Potentiometric Titration

POTENTIOMETRIC TITRATION Rady J. Remigio

February 4, 2008

DATA AND RESULTS TABLE 1. STRONG ACID-STRONG BASE TITRATION pH V NaOH (mL)

Phenolphthalein indicator 0 2.13 1 2.16 2 2.2 3 2.25 4 2.30 5 2.35 6 2.42 7 2.52 8 2.62 9 2.77 10 3.00 11 3.62 11.1 3.84 11.2 4.39 11.3 5.65 11.4 6.25 11.5 6.84 11.6 9.05 11.7 9.87 11.8 10.22 11.9 10.47 12 10.67 16 11.90 20 12.10 Concentration of NaOH=0.2N, HCl=0.1N

Methyl red indicator 2.14 2.17 2.21 2.26 2.31 2.37 2.44 2.52 2.62 2.76 3.00 3.64 3.88 4.31 5.79 6.25 6.72 9.08 9.93 10.28 10.51 10.69 11.89 12.08

TABLE 2. WEAK ACID-STRONG BASE TITRATION V NaOH 0 1 2 3 4 5 6 7 8 9 10 11 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9 12 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 12.9 13 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.9 14 17 20

pH phenolphthalei methyl red n 4.10 4.04 4.36 4.29 4.57 4.52 4.72 4.7 4.86 4.84 4.99 4.98 5.11 5.10 5.22 5.22 5.34 5.34 5.47 5.47 5.61 5.61 5.79 5.78 5.81 5.79 5.82 5.81 5.84 5.84 5.86 5.86 5.88 5.88 5.91 5.91 5.94 5.93 5.97 5.96 5.99 5.99 6.03 6.02 6.06 6.05 6.10 6.08 6.14 6.13 6.17 6.16 6.22 6.20 6.28 6.25 6.32 6.30 6.38 6.35 6.48 6.43 6.55 6.52 6.63 6.61 6.74 6.72 6.93 6.89 7.26 7.22 8.85 7.89 9.44 9.38 10.02 10.32 10.56 10.73 10.68 11.88 11.84 12.12 12.09 wt wt KHP=0.6066g KHP=0.6101g

Graph 1 pH vs mL NaOH (Phenolphthalein) 14

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pH

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Volume NaOH (mL)

Graph 2 ΔpH/ΔV vs. mL NaOH (Phenolphthalein) 25

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ΔpH/ΔV

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0 0

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15 Volume NaOH (mL)

Graph 3 2nd Derivative Curve (Phenolphthalein) 2000

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Δ2pH/ΔV2

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-2000 Volume NaOH (mL)

Graph 4 pH vs mL NaOH (Methyl Red) 14

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pH

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15 Volume NaOH (mL)

Graph 5 ΔpH/ΔV vs mL NaOH (Methyl Red) 25

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ΔpH/ΔV

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Volume NaOH (mL)

Graph 6 Second Derivative Curve (Methyl Red) 2500

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Δ^2pH/ΔV^2

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Graph 7 pH vs mL NaOH 14

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8 pH

ph≈5. 6

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Volume NaOH (mL) Phenolphthalein indicator

Methyl Red indicator

Graph 8 ΔpH/ΔV vs mL NaOH 18

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ΔpH/ΔV

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8

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15 Volume NaOH (mL) Phenolphthalein

Methyl Red

Graph 9 Second Derivative Curve 1500

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Δ^2pH/ΔV^2

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0 0

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-1500 Volume NaOH (mL) Phenolphthalein

Methyl Red

DISCUSSION OF RESULTS Many acid-base titrations are difficult to accomplish using a visual indicator for several reasons, for example, there may not be a suitable color change available for a particular type of titration or the solutions themselves may be colored, opaque or turbid. It may be desired to automate a series of replicate determinations. In such situations, potentiometric titration, using a glass hydronium ion selective electrode, a suitable reference electrode and a sensitive potentiometer (a pH meter) may be advantageous. Any acid-base titration may be conducted potentiometrically. Two electrodes, after calibration are immersed in a solution of the analyte. One is an indicator electrode, selective for H3O+ and the other a stable reference electrode. The potential difference, which after calibration is pH, is measured after the successive addition of known increments of acid or base titrant. The critical problem in a titration is to recognize the point at which the quantities of reacting species are present in equivalent amounts. The titration curve can be followed point by point, by plotting successive values of pH against the corresponding volume of titrant added. The experiment is divided into two parts – strong acid-strong base, and weak acid-strong base titrations. For the first part, approximately 0.2N NaOH and 0.1N HCl are used. For the second part, KHP was used for the weak acid solution and the same NaOH solution for the base. In the first part, 25.00ml of HCl is put in a beaker and diluted to about 100ml with distilled water. The beaker, with a magnetic bar is placed in the magnetic stirrer. The electrodes are then inserted and adjusted into the solution. A burette is set up containing the NaOH with the tip touching the side to prevent spattering. The pH of the solution is then measured before addition of the NaOH titrant. Then 1ml of the base solution is

added and the pH measured. The pH values as well as the burette readings are then recorded. The recording of pH and burette readings are continued after the addition of 1ml until the total volume added reached 11, where the equivalence point is almost reached. From this point, successive 0.1ml portions of the titrant are added until the equivalence point is passed. After the volume of 12, two more readings are recorded after adding 4ml each until the total volume added reached 20ml. Finally, the plots of pH vs. ml of NaOH, ΔpH/ΔV vs. ml NaOH, and Δ2pH/ΔV2 vs. ml NaOH are made and the volume ratio of the two solutions determined. The titrations and recordings were used for two indicators-phenolphthalein and methyl red. In the second part, 0.60 g standard potassium acid phthalate is weighed to the nearest 0.1mg and dissolved in about 100ml of distilled water. It is then titrated with the sodium hydroxide solution. The same procedures as in the first part are then followed. The two tables show the volume and pH readings for the titrations. Graphs 1 and 4 show the plot of pH vs. mL NaOH. Graphically, the inflection point in the steeply rising portion of the curves can be used to estimate the end point. Graphs 2 and 5 show the plot of ΔpH/ΔV vs. ml NaOH. The endpoint can be estimated from the maximum, which corresponds to the point of inflection. Graphs 3 and 6 show the plot of second derivative of pH over the change in volume, squared, Δ 2pH/ΔV2 vs. ml NaOH. The point at which the second derivative crosses zero is the inflection point, which is taken as the end point of the titration. At the equivalence points, which is about 11.5mL NaOH added, the volume ratio is 11.5/25=0.46 or almost 1:2. Graphs 7-9 show the same plots but for the weak acid-strong base titrations. The graphs of strong acid-strong base and weak acid-strong base are almost the same but the curves have differences. The pH vs. volume graphs have steeper and longer rising portions in strong acid-strong base than in weak acid-strong base. In the first derivative graphs, the inflection point in the weak acid-strong base is clearer for there is only one inflection but it is smaller and narrower. Comparison of Phenophthalein & Methyl Red 14

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Transition range of phenolphthalei

pH

8

Transition range of methyl red

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Among the indicators, phenolphthalein is better to use in a weak acid-strong base titration because of its transition range of about pH 8.3-10, which is where the color change occurs, is close to the equivalence point, where the graph shows an abrupt change. The transition range of the methyl red indicator, which is about 4-6, does not fall within the area where the equivalence point occurs. Its color change would occur before reaching the equivalence point, therefore providing greater errors. From the plot of pH vs. mL NaOH, the equivalence point is reached by adding about 13.5mL of NaOH. Half this volume can be used to determine the pH, which equals the pKa of the acid. At half the volume, which is 6.75, the pH is about 5.25. Therefore, pKa=5.25. REFERENCES Skoog, et.al., Fundamentals of Analytical Chemistry, 8th ed, Brooks/Cole, 2004. www.towson.edu