Lab 8 Report

d. The aqueous solution on the inside is mixed with the aqueous Chlorine to make aqueous AgCl and solid silver to produce aqueous silver ions, aqueous Chlorine ions, and a solid silver precipitate. e. Paraphrase the last paragraph on page 352 that is right before Figure 15-14 and continues on the top of page 353. A glass electrode is considered an ion-selective electrode Combination electrodes use both glass and reference electrodes in a single unit. A thin glass bulb or tip at the bottom of the electrode is what measures pH. The reference electrode includes Ag wire and AgCl paste. This is the left side of the line diagram. The right side represents the Ag|AgCl core of the electrode. These two reference electrodes measure the electric potential difference across the glass membrane. The porous plug acts as a salt bridge by allowing the passing of electrolyte. Experimental Step 1. NaCl solution Mass of NaCl (g): 5.9971 g Calculate the molarity of the NaCl solution here using the equation editor: 5.9971 ? × 1 ??? ???? 58.443 ?/??? × . 1026 1000 ? = .1026 ? Step 2. The pH Electrode Standardization: Table 1: Standardization Data Collected Data Temperature (°C) 20.4 pH 4.00 (mV) 145.29 pH 7.00 (mV) -23.9 Electromotive Efficiency (%) as given by the instrume 96.2% Graph the data in Table 1 so it looks like that found in Figure 15-19 and label it Figure 1: Two-Point calibration of a pH electrode.

D 0 9.98 × 10 −6 5.00 4.56 148.6 E 0 9.98 × 10 −7 6.00 4.85 126.6 B-NaCl .1026 9.98 × 10 −4 3.00 3.33 219.3 C-NaCl .1026 9.98 × 10 −5 4.00 4.19 167.1 D-NaCl .1026 9.98 × 10 −6 5.00 4.02 137.6 E-NaCl .1026 9.98 × 10 −7 6.00 4.24 115.1 AA 0 .0083 3.42 3.04 192.1 BB 0 8.3 × 10 −4 3.08 3.58 163.4 CC 0 8.3 × 10 −5 4.08 3.63 150.5 DD 0 8.3 × 10 −6 5.08 4.02 121.5 EE 0 8.3 × 10 −7 6.08 4.29 98.5 BB-NaCl .1026 8.3 × 10 −5 3.95 3.36 171.3 CC-NaCl .1026 8.3 × 10 −6 4.52 3.75 143.1 DD-NaCl .1026 8.3 × 10 −7 5.21 4.18 121.1 EE- NaCl .1026 8.3 × 10 −8 6.10 4.49 104.6 Sample calculations for Table 2: (Note that the concentration of NaCl is fixed to what was created – ignore the minor dilution caused by creating Solution B from Solution A.) Show a calculation example for solutions C, C-NaCl, CC and CC-NaCl. C: − log(9.98 × 10 −5 ) = ?. ?? CC: − log(8.3 × 10 −5 ) = ?. ?? CC-NaCl: 1.753 × 10 −5 = 𝑥 2 8.3×10 −5 = 3.04 × 10 −5 → − log(3.04 × 10 −5 ) = ?. ?? Table 3 : Calculated ionic strength, μ, calculated activity coefficient, γ, calculated pH based on γ and μ. Solution μ γ Calculated pH based on γ and μ A .00998 .913 2.04 B 9.98 × 10 −4 .967 3.02

Post-Lab Questions (Answer in complete sentences): 1. What is electromotive efficiency measuring? Electromotive efficiency is a measure of how accurate a pH electrode is in measuring the pH of solutions. 2. How do the measured and expected values compare in Table 2. Is there a difference when NaCl is present? The measure and expected values for Table 2 were sometimes close and sometimes not. When NaCl was present, there was a not a common trend in pH change. The measured pH for solutions without NaCl were no closer to the expected pH than the solutions with NaCl. From this, we can conclude that salts do not have a significant effect on the pH of solutions. 3. How do the calculated values from Table 3 compare to the measured values in Table 2? When ionic strength and activity coefficients are considered, the pH for A-E were still somewhat far from what was measured. 3A was only .05 more than 2A, and 3C was .02 higher than 2C, but 3B was .99 more than 2B. It was a bit inconsistent. The most noticeable difference, however, was between the Table 3 pH ’s for the NaCl solutions and the measured ones from Table 2. The pH was unchanged for B-E of NaCl in table 3 but changed in table 2. 4. What happened to the pH measurement in the absence and presence of salt? What is attributed to the effect observed? The pH measurements for solutions without salt were more acidic than those with salt. This is because of the common ion effect, where ionization is suppressed due to both electrolytes having a common ion. In this case, the common ion was chloride. 5. The measurement of pH is really the measurement of activity. It is not common practice to find the activity coefficients when we measure pH. Explain why it is commonly ignored. Activity is commonly ignored because the pH with activity coefficients are generally very close to measured values, which was demonstrated in the data. This fact would be better supported if our meter had been properly calibrated. 6. How do the calculated pH values in Table 4 compare to those measured and listed in Table 2? The pH’s of table 4 and table 2 were supposed to be closer together, but again, our meter was poorly calibrated. Generally, using a calibration curve to calculate pH would yield similar results to the pH’s actually measured by the calibrated meter. 7. There are 9 errors in pH measurements as found on page 355-356 of the textbook. Explain how any or all of them were an error in the measurements in this lab experiment. The errors are standard, junction potential, junction potential drift, sodium error, acid error, equilibrium time, hydration of glass, temperature, and cleaning. 1. For our data, I think that the acid error applies because our pH was more acidic than expected for D, E, D-NaCl, E-NaCl, AA, CC, DD, EE, and NaCl BB-EE. The