Coulometric Titration of Acid

Written by Morgan

I. Introduction:

This experiment is concerned with determination of the concentration of a standard solution of HCl. Coulometric titrations are performed using MicroLab FS 522 with MicroLab Model 290 sensor adaptor module to gather data regarding electric current variability. The theories underlying this variety of titration include redox reactions, current and total charge, and the techniques of coulometric measurement of electrolysis can be found in the following materials¹:

1. D.C. Harris, Quantitative Chemical Analysis, 8^th ed., W.H. Freeman, 2010, Chapter 13, pp. 318-321.

II. Experimental Procedure:

Coulometric Titration of Acid, instructor’s handout for experiments scheduled for April 2, 2013 was used. Generally procedures given in this handout were used.

Deviations:

1. All steps pertaining to MicroLab system set-up were omitted. Procedure was pre-programmed in the specific “coulometric titration” MicroLab program used.
2. No analyses of acetic acid in vinegar or HCl in toilet bowl cleaner were performed.
3. Voltage was set to 2.500 Volts for all trials.
4. Four trials were performed instead of three, due to large deviations in first three trials. All four were used in calculating average results.

III. Data and Results:

IV. Calculations:

• Faraday’s constant (96,485 C/mol)
• Pertinent rxns: H₂O + e^– à ½ H₂ (g) + OH^– (aq)   and    H3O⁺ (aq) + OH^– (aq) à 2H₂O
• Find moles of acid consumed at endpoint

Trial 1:

At endpoint (displayed by solution color change due to indicator):

Time = 200 seconds, 3229.741 mC

Moles e- = 3229.741 mC *(1C/1000mC) *(1 mol e-/96485 C) = 3.35E-05 moles e-

Moles HCl consumed = 3.35E-05 (1:1 ratio)

Volume HCl = 5.0E-04 L

Molarity of Acid = moles HCl / volume HCl = (3.35E-05 mol/5.0E-04 L)

= 0.06695 M HCl

Trial 2:

Time = 500 seconds, 7155.126 mC

Moles e- = 7155.126 mC *(1C/1000mC) *(1 mol e-/96485 C) = 7.4E-05 moles e-

Moles HCl consumed = 7.4E-05 (1:1 ratio)

Volume HCl = 5.0E-04 L

Molarity of Acid = moles HCl / volume HCl = (7.4E-05 mol/5.0E-04 L)

= 0.1483 M HCl

Trial 3:

Time = 96 seconds, 1241.451mC

Moles e- = 1241.451mC *(1C/1000mC) *(1 mol e-/96485 C) = 1.29E-05 moles e-

Moles HCl consumed = 1.29E-05 (1:1 ratio)

Volume HCl = 5.0E-04 L

Molarity of Acid = moles HCl / volume HCl = (1.29E-05 mol/5.0E-04 L)

= 0.02573 M HCl

Trial 4:

Time = 162 seconds, 2502.043 mC

Moles e- = 2502.043 mC *(1C/1000mC) *(1 mol e-/96485 C) = 2.59E-05 moles e-

Moles HCl consumed = 2.59E-05 (1:1 ratio)

Volume HCl = 5.0E-04 L

Molarity of Acid = moles HCl / volume HCl = (2.59E-05 mol/5.0E-04 L)

= 0.05186 M HCl

Average:

0.06695 M + 0.1483 M + 0.02573 M + 0.05186 M

= 0.07321 M

Standard Deviation:

Sqrt[∑(0.06695 – 0.07321)² + (0.1483 – 0.07321)² + (0.02573 – 0.07321)² + (0.05186 – 0.07321)²/3]

=0.05288 M

V. Conclusions and Discussion:

The majority of the discussion that follows will focus on this question: Given a consistent Voltage at 2.500 V, what caused deviations in Trial results at endpoint to be consistently large?

In this analysis of HCl, the electrolyte NaBr was added to prevent Ag+ formed at the anode from interfering at the cathode. The Br- stabilized the Ag cations, precipitating AgBr, so that the reduction of water alone can occur at the cathode. The presence of Br ions allowed for the setup of the non-isolated electrodes in the analyte solution. However, the oxidation of Br anions at the anode interferes with that of H20, resulting in deviation among trials by this reaction’s variance. This was negligible though, because the precipitation of AgBr in this system was favored over Br- oxidation. So, neither of these appear as main sources of error in the experiment.

A second potential cause of large deviation in trial results was the variation in amount of HCl added per trial. Although a precise micropipetting device was used, amounts could not have all been the exact same. At this point, this cannot be discredited as a potential main source of error.

The judgement of endpoint time by observation of the indicator does accounted for the level of deviation in times either.

VI. References:

1. D.C. Harris, Quantitative Chemical Analysis, 8^th ed., W.H. Freeman, 2010, Chapter 13, pp. 318-321.