sodium Hypochlorite (NaOCl) to Parts Per Million (ppm) of Hypochlorous Acid (HOCl) for Bleach Solution Used to Prepare the Stock Solution OCl
]
%
W V
concentration
of
NaOCl
)
)
)
)
[
−
= (
/
× (
1 g
1000 mg
× (
1 L
1000 mL
× (
moles
of
HOCl
moles
of
NaOCl
OCl
]
(
) ± (
)]
)
)
)
[
−
= [
8.25 g
100 mL
0.05 g
100 mL
× (
1 g
1000 mg
× (
1 L
1000 mL
× (
1 mol
1 mol
OCl
]
2 500 ppm
.6060606061%
[
−
= 8
± 0
OCl
]
2 500 00 ppm
[
−
= 8
± 5
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10 Figure 6: Calculation for the Concentration of the Stock Solution OCl
]
Initial
concentration
)
[
−
= (
×
(
V olume
prepared
)
(
V olume
of
bleach
used
)
OCl
]
82 500 00 ppm
)
[
−
= (
± 5
×
(1.00 ± 0.01 mL
) (1000.0 ± 0.6 mL
)
OCl
]
2.5 ppm
.6660606061%
[
−
= 8
± 1
OCl
]
2.5 .3745 ppm
[
−
= 8
± 1
OCl
]
3 ppm
ppm
[
−
= 8
± 2
Figure 7: Sample Calculation for the Volume of Stock Solution Required to Prepare Standard Solutions Using Values for the 4 ppm Standard Solution Volume f
stock
solution
Concentration
of
standard
solution
)
o
= (
×
(
V olume
prepared
)
(
Concentration
of
stock
solution
)
Volume f
stock
solution
4.00 ppm
)
o
= (
×
(82.5 ppm
)
(100.0 mL
) Volume f
stock
solution
.8484848485 mL
o
= 4
Volume f
stock
solution
.85 mL
o
= 4
Figure 8: Sample Calculation for the Concentration of Standard Solutions with Uncertainty Using Values for the 4 ppm Standard Solution OCl
]
Concentration
of
stock
solution
)
[
−
= (
×
(
V olume
prepared
)
(
V olume
of
bleach
used
)
OCl
]
82.5 .3745 ppm
)
[
−
= (
± 1
×
(4.85 ± 0.01 mL
) (100.00 ± 0.08 mL
)
OCl
]
.00125 ppm
.9523106061%
[
−
= 4
± 1
OCl
]
.00125 .0780924242 ppm
[
−
= 4
± 0
OCl
]
.00 ppm
.08 ppm
[
−
= 4
± 0
11 Table 1: Calculated Volumes Required To Prepare Standard Solutions for Concentrations of 1 ppm Through 10 ppm at Intervals of 1 ppm with Uncertainties. Sample Number Initial Concentration (c 1 ) Volume of Stock Solution (v 1 ) Concentration of Standard Solution (c 2 ) Volume of Standard Solution Prepared (v 2 ) 1 83 ± 2 ppm 1.21 ± 0.01 mL 1.00 ± 0.03 ppm 100.00 ± 0.08 mL 2 83 ± 2 ppm 2.42 ± 0.01 mL 2.00 ± 0.05 ppm 100.00 ± 0.08 mL 3 83 ± 2 ppm 3.64 ± 0.01 mL 3.00 ± 0.07 ppm 100.00 ± 0.08 mL 4 83 ± 2 ppm 4.85 ± 0.01 mL 4.00 ± 0.08 ppm 100.00 ± 0.08 mL 5 83 ± 2 ppm 6.06 ± 0.07 mL 5.0 ± 0.2 ppm 100.00 ± 0.08 mL 6 83 ± 2 ppm 7.27 ± 0.05 mL 6.0 ± 0.2 ppm 100.00 ± 0.08 mL 7 83 ± 2 ppm 8.48 ± 0.05 mL 7.0 ± 0.2 ppm 100.00 ± 0.08 mL 8 83 ± 2 ppm 9.70 ± 0.04 mL 8.0 ± 0.2 ppm 100.00 ± 0.08 mL 9 83 ± 2 ppm 10.9 ± 0.2 mL 9.0 ± 0.3 ppm 100.00 ± 0.08 mL 10 83 ± 2 ppm 12.1 ± 0.2 mL 10.0 ± 0.2 ppm 100.00 ± 0.08 mL 3.3 Raw Quantitative Data Table 2: Recorded Absorbance Values of Prepared Standard Solutions for Concentrations of 1 ppm Through 10 ppm at Intervals of 1 ppm With Absolute Uncertainties for Time and Relative Uncertainties for Absorbance Concentration of Standard Solution (c 2 ) Absorbance (A) 1.00 ± 0.03 ppm 0.113 ± 13% 2.00 ± 0.05 ppm 0.254 ± 13% 3.00 ± 0.07 ppm 0.353 ± 13% 4.00 ± 0.08 ppm 0.464 ± 13% 5.0 ± 0.2 ppm 0.633 ± 13% 6.0 ± 0.2 ppm 0.758 ± 13% 7.0 ± 0.2 ppm 0.820 ± 13% 8.0 ± 0.2 ppm 0.904 ± 13% 9.0 ± 0.3 ppm 1.006 ± 13% 10.0 ± 0.2 ppm 1.108 ± 13%
12 Table 3: Recorded Absorbance Values of Samples Exposed to UV-C Light for the Initial Value (time=0) Through 180 Minutes at Intervals of 15 Minutes With Absolute Uncertainties for Time and Relative Uncertainties for Absorbance Time (t) Absorbance (A) 0 ± 2 min 0.851 ± 13% 15 ± 2 min 0.770 ± 13% 30 ± 2 min 0.665 ± 13% 45 ± 2 min 0.356 ± 13% 60 ± 2 min 0.359 ± 13% 75 ± 2 min 0.276 ± 13% 90 ± 2 min 0.289 ± 13% 105 ± 2 min 0.195 ± 13% 120 ± 2 min 0.315 ± 13% 135 ± 2 min 0.283 ± 13% 150 ± 2 min 0.224 ± 13% 165 ± 2 min 0.146 ± 13% 180 ± 2 min 0.110 ± 13% Table 4: Recorded Absorbance Values of Samples of the Control Group (No UV-C Light Exposure) for the Initial Value (time=0) Through 180 Minutes at Intervals of 15 Minutes With Absolute Uncertainties for Time and Relative Uncertainties for Absorbance Time (t) Absorbance (A) 0 ± 2 min 0.851 ± 13% 15 ± 2 min 0.804 ± 13% 30 ± 2 min 0.677 ± 13% 45 ± 2 min 0.467 ± 13% 60 ± 2 min 0.376 ± 13% 75 ± 2 min 0.366 ± 13% 90 ± 2 min 0.365 ± 13% 105 ± 2 min 0.348 ± 13% 120 ± 2 min 0.388 ± 13% 135 ± 2 min 0.345 ± 13% 150 ± 2 min 0.398 ± 13% 165 ± 2 min 0.399 ± 13% 180 ± 2 min 0.342 ± 13%
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13 3.4 Processed Quantitative Data Figure 9: Sample Calculation for the Value of the Constant (k) Using Values for the 4 ppm Standard Solution Absorbance
) (
Concentration
of
Standard
Solution
) k
= (
/
0.464 ± 13%)
(4.00125 ppm
.9523106061% ) k
= (
/
± 1
.1159637613 ppm
4.9523106061%
k
= 0
1
−
± 1
.1159637613 0.0173446803 ppm
k
= 0
± 1
−
.12 0.02 ppm
k
= 0
± 1
−
Table 5: Calculated Constant (k) Values of Prepared Standard Solutions for Concentrations 1 ppm Through 10 ppm at Intervals of 1 ppm and Calculated Average Value With Uncertainties Concentration of Standard Solution (c 2 ) Absorbance (A) Constant (k) 1.00 ± 0.03 ppm 0.11 ± 0.02 0.11 ± 0.02 ppm -1 2.00 ± 0.05 ppm 0.25 ± 0.04 0.13 ± 0.02 ppm -1 3.00 ± 0.07 ppm 0.35 ± 0.05 0.118 ± 0.009 ppm -1 4.00 ± 0.08 ppm 0.46 ± 0.07 0.12 ± 0.02 ppm -1 5.0 ± 0.2 ppm 0.63 ± 0.09 0.13 ± 0.02 ppm -1 6.0 ± 0.2 ppm 0.8 ± 0.1 0.13 ± 0.02 ppm -1 7.0 ± 0.2 ppm 0.8 ± 0.2 0.12 ± 0.02 ppm -1 8.0 ± 0.2 ppm 0.9 ± 0.2 0.11 ± 0.02 ppm -1 9.0 ± 0.3 ppm 1.0 ± 0.2 0.11 ± 0.02 ppm -1 10.0 ± 0.2 ppm 1.1 ± 0.2 0.11 ± 0.02 ppm -1 Average N/A 0.12 ± 0.02 ppm -1
14 Figure 10: Sample Calculation for the Concentration of a Sample Over Time Using Values for the Sample Exposed to UV-C Light for 150 Minutes Absorbance
) (
k
) c
= (
/
0.224 ± 0.02912)
(0.1179320635 ± 0.0181856909 ppm
) c
= (
/
1
−
.8993986314 ppm
± 28.4204805666% c
= 1
.8993986314 ± 0.5398182189 ppm
c
= 1
.9 ± 0.6 ppm
c
= 1
Table 6: Calculated Values for Samples Exposed to UV-C Light for the Initial Value (time=0) Through 180 Minutes at Intervals of 15 Minutes With Uncertainties Time (t) Absorbance (A) Concentration (c) 0 ± 2 min 0.9 ± 0.2 7 ± 3 ppm 15 ± 2 min 0.8 ± 0.2 7 ± 2 ppm 30 ± 2 min 0.67 ± 0.09 6 ± 2 ppm 45 ± 2 min 0.36 ± 0.05 3.0 ± 0.9 ppm 60 ± 2 min 0.36 ± 0.05 3.0 ± 0.9 ppm 75 ± 2 min 0.28 ± 0.04 2.3 ± 0.7 ppm 90 ± 2 min 0.29 ± 0.04 2.5 ± 0.7 ppm 105 ± 2 min 0.20 ± 0.03 1.7 ± 0.5 ppm 120 ± 2 min 0.34 ± 0.05 2.7 ± 0.8 ppm 135 ± 2 min 0.28 ± 0.04 2.4 ± 0.7 ppm 150 ± 2 min 0.22 ± 0.03 1.9 ± 0.6 ppm 165 ± 2 min 0.15 ± 0.02 1.2 ± 0.4 ppm 180 ± 2 min 0.11 ± 0.02 0.9 ± 0.3 ppm
15 Figure 11: Graph for the Concentration of Free Chlorine in Pool Water as a Function of Time When Continually Exposed to UV-C Light Table 7: Calculated Concentration Values for Samples Not Exposed to UV-C Light (Control Group) for the Initial Value (time=0) Through 180 Minutes at Intervals of 15 Minutes With Uncertainties Time (t) Absorbance (A) Concentration (c) 0 ± 2 min 0.8 ± 0.2 7 ± 3 ppm 15 ± 2 min 0.8 ± 0.2 7 ± 2 ppm 30 ± 2 min 0.68 ± 0.09 6 ± 2 ppm 45 ± 2 min 0.47 ± 0.07 4 ± 2 ppm 60 ± 2 min 0.38 ± 0.05 3 ± 1 ppm 75 ± 2 min 0.37 ± 0.05 3.1 ± 0.9 ppm 90 ± 2 min 0.37 ± 0.05 3.1 ± 0.9 ppm 105 ± 2 min 0.35 ± 0.05 3.0 ± 0.9 ppm 120 ± 2 min 0.39 ± 0.06 3 ± 1 ppm 135 ± 2 min 0.35 ± 0.05 2.9 ± 0.9 ppm 150 ± 2 min 0.40 ± 0.06 3 ± 1 ppm 165 ± 2 min 0.40 ± 0.06 3 ± 1 ppm 180 ± 2 min 0.34 ± 0.05 2.9 ± 0.9 ppm
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16 Figure 12: Graph for the Concentration of Free Chlorine in Pool Water as a Function of Time With No Exposure to UV Light (Control Group) Figure 13: Calculation for the Decay Rate of Free Chlorine in Samples Exposed to UV-C Light 6.3041)
e
)
y
= (
× (
0.01
x
−
[( e ) -0.01 ] x 6.3041)
y
= (
×
6.3041)
0.9900498337)
y
= (
× (
x
ecay
rate
.9900498337
d
= 1
−
0
ecay
rate
.0099501663
d
= 0
ecay
rate
.00995
d
= 0
ecay
rate
.995%
d
= 0
17 Figure 14: Calculation for the Decay Rate of Free Chlorine in Control Group Samples (No UV-C Light Exposure) 5.6136)
e
)
y
= (
× (
0.004
x
−
[( e ) -0.004 ] x 5.6136)
y
= (
×
5.6136)
0.9960079893)
y
= (
× (
x
ecay
rate
.9960079893
d
= 1
−
0
ecay
rate
.0039920107
d
= 0
ecay
rate
.00399
d
= 0
ecay
rate
.399%
d
= 0
18 4. Discussion 4.1 Summary of Results In this experiment, the concentration of free chlorine in samples exposed to UV-C light, and samples not exposed to UV light over time was tested over a 3-hour period, with readings at 15-minute intervals. The experimental data demonstrated that the change in concentration of free chlorine in samples exposed to UV-C light can be modelled by the equation 6.30)
e
)
y
= (
× (
0.01
x
−
with a decay rate of 0.995% per minute. The variation in free chlorine concentration in samples of the control group followed the equation with a decay rate of 0.399% per 5.94)
e
)
y
= (
× (
0.004
x
−
minute. The coefficient of determination values, 0.8483 for the trend in UV samples and 0.6116 in control samples, though high enough to be confident in the presence of a trend, indicated that there was a relatively high level of variance in the results. The high level of uncertainty also deters from the confidence of the mathematical trend, particularly for the control group. 4.2 Comparison to Accepted Scientific Context The trends demonstrated as a result of processing the collected data are comparable to the results of several other experiments. The “Investigation and Modeling of Solar UV-Induced Chlorine Decay in Disinfection Contact Basins at Full-Scale Water Treatment Plants”
was able to explain the decay of free chlorine in the samples of the control group. According to authors Gu and Neethling (2008), the chlorine breakdown in covered samples was “mainly due to chlorine demand exerted by disinfection” and is called “base line chlorine demand” (p. 182). They then
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19 went on to explain that decay in the UV-exposed samples was both due to this ‘base line chlorine demand’ as well as the photodecomposition of free chlorine because of the UV light. Photolysis or photodecomposition is the breaking of chemical bonds within a molecule due to the absorption of light energy (Speight, 2017, p. 325). This results in the molecule separating into smaller molecules or the atoms within it assuming their elemental form. In addition to the loss of free chlorine in UV-exposed samples due to the “base line chlorine demand” and photolysis, UV-C light is used as a disinfecting agent, as explained in section 1.1.2 Use of UV-C Light ; this could potentially reduce the amount of chlorine consumed for disinfection purposes by aiding the disinfection process. The exponential trend can be compared to one observed in the study “A Potential New Method for Determination of the Fluence (UV Dose) Delivered in UV Reactors Involving the Photodegradation of Chlorine”. In Figure 15 , taken from the study, a clear, exponential decrease in free chlorine concentration over time when exposed to UV-C light is visible. Figure 15: “Time profiles of free chlorine and its photodegradation products”, Retrieved From “A Potential New Method for Determination of the Fluence (UV Dose) Delivered in UV Reactors Involving the Photodegradation of Chlorine”. (Feng, Smith, & Bolton, 2010, p. 331). Red markers were added to data points relevant to this study.
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20 Although the trend demonstrated in this study as well as that from this secondary source were similar in that they both portrayed clear exponential decay, the rates at which free chlorine was consumed in the two works were not easily comparable. This can be explained by the evidence which portrays that “free chlorine can be degraded more rapidly by UV light in the presence of organic matter in water sample” (Feng, Smith, & Bolton, 2010, p. 331). This means that the effect of UV-C light on free chlorine decay, though clearly present, cannot be modelled by a single universal equation because its effect is heavily dependent on the contents of the water. 4.3 Extensions and Further Investigations 4.3.1 Impacts of Chlorine Photolysis Although the breakdown of free chlorine due to UV-C light is seemingly no more than a nuisance towards attempts to disinfect water, chlorine photolysis can be “applied to simultaneously inactivate chlorine-resistant pathogens and transform organic contaminants of concern” (Remucal & Manley, 2016, p.566). Remucal and Manley outline three mechanisms through which combined UV and chlorine treatment can aid in the disinfection process in their critical review “Emerging Investigators Series: The Efficacy of Chlorine Photolysis as an Advanced Oxidation Process for Drinking Water Treatment”. The first two, aforementioned in sections 1.1.1 Use of Chlorine in Pools and 1.1.2 Use of UV-C Light
, are chlorine and UV-C light being used individually as disinfecting agents in pool and drinking water. The third mechanism explained in Remucal and Manley’s study is “reactive species-mediated indirect photolysis” (p.
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21 566). This alludes to the reactions of breakdown products of chlorine photolysis which result in pathogen inactivation and the decomposition of organic compounds. Three main circumstances were explained in the review that are applicable to the circumstances of this study. First, the irradiation of HOCl at a wavelength of less than 400 nanometers. The most likely situation is outlined in Figure 16
. The products of the initial and final reaction in the chain result in the hydroxyl radical which is highly reactive and thus easily breaks down contaminants. Figure 16: Possible Chain Reaction for the Photolysis of HOCl Compiled Using Equations from “Emerging Investigators Series: The Efficacy of Chlorine Photolysis as an Advanced Oxidation Process for Drinking Water Treatment” (Remucal & Manley, 2016, p.568) Next, the irradiation of OCl - at a wavelength less than 320 nanometers can result in two main situations outlined below in Figure 17 and Figure 18
, both resulting in reactive hydroxyl radicals at the end of the chain. Figure 17: Possible Chain Reaction for the Photolysis of OCl - Compiled Using Equations from “Emerging Investigators Series: The Efficacy of Chlorine Photolysis as an Advanced Oxidation Process for Drinking Water Treatment” (Remucal & Manley, 2016, p.568)
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22 Figure 18: Possible Chain Reaction for the Photolysis of OCl - Compiled Using Equations from “Emerging Investigators Series: The Efficacy of Chlorine Photolysis as an Advanced Oxidation Process for Drinking Water Treatment” (Remucal & Manley, 2016, p.568) This can be both advantageous and disadvantageous to the water treatment process. As previously stated, the hydroxyl radical is highly reactive, thus facilitating quick disinfection. A combined chlorine-UV treatment process would also allow for the “inactivat[ion of] protozoa… which are difficult to treat by chlorination” (Feng, Smith, & Bolton, 2007, p. 277). However, according to Remucal and Manley (2016), “it is also inefficient as it reacts quickly with many compounds present in natural waters” (p. 568). Additionally, re-chlorination would likely be required more often as the chlorine would be utilized at a quicker rate. Apart from efficiency, another concern related to the photolysis of chlorine is health, particularly in the disinfection of drinking water. Feng, Smith, & Bolton (2010) mentioned that regulations on free chlorine concentration in drinking water were put in place by Health Canada because “high levels of chlorate ions” resulting from hypochlorite photolysis “have been associated with hemolytic anemia” (p. 330).
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23 4.3.2 Preventing Chlorine Photodecomposition Cyanuric acid (
Figure 19
) is a commonly used additive to swimming pool water, particularly in outdoor pools. It is added with the intention of slowing chlorine decay which is caused by the UV light emitted by the sun. It reacts reversibly with chlorine such that the chlorine can bind to it, prohibiting photolysis, but detach over time for disinfection (Huthmacher & Most, 2000, p. 2). Figure 19
: Lewis Structure of Cyanuric Acid Retrieved from “Cyanuric Acid and Cyanuric Chloride” (Huthmacher & Most, 2000, p. 2) Cyanuric acid can react with chlorine in many ways, forming a variety of chloroiso-cyanurate compounds. Some common examples found in swimming pools that were presented in “Cyanuric Acid and Cyanuric Chloride” can be observed in the following table (Huthmacher & Most, 2000, p. 2).
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24 Table 8: Common Chloroiso-Cyanurates Found in Pool Water from “Cyanuric Acid and Cyanuric Chloride” (Huthmacher & Most, 2000, p. 2)
Name Lewis Structure Sodium dichloroisocyanurate Potassium dichloroisocyanurate Trichloroisocyanurate (TCCA)
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25 Another common method for slowing down the photolysis of chlorine caused by UV light is the use of pool covers. Pool covers work by minimizing the exposure of chlorinated water to the sun, which emits UV light rays. The “Investigation and Modeling of Solar UV-Induced Chlorine Decay in Disinfection Contact Basins at Full-Scale Water Treatment Plants”
tested the use of covers at water-treatment basins, and found that they played a very significant role in the regulation of chlorine decay; the uncovered basins demonstrated a notably higher level of chlorine consumption than those which were covered ( Figure 20
). In addition to photolysis, they also noted that “there could be some chlorine loss as a result of evaporation in the open basin” (Gu & Neethling, 2008, p. 182). Figure 20: “Comparison of chlorine consumption through the contact basin at peak sunlight intensity (i.e., 11:00 a.m. to 1:00 p.m.) in the uncovered and covered chlorine DCBs at EDHWWTP” from “The Investigation and Modeling of Solar UV-Induced Chlorine Decay in Disinfection Contact Basins at Full-Scale Water Treatment Plants” (p. 182)
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26 5. Evaluation 5.1 Strengths 5.1.1 Frequent Readings One of the main strengths of this study was the frequency and number of data readings taken. The concentration of free chlorine for both samples exposed to UV-C light and samples of the control group were recorded at 15-minute intervals, for a total of 12 data points for each group over a 3 hour period. This minimized the effect of random error on the conclusion and allowed for a higher level of precision when it came down to retrieving an equation dictating the resulting trend. 5.1.2 Purity of Water The purity of the water used to prepare the standard solutions played an important role in the accuracy of results. Chlorine is not only used to disinfect pool
water; regular tap water is known to contain forms of chlorine for disinfection purposes as well. Thus, in order to minimize random error caused by unknown concentrations of chlorine in tap water, distilled water was used to prepare the standard solutions. Had the chlorine from tap water reacted with the DPD, the absorbance value would have increased and become inconsistent with the “known” concentration value being used in calculations. This would have resulted in a higher calculated value for the constant (k), eventually leading the calculated concentrations of the UV and control samples to being lower than they should be. Though not eliminating systematic error, the distilled water was taken from the same source to further mitigate the effect of random error on the conclusion.
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27 5.2 Weaknesses 5.2.1 Uneven UV Exposure The greatest weakness of this experiment was the possibility of uneven UV exposure among samples in the UV sanitizing cabinet. The samples were all kept as centred as possible in the cabinet in an attempt to maximize UV-C light exposure. However, an extra sample was kept in the far corner of the cabinet with the sole purpose of testing whether UV-exposure was equally distributed throughout the cabinet. This sample did not fit the trend pulled from this study in that the concentration of free chlorine in the sample did not decrease as much as the trend dictated it would. This likely means that the UV light is more concentrated in the middle of the cabinet, where the tested samples were placed. However, this also indicates that there was likely inconsistency in UV exposure among the samples used in the experiment. This would result in the concentrations of certain samples being higher than they should be, as they were not exposed to the direct UV light that other samples were affected by. This can be controlled by either exposing the samples to a different UV light source which is known to produce more consistency in exposure or by only exposing one, larger sample to the UV light, keeping it in the same position throughout the experiment, and taking samples from it for each data point. 5.2.2 Temperature A major factor that was not considered throughout the experiment was the effect of temperature on the photolysis of free chlorine. According to the 2010 study by Yangang Feng, Daniel Smith, and James Bolton, “A Potential New Method for Determination of the Fluence (UV dose) Delivered in UV Reactors Involving the Photodegradation of Free chlorine”, the
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28 quantum yield for the photodegradation of chlorine is notably lower in cooler conditions (p. 331). Though the temperature was not measured during the experiment, it could be felt that the UV light cabinet was significantly colder than the ambient temperature of the laboratory, including the area in which the control samples were kept. Consequently, the comparison between the decomposition of free chlorine in samples exposed to UV light and those in the control group may not be accurate, as the colder temperature of the UV-exposed samples may have resulted in a slower breakdown of free chlorine. In future studies, the temperatures of the UV-exposed samples and the control group samples should be kept the same in order to be able to draw a valid, numerical comparison between the two groups. However, this would not affect the general conclusion that UV light increases the rate of decay of free chlorine as a higher temperature should theoretically further increase the rate of decay. 5.2.3 pH
As explained in section 1.1.1 Use of Chlorine in Pools
, the pH of the water determines whether HOCl or OCl - is the dominant species. The higher reactivity of HOCl was also mentioned in this section. pH was not tested during the experiment, and it is thus not known which of the two is the dominant species. If HOCl were more present than OCl - , the degradation of chlorine may have occurred at a quicker pace because HOCl is much more likely to react with contaminants and destroy them. Because of this, pH would be something worth testing if this experiment were ever to be carried out again. The governing ministry of the region in which this experiment was conducted requires that the pH of the water in public pools rest within the range of 7.2 and 7.8. As seen in Figure 1
of section 1.1.1 Use of Chlorine in Pools
, this would mean
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29 that there should be a relatively similar concentration of HOCl and OCl - in the pool water at any given time.
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30 6. Conclusion The question that this study aimed to answer was “what is the effect of UV-C light exposure over time on free chlorine concentration of local pool water?” After processing the data produced during the experiment and comparing the results to those from other studies, it can be confidently stated that UV-C light exposure increases the rate at which chlorine concentration decreases. The resulting equation, , produced a decay rate in chlorine 6.30)
e
)
y
= (
× (
0.01
x
−
concentration of approximately 0.995% per minute. The function representing the concentration of free chlorine in samples of the control group over time was, , with a 5.94)
e
)
y
= (
× (
0.004
x
−
decay rate of 0.399% per minute. This is in line with the hypothesis that samples of both groups would experience a decrease in concentration, but that the samples exposed to UV-C light would experience a quicker decrease. However, although the analysis strongly suggests a decline in the concentration of chlorine due to UV-C light exposure, these equations do not represent the rate of decline in all situations, as the intensity of the UV light and the amount of contamination in the sample, among other things, would play a role in how quickly free chlorine would break down. Alongside this, the confidence in the mathematical equations obtained is lessened by relatively high uncertainty values and variance levels.
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31 7. Bibliography Feng, Y., Smith, D. W., & Bolton, J. R. (2010). A Potential New Method for Determination of the Fluence (UV Dose) Delivered in UV Reactors Involving the Photodegradation of Free Chlorine. Water Environment Research, 82
(4), 328-334. Retrieved from http://www.jstor.org/stable/25679783 Feng, Y., Smith, D. W., & Bolton, J. R. (2007). Photolysis of aqueous free chlorine species (HOCl and OCl - ) with 254 nm ultraviolet light.
Journal of Environmental Engineering and Science, 6 (3), 277-284. doi:10.1139/s06-052 Gu, A., & Neethling, J. (2008). Investigation and Modeling of Solar UV-Induced Chlorine Decay in Disinfection Contact Basins at Full-Scale Wastewater Treatment Plants. Water Environment Research, 80 (2), 179-185. Retrieved from http://www.jstor.org/stable/23804231 Hach. (n.d.). Chlorine, Free and Total. Retrieved from: https://www.hach.com/asset-get.download.jsa?id=7639984177 Holmquist, D. D., Randall, J., Volz, D. L. (2007). Determining the Free Chlorine Content of Swimming Pool Water. In Chemistry with Vernier
(33). Beaverton, OR: Vernier Software & Technology. Huthmacher, K. and Most, D. (2000). Cyanuric Acid and Cyanuric Chloride. In Ullmann's Encyclopedia of Industrial Chemistry, (Ed.). doi:10.1002/14356007.a08_191 Hydro Instruments. (n.d.). Basic Chemistry of Chlorination. Retrieved from: http://www.hydroinstruments.com/files/Basic%20Chemistry%20of%20Chlorination.pd f
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32 Kleijnen, R. G., Hoofwijk, B. L., Pol, D. G. J., Heintges, G. H. L., & De Visser, J. F. (2011). Technical aspects of water disinfection. In The Chlorine Dilemma
(4). Retrieved from: https://web.archive.org/web/20140222152238/http://students.chem.tue.nl/ifp42/Downlo ads/Final%20Report%20MDP%20Group%201%20-%20The%20Chlorine%20Dilemm a.pdf# Minnesota Rural Water Association. (1994). Iron and Manganese. In Minnesota Water Works Operations Manual
(14). Retrieved from https://www.mrwa.com/WaterWorksMnl/Chapter%2014%20Iron%20and%20Mangane se.pdf Olsen, K. (2007). Clear Waters and a Green Gas: A History of Chlorine as a Swimming Pool Sanitizer in the United States. Bulletin for the History of Chemistry, 32 (2), 129-140. Retrieved from http://acshist.scs.illinois.edu/bulletin_open_access/v32-2/v32-2%20p129-140.pdf Philips Lighting. (2018). TUV T8. Retrieved from https://www.assets.lighting.philips.com/is/content/PhilipsLighting/fp928039404005-pss -global Remucal, C. K., & Manley, D. (2016). Emerging investigators series: the efficacy of chlorine photolysis as an advanced oxidation process for drinking water treatment.
Journal of Environmental Science: Water Research & Technology, 2 (4), 565-579. doi:10.1039/C6EW00029K Speight, J. (2017). Chemical Transformations in the Environment. In Environmental Organic Chemistry for Engineers (7). doi:10.1016/B978-0-12-804492-6.00007-1
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33 Victorin, K., K. -G. Hellström, & Rylander, R. (1972). Redox Potential Measurements for Determining the Disinfecting Power of Chlorinated Water. The Journal of Hygiene, 70
(2), 313-323. Retrieved from http://www.jstor.org/stable/3861426 Wyckomar Inc. UV Purification Services. (2012). Ultraviolet Sterilization Technology. Retrieved from: http://www.wyckomaruv.com/bin/info/Wyckomar_UVTechnology.pdf
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34 8. Appendix 8.1 Apparatus -
1 mL pipette ± 0.01 mL (1) -
5 mL pipette ± 0.01 mL (1) -
10 mL pipette ± 0.1 mL (1) -
100 mL volumetric flask ± 0.08 mL (10) -
1000 mL volumetric flask ± 0.6 mL (1) -
100 mL beaker (25) -
25 mL graduated cylinder (2) -
Wax flask stoppers/covers (11) -
Phone used as timer (1) -
Vernier cuvettes (15) -
Lint-free tissues (~31) -
Pipette pump (1) -
Distilled Water (~2500 mL) -
8.25% Household bleach (1.00 mL) -
Stirring rod (2) -
Pool water sample (~1500 mL) -
Hach DPD Free Chlorine Reagent Powder Pillow (25) -
Wardsci UV sanitizing cabinet, containing Philips TUV 25W 1SL/25 lamp (1) -
Vernier SpectroVis Plus Spectrophotometer ± 13% (1) 8.2 Averaging The average value of the constant (k) and its uncertainty was determined using the following formula: μ =
sum
of
all
data
values
number
of
data
values
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- When 14.3 g of a certain molecular compound X are dissolved in 85.0 g of benzene C6H6, the freezing point of the solution is measured to be 0.5 °C. Calculate the molar mass of X. molal freezing point depression constant (Kf)°C·kg·mol−1 of benzen is 5.07 If you need any additional information on benzene, use only what you find in the ALEKS Data resource. Also, be sure your answer has a unit symbol, and is rounded to 2 significant digits.arrow_forwardgrammarly-G X * Content oard.odu.edu/ultra/courses/_393452_1/cl/outline F2 Question Completion Status: X GA student wan X 1 QUESTION 3 A student wants to determine the solubility of water and copper (1) bromide (a green solid) in ethyl acetate (a colorless organic liquid). She places 1 mL of ethyl acetate into each of two test tubes. To one, she adds 5 drops of water to it. To the other, she adds a small scoop of copper(1) bromide. 8.0 F3 2 - Which test tube represents the ethyl acetate and A. Test tube 1 copper(1) bromide if the copper(1) bromide is insoluble in ethyl B. Test tube 2 acetate? C. Test tube 3 D. Test tube 4 - Which test tube represents the mixture of the ethyl acetate and water if the water is insoluble in ethyl acetate? $ Question Com X C Chegg Search x 3 Which test tube represents the mixture of the ethyl acetate and water if the water is soluble in ethyl acetate? - Which test tube represents the ethyl acetate and copper(1) bromide if the copper(1) bromide is soluble…arrow_forwardThe solubility of CaF, in water at 25 °C is measured to be 0.017 . Use this information to calculate K, for CaF,. sp Round your answer to 2 significant digits.arrow_forward
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