Lab #9 Report (Redo)

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Jan 9, 2024

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McKenna Nichols BME 447 Loeffler 25 October 2023 Chapter 10 Lab Report: Electrochemical Sensors Results Task 1: Buffer Preparations and Their pH Measurements pH From Meter Expected pH pH from pH Strip 9.59 10 10 6.78 7 7 3.86 4 4 pH From Meter Back Calculated pH pH from pH Strip 2.6 (flat soda) 2.67 3 3.81 (non-flat soda) 3.94 5

Task 2: pH Meter Circuit pH Value Output Voltage (mV) Theoretical Voltage (mV) 4 108 177 7 20 0 10 145 -177 Back Calculation Output Voltage (mV) Theoretical Voltage (mV) 2.845 175 (flat soda) 253.7 2.408 194 (non-flat soda) 279.07 Task 4: Fluoride Ion Selective Electrode with Circuit Fluoride Concentration Output Voltage (mV) 10 -145 5 -109

1 -73 0.5 -41 0.1 -9 0.01 41 Back Calculated Fluoride Concentration Output Voltage (mV) -2.35 72.1 (tap water) -1.94 46.7 (fluoride solution) Discussion In task one of this lab, we began by taking the pH measurements of the solution with a pH of 10, 7, and 4. We then took the pH of these same solutions using the meter making any necessary calibrations. We then graphed the pH reading using the meter versus the expected pH. We then used both the pH strips and the pH meter to take measurements for our two unknown samples, flat soda and non-flat soda. Using the plot we made, we then used the pH measurements

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of our unknown solution to back calculate the expected pH. From here we can compare our measured values to our expected values. Based on our results we observed that the pH of the flat soda when measured with the meter was 2.6 but when we back-calculated this value using our graph we found the value to be 2.67. The non-flat soda had a pH value of 3.81 when measured with the meter and of 3.94 when back-calculated. We did expect the non-flat soda to be more acidic than the flat-soda due to its greater concentration. However, we observed the flat soda 𝐶𝑂 2 to be more acidic. This could be attributed to the soda being opened several days before our testing, meaning the non-flat soda may have actually been flat, thus hindering the experiment from working as expected. In task two of this lab, we began by constructing the circuit below. We then calculated the theoretical voltage values for each of the buffer solutions using the expected and measured values of pH. We then measured the values of for the three buffers 𝑉 ??? and then we plotted these values versus the expected pH. We then repeated all of these measurements for the unknown samples, flat soda and non-flat soda. We were then able to back calculate the pH of the unknown samples using the graph that we created. Then we were able to compare the measured and theoretical values we obtained. The pH electrode is 0 mV at a neutral

pH. This means that we expect that the more acidic the solution, the more positive that the voltage output should be and the more basic the solution, the more negative that the voltage output should be. We observed that the voltage output of the flat soda to be 175 mV with a pH of 2.845. This voltage output is much lower than the theoretical voltage at 253.7 mV. We then observed that the non-flat soda had a voltage output of 194 mV and a pH value of 2.408. The theoretical voltage output for this solution was 279.07 mV. This theoretical value is also much higher than the measured. This could be attributed to the fact that the soda had been left out for several days prior to the experiment which resulted in the results being skewed. In task four of this lab, we began by using the same circuit we used before but instead we used the fluoride probe. We used this probe to measure the value of for both the tap water 𝑉 ??? and the toothpaste sample. Then we used the data that was given to us to plot the voltage output versus the concentration of fluoride. Using this graph, we back calculated the fluoride concentration of our unknown samples. We calculated the fluoride concentration of the tap water to be -2.35 and the fluoride concentration of the fluoride solution to be -1.97. This makes sense because fluoride is more basic which means that the voltage output should be more negative when the fluoride concentration increases. Our results show that a lower concentration of fluoride, meaning a more acidic solution, has a more positive voltage output. The tap water has a fluoride concentration of -2.35 which is lower than the fluoride concentration of the toothpaste which is -1.97. This means that the tap water is more acidic than the toothpaste and therefore should have a more positive voltage output. Our results show this because the toothpaste has a voltage output of 46.7 mV and the tap water has a voltage output of 72.1 mV which is greater (more positive). Another observation that is important to note is that Tucson puts fluoride in their water for dental health which is why the tap water pH is so high.

Review Questions 10.4 Why should the standard curve be plotted against the log concentration of fluoride? (Hint: Use Nernst equation). The standard curve should be plotted against the log concentration of fluoride because in the natural log in the Nernst equation causes the Voltage of the pH to be linearly proportional to the log of the ionic concentration. This means that you must take the log of the concentration to relate it to the voltage because of the relationship in the Nernst equation. 10.5 Can you use 0 mg/mL fluoride solution (=deionized water) for the standard curve? Briefly explain why. 0 mg/mL fluoride solution cannot be used for the standard curve because there must be some concentration for the Nernst equation to work because the natural log of zero does not exist. 10.7 Why is the slope of Fig. 10.20 negative, while that of Fig. 10.19 is positive? (Hint: compare with the results of the pH meter.) According to equation 18 in the textbook, , we expect a decrease of voltage 𝐸 = 𝐸 ? − 0. 059?𝐻 by 59 mV per each pH unit that the solution increases. This means that as our pH gets greater, the voltage reading will get more negative. This is an indication of an inverse relationship.

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