Lab 8 Report

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Kennesaw State University *

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Chemistry

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Feb 20, 2024

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REPORT SHEET LAB 8: How does the pH electrode work? Date of the experiment: 10/19/23 Student Name: X Prelab Questions: 1. What is the Nernst equation and how is it related to pH? ? = ? ° − 𝑅𝑇 𝑛? ln ( 𝐴 𝑚 𝐴 0 ) Cell reactions with H ⁺ will have a value of E will dependent on the pH. The Nernst equation can predict the effect of pH on the cell reaction. Provide Equations 14-14, 14-16, 15-5 and 15-6 from your textbook. ??????: ? = ? ° − 𝑅𝑇 𝑛? ln ( 𝐴 ? ? 𝐴 ? ? ) ?????? ?? 25 ????𝑖??: ? = ? ° − 0.05916𝑉 𝑛 log ( 𝐴 ? ? 𝐴 ? ? ) ??????𝑖? 𝑃?????𝑖?? ?𝑖???????? ?? 25 ????𝑖??: ? = ???????? − 0.05916𝑉 𝑛 log𝐴 0 𝑅??????? ?? ????? ?????????: ? = ???????? + ?(0.05916𝑉)log𝐴 𝐻 + (????𝑖??) ? = ???????? − ?(0.05916𝑉)pH (????𝑖??) a. Define all the terms in those equations in a. (14-14): 𝑬 ° = 𝑅?????𝑖?? 𝑃?????𝑖?? R = Gas constant (8.314 J/mol * k) T = Temperature (K) F = Faraday Constant (9.649e-4) c/mol) n = number of electrons 𝚨 𝒊 = Activity Constant of Species (15-5): n = Charge of Analyte Ion (15-5 & 15-6): β = Electromotive Efficiency (1.00) 𝚨 ? = Activity in Outer (unknown) b. Explain why Equation 15-5 and 15-6 are different from 14-16. Equation 15-6 is for measuring glass electrode response and 15-5 is for electric potential difference for ion selective electrodes. 14- 16 is an equation that doesn’t consider electromotive efficiency like 15-6 does or the pH outside the electrode, like 15-5 does, but all equations are Nernst-like. c. Explain how Equation 14-16 is different from 14-14. 14-16 is only used at 25 ◦ C (room temperature conditions) whereas 14-14 can be applied at any temperature. 2. How is the Nernst equation displayed graphically? a. Equation 15-6 is graphed in Figure 15-19 on page 355. Using the points provided on the graph, and estimating as best you can, find the slope of the line. ??𝑖???: (4, 200)& (7.2, 0) ? = 0−200 7.2−4 = −62.5 mV b. With reference back to Equation 14-14, describe what would happen to the slope if the temperature was increased from 25°C to 37°C (body temperature).

If the temperature increases, this would cause the slope to increase relatively. The voltage, 0.05916V would increase with the temperature, and therefor the slope would become steeper/more negative. 3. How is activity involved with pH? a. Give the ionic strength equation (Equation 8-3). ? = 1 2 (? 1 ? 1 2 + ? 2 ? 2 2 + ⋯ ) b. Give the Extended Debye-Hückel equation (Equation 8-6) ???? = −0.51𝑧 2 √ 𝜇 1−𝛼( √ 𝜇 305 ⁄ ) c. Give the pH equation as defined in Equation 8-8. ?? = −???𝐴 𝐻 + d. Referring back to equation 15-6, how would an activity coefficient of 0.90 affect results compared to an activity coefficient of 1.00? Increasing ionic strength result s in the decrease of activity. Therefore, a γ of 0.90 would have a higher electric potential difference. 4. How does the pH electrode work? a. Draw Figure 15-14 in your notebook and label all the parts (for pre-lab question, you draw it in your notebook; for your report, take a picture 15-14). b. Explain the need for the air inlet in Figure 15-14. The air inlet allows air for the electrolyte to drain through the porous plug. It allows pressure to be released. c. Write out the line diagram found on page 352.

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.

What is the slope of the two-point calibration of the pH electrode? -56.397 What is the electromotive efficiency based on the slope? 95.32% Show your calculation: ∆? ∆𝑝𝐻 = −23.9−145.29 7−4 = −56.397 mV −? × 0.05916𝑉 = −0.056397𝑉 → −? = − 0.056397 0.05916 = −.9532 = −? → .9532 Standard Operating Procedure – How to calibrate a pH meter 1. Turn the meter on using the power/light button 2. Press mode to ensure pH is selected 3. If there is a colored ring at the top (purple for this model), rotate it so the color black shows through the hole instead of white 4. Remove the storage bottle from the tip of the electrode and push the cap ring up the probe 5. Gently rinse the probe with DI water and wipe the probe, but be careful to only BLOT the tip where the membrane is 6. Submerge the tip of the electrode into pH 4 buffer solution and swirl the probe while pH is stabilizing 7. Once stable, press STD to standardize, then press the middle grey button to clear the history. Wait for the meter to say “Press STD to standardize,” then press STD 8. Remove the electrode from pH buffer, wash with DI and gently dry with a Kimwipe, and submerge in pH 7 buffer solution

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