Experiment 4 - Potentiometric Titration

CHE130L Analytical Chemistry Laboratory

MALAYAN COLLEGES LAGUNA

EXPERIMENT NO. 4 POTENTIOMETRIC TITRATION I.

OBJECTIVES Upon completion of the experiment, the student should be able to:

II.



calibrate an electrometric pH meter;

 

use potentiometric measurements to determine the pH of an unknown; determine the factors affecting pH of solutions; and



determine the ionization constant of a weak acid by potentiometric titration.

A. LABORATORY EQUIPMENT / INSTRUMENTS Equipment/ Accessories 250-mL beaker pH meter Magnetic stirrer Iron stand Pipet Rubber aspirator Stirring rod Iron stand Iron clamp

Quantity 2 1 2 1 1 1 1 1 1

B. CHEMICALS AND REAGANTS Chemical/ Reagent Standard NaOH solution Distilled water in wash bottle Standard buffer solutions (ph 4.0 and pH 7.0) Unknown acid

III.

DISCUSSION OF FUNDAMENTALS Introduction In the previous experiment, we are already concerned of the different concepts that underlies about acid-base titrations. Acid-base titrations, by concept, is also called a neutralization reaction, since it involves an acid and a base (that is regardless of strength, either one is weak or one is strong or either weak or both strong and ad infinitum), and regularly have the products of salt and water. Experiment 4: Potentiometric Titration

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EXPERIMENT NO. 4 POTENTIOMETRIC TITRATION The completion of the reaction is reached when the number of moles of the acid is equal to the moles of the base, or technically the equivalence point is reached. In this point, if the stoichiometric ratios of the acid and base are equal, we can solve for the concentration of the unknown by using the dilution equation, that is, M1 V1=M2 V2. For diprotic and triprotic acids and bases that are titrated with a strong acid/base, however, have many equivalence points, depending on how protic it is(i.e. a diprotic acid/base has two equivalence points, whereas a triprotic acid/base has three) since the acid/base has to converted to its less acidic/basic form, or its intermediates. A good example would be a diprotic acid titrated using a strong base (in this case, sodium hydroxide), or H 2 A. The reaction would proceed, forming HA-, and ultimately forming the base, A2-. But in this titration, you cannot determine its actual pH at a certain aliquot of the titrant, since what we are using in this type of titration is an indicator, an organic compound added to the solution to see the equivalence point by a change of color. In doing so, a pH meter is used to monitor the measurement of pH of the titration process at hand. The pH meter measures the [H+] concentration of the solution, so if the titration goes from base to acid, the pH must be computed as pH=14-pOH, where pOH is the initial reading of pH. By using a pH meter in an acid-base titration to monitor the pH variations of the titration process, we call this process a potentiometric titration, wherein it finds the equivalence point not through an indicator, but through a graph that signifies the relationship between the recorded amount of titrant used, and the measured pH of the system.

Discussion Potentiometric titration as has been introduced in the previous experiment also needs the slow addition of a titrant to an analyte to determine the endpoint. Potentiometric titration makes use of a pH meter to monitor the pH of the resulting solution upon addition of the titrant. A plot is then made of the resulting pH against volume of added titrant. In cases where the change in pH at the equivalence point are too small to obtain a sharp Figure 1. Sample of titration curve endpoint, the slope of the titration curve can be plotted against the volume of added titrant and a sharp peak corresponding to the equivalence point can be obtained The tip of the sharp peak, known as the inflection point, corresponds to a value where the slope goes through a maximum (increasing to decreasing values).

Experiment 4: Potentiometric Titration

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EXPERIMENT NO. 4 POTENTIOMETRIC TITRATION Aside from determining amounts of analytes, titration data, especially the titration curve can help in the determination of identities of unknown acids and bases. Various portions of the curve show species that are present during the course of the titration. The equivalence point also known as the neutralization point shows all analyte has been used up, and after which, the titrant is in excess. For the titration of an unknown weak acid, the pH at the midpoint (half-way point), also termed as the point of 50% neutralization point, gives the ionization constant of the acid. Figure 2. 1st plot derivative of sample titration curve

Applications The potentiometric titration method has many applications. These applications, however, seek proper tests to satisfy the conditions for quantitative analysis, i.e. starting a test of a sample with known concentration, etc. Potentiometry is mainly applied to acid-base, precipitation and redox reactions. On acid-base reactions, its practical application would be on the field of knowing the pH levels of the different manufactured food and many others, as well as common household items. For precipitation reactions, the determination of how chlorinated the water sample is done by finding the chloride ions in different water samples, and comparing them. The chloride ions are generally insoluble to water, so by a precipitation reaction, it will do the trick. Lastly, for redox reactions, potentiometry is used to determine the ferrous (Fe 2+) ions in certain medicinal tablets, as iron goes on two states, one having a 2+ charge(ferrous) and one with a 3+ charge(ferric). By the redox reaction and its counterpart potentiometric titration, we can successfully determine if the iron in the tablet is ferrous or is ferric.


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EXPERIMENT NO. 4 POTENTIOMETRIC TITRATION IV.

METHODOLOGY

The pH meter was calibrated according to the instructions given.

The pHs of the standard buffers used for calibration were recorded.

Figure 3. Calibration of the pH meter

After calibrating, the pH of the unknown acid was recorded. When the results stray far from the expected, the probe was calibrated using the two buffers. The acid was titrated potentiometrically with 0.50-mL aliquots until there was no change in its pH. Volume and pH were recorded with every addition.

After every pH measurement, the probe was washed with distilled water.

A graph showing the relationship between the pH and titrant volume was made.

Figure 4. Potentiometric titration of unknown acid


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EXPERIMENT NO. 4 POTENTIOMETRIC TITRATION V.

DESCRIPTION OF THE APPARATUS / SET – UP

Figure 5. Potentiometric titration set-up

Figure 6. pH meter device

A pH meter is an instrument that measures the hydrogen ion concentration of the solution it will be used into. The experiment we’ve done uses a portable version, one that can run without plugging in an electric socket since it runs on batteries. Beside the probe is a metal that detects the temperature, which can be seen on the display screen. The probe, metal and the surrounding parts are washed with distilled water as to remove the remaining traces of the last measured sample. The pH meter has two modes, calibrate and measure. Before measuring any sample, it is a good practice to always calibrate the probe with the green and red buffer solutions of 4.01 and 7.00 pH measurements. This is to ensure the accuracy and precision of the results of the measuring to be done. VI.

DATA SHEET TITRANT VOLUME 6 6.5 7 7.5 8 8.5

pH 1.94 1.99 2.26 2.34 2.4 2.52


TITRANT VOLUME 22 22.5 23 23.5 24 24.5

pH 7.52 7.64 7.71 7.86 7.99 8.13 Page | 5



EXPERIMENT NO. 4 POTENTIOMETRIC TITRATION 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 19 19.5 20 20.5 21 21.5

VII.

2.64 2.64 2.55 2.82 2.71 2.85 2.8 3.04 3.28 3.45 4.14 4.48 4.95 5.45 5.78 6.24 6.45 6.89 6.95 6.98 7.02 7.05 7.12 7.2 7.37 7.51

25 25.5 26 26.5 27 27.5 28 28.5 29 29.5 30 30.5 31 31.5 32 32.5 33 33.5 34 34.5 35 35.5 36 36.5 37 37.5

8.3 8.6 8.82 9.18 9.33 9.72 9.9 9.97 10.07 10.11 10.34 10.57 10.74 10.82 10.85 11.05 11.12 11.21 11.45 11.53 11.6 11.6 11.61 11.62 11.65 11.65

SAMPLE COMPUTATIONS (

)

|

|

(

)

Probable identity of acid: Citric acid (7.4 x 10-3) or Phosphoric acid (7.5 x 10-3)


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CHE130L Analytical Chemistry Laboratory VIII.


EXPERIMENT NO. 4 POTENTIOMETRIC TITRATION RESULTS AND DISCUSSIONS Titration is the controlled neutralization of an acid and a base. If we consider the titration of a weak acid by a strong base, we can analyze the situation completely and determine all the concentrations of the aqueous species at any volume addition of the titrant. From the figure, one can notice that the titration of a weak acid with a strong base is Figure 7. Titration curve of weak acid titrated by a strong base different from that of a strong base to a relatively strong acid titration, primarily in the beginning of the titration, or before the equivalence point is reached. Moreover, the equivalence point itself is shifted upward in pH, making the equivalence point arrive in the basic pH.

The experiment started with the calibration of the pH meter. A pH meter is an instrument that measures the hydrogen ion concentration of the solution it partakes unto, and if this should be done, one must need to calibrate it first to ensure accuracy for the proceeding myriad of measurements to follow. There are various types of pH meters, and the one in this experiment is the portable version, one that can run without plugging it in since it runs on batteries. By having two standardized buffer solutions of different concentrations (one preferably neutral and the other acidic/basic), the pH meter is calibrated accordingly by the following steps:

Figure 8. Example of a portable pH meter.

1. Turn on meter and set MODE to CAL 2. Wash probe with distilled water. 3. Put the probe on one solution. When display registers ‘READY’, press enter. Read until the desired

concentration is registered. 4. Rinse the probe with distilled water and slightly shake to dry. 5. Repeat 3 for the other solution. When finished, put MODE to MEAS (Measure). Experiment 4: Potentiometric Titration

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EXPERIMENT NO. 4 POTENTIOMETRIC TITRATION The titration process is started with the usage of the standardized NaOH solution from Experiment 3(Acid-base titrations). The NaOH solution is used to fill the burette to the 0.00 mL mark. By having a known volume of the unknown acid, the initial pH is measured by the pH meter. This initial pH will be used for the computation of the ionization constant of the unknown acid. After this, the potentiometric titration process will now commence. Potentiometric titration is mainly used in this because it does not need an indicator to determine the equivalence point, but by titrating in aliquots, not considering whether it will break through the equivalence point or not. By using 1.50, 0.50 and 0.20 (from which our group only did the second aliquot) mL aliquots at different containers, the acid is to be titrated with it, and measuring the pH for every aliquot that is added to it. In doing this, one can observe that the change in pH is small, but there would be a sharp change in pH upon the middle of the titration. This sharp change is characterized by the solution’s tendency to reach the equivalence point of the titration process. The titration is stopped when the pH of the system is constant. This pH is referred to as the pH of the titrant, from which it will be already constant since all of the unknown acid has been converted to its salt and water form by reacting with NaOH. To understand what the values basically mean, the values are plotted as the pH on the y-axis, and the volume of the titrant (cumulative) on the x-axis. This graph is now what we call as the titration curve. By observation, the graph looks like an intestine shaped like an ‘S’. This type of graph is what we call a sigmoidal curve, a logistic curve (or one that is related to exponential functions). The end point will be found in this graph, as the location of the sudden change in pH will be clear. Since this experiment will be under the preference that it assumes the acid is a hypothetical monoprotic acid HA, and if was made to react to a strong base, then the resulting graph would only have one equivalence point.


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EXPERIMENT NO. 4 POTENTIOMETRIC TITRATION

14 12 10

pH

8 6 4 2 0 0

5

10

15

20

25

Volume of titrant (mL)

30

35

40

Figure 9. pH vs. Volume of titrant

Based on the graph will be the determination of the ionization constant K a of the unknown acid. This will come in two ways. First would be the calculation of the ionization constant using titration data. By getting the volume of the titrant at the equivalence point, on the half-equivalence point, and the pH value of 50% neutralization, we can compute for the pH of the system by using the equation:

and

. After this, the other method would be the

calculation of the ionization constant based on initial pH. By having the assumed concentration of the unknown acid to be equal to the concentration of the titrant and its initial pH, the pH will be computed by the following equation:

. By comparing both values to some known

acid’s ionization constants, the identity of the unknown acid will be clarified.


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EXPERIMENT NO. 4 POTENTIOMETRIC TITRATION IX.

SUMMARY AND CONCLUSIONS This experiment concluded the concepts involving potentiometric titration. By specifically using a calibrated pH meter using buffer solutions of standardized concentrations, the pH of the solution is constantly measured as to adding an exact amount of the titrant that is measured accordingly to its equivalent aliquot. This is done until the pH of the system is constant and more or less basic (since we are handling an analyte that is classified as an unknown acid, which is being titrated by a strong base). After plotting the values of pH against volume of the titrant, we form a graph that will determine the equivalence point of the reaction. By using the values of the volume @50% neutralization, the equivalence point and the initial value of the pH, we can co mpute for the ionization constant of the unknown acid, as well as identifying the molecular formula of the acid and identifying the acid itself. Errors should be avoided especially on the consistency of putting an exact amount of the titrant, as this will in small amounts, will collectively deter the position as to where the equivalence point should be, therefore getting an incorrect ionization constant in the following changes, and it further deteriorates the consistency of the experiment itself.

X.

POST LAB QUESTIONS 

Which of the two methods of calculation can give the more accurate value for the ionization constant? Explain your reason. Cite possible errors incurred. Answer: The more accurate method of knowing the ionization constant is the one using the initial pH, by the equation:

. This is more accurate than the other one, since the

50% neutralization volume in our graph is not clearly determined at the first sight of it, therefore it will leave us to do interpolation, which might not be needed to do for there is a given formula for getting the ionization constant, and there wouldn’t be any need to do that. Moreover, the initial pH is among the most accurate of all the pH measurements, since it is the first to be measured by the freshly calibrated pH meter, and in effect, will have the most accurate computed ionization constant, Ka .


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EXPERIMENT NO. 4 POTENTIOMETRIC TITRATION XI.

REFERENCES Christian, Gary D. 2004. Analytical chemistry (6th ed.). John Wiley and Sons Inc. Hage, David S. and James D. Carr. 2011. Analytical chemistry and quantitative analysis. New Jersey: Pearson Prentice Hall. Skoog, Douglas et. al. 2004. Fundamentals of Analytical Chemistry (8 th ed.). Singapore: Thomson Learning.


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Experiment 4 - Potentiometric Titration

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