Title

Potentiometric Measurement – Determination of the pH of an alkali solution, a buffer solution , a hair shampoo, a hand wash and distilled water.

Objective

To carry out potentiometric measurement to determine whether a solution is controlled by the buffer system or by a strong electrolyte. To prepare the samples with different concentration by adding distilled water. Introduction

The potentiometric measurement is to determine the difference in potential between a working (an indicator) electrode and a counter (reference) electrode by using the potentiometer. The working or indicator electrode represents the cathode which locates at the right half-cell whereas the counter or reference electrode represents the cathode which locate at the left half-cell.

In most of the chemical processes, pH is one of the most general laboratory measurements because many chemical processes are dependent on pH. The speed rate of chemical reactions can often be significantly altered by changing the pH of the solution. Apart from that, the solubility of many chemicals in solution and their bio-availability is dependent on pH. The physiological chemistry of living organisms usually has very specific pH boundaries.

The term pH derives from a combination of pH for the power and H for the symbol of the element Hydrogen. pH is the negative log of the activity of hydrogen ions.

pH = 〖log〗_(10 a) H^+

At a given temperature, pH represents the activity of hydrogen ions in a solution. The term activity is used because pH reflects the amount of available hydrogen ions, not concentration of hydrogen ions.

Although the litmus paper is the most common used to determine the pH value of the solution, the only trustworthy way to measure the pH is by using the potentiometric electrodes. By differing the concentration of Hydrogen ion (H^+), those electrodes essentially monitor the changes in potential which is also called the voltage. The reasons of using the electrodes to measure the pH are they have the wide range, highly precision, unaffected by the sample colour or turbidity, low cost, fast response and real time in-situ measurement.

The majority of pH electrodes available are the combination electrodes which they combine the reference and pH sensing elements into single electrode. Separate pH and reference electrodes can still be used, but this tends to be for the highest precision, research measurements. Physically, combination pH electrodes are much more practical to use.

The most important step towards accurate pH measurements is by making sure the pH electrode is calibrated probably. Calibration adjust the slope and offset of the slope is produced using the Nernst equation and is usually expressed as a percentage of a theoretically perfect slope (i.e. 100%). However, the limitation of using Nernst Equation is the temperature should maintain at 250C because it is temperature dependent. Besides that, the presence of additional potentials in the electrochemical cells which also called the liquid junction potential, it could be minimized by a salt such as KCl.

Normally, pH buffers are used as standards to calibrate pH electrode. There are a large number of buffers available of different pH values (pH 1.68, pH 4.01, pH 6.86, pH 7.00, pH 9.18 and pH 10.01 at 25 0C) and of varying quality, so it is important to know what should be using. Well, in this experiment, we are using the buffer solutions with pH 4.00, pH 7.00 and pH 10.00 only.

A calibrated glass pH electrode is used to measure the pH of commercially available shampoos; both concentrated and diluted (i.e. 1:10 and 1:100). An estimate is made whether the pH is controlled primarily by a buffered solution or a strong electrolyte.

Procedures

Calibration of the electrodes The experiment had been performed by the first calibrating the electrodes at pH7 and making approximate pH measurements of the various samples. The glass electrode had been soaked in0.1M KCl solution at least one day prior to its use; it had been stored in the dilute KCl solution when not in used. Only three buffers solution were used for calibrations such as pH 4, 7 and 10. This would consist essentially of adjusting the meter to read the pH of the standard buffer at room temperature with the electrodes immersed in the buffer solution. The electrodes were rinsed with distilled water after calibration and it was blotted with tissue paper; they were not being wiped as this would impart a static change to the glass membrane. The pH meter was turned to “standby” carefully when removing the electrodes from the solution. If only a small quantity of buffer was used, it would be better to discard it rather than chance contamination of the entire supply.

pH Measurement of samples A 1:10 dilution of each sample was prepared in a 100-ml beaker by pipetting 5.00ml into 45.0ml distilled water measured from a 50-ml burette. A 1:100 dilution of the sample was prepared in a 100ml beaker by pipetting 5.00ml of the 1:10 diluted sample into 45.0ml distilled water measured from a 50ml burette. The pH of first the diluted samples (1:10 and 1:100 dilution) and then the concentrated sample were measured. If a small beaker was used, it might be possible to make the measurement with 5ml of the sample; it was just enough to cover the bulb of the glass electrode. For the combination pH reference electrode, a salt bridge wick for the reference electrode was above the pH glass bulb and so the probe will have to be immersed a bit deeper to make contact with both electrode. There would be two wire leads from the probe, one for each electrode. The electrode was immersed in the test solution and it was swished or agitated for a few seconds. The pH reading is allowed to equilibrate and the nearest 0.01pH was recorded. The electrode was rinsed well between measurement and it was blotted off with the water. It was best to make the measurements from low pH to high, or vice versa. If the pH reading of any of the samples fell outside the range of 4 to 10, the electrodes had been recalibrated with a buffer closer to the sample pH.

Method

Potentiometric Measurement Techniques

Apparatus and Materials

pH meter Standard pH 4.01, 7.00, and 10.01 buffers 〖NH〗_3-〖NH〗_4 Cl buffer solution Diluted ammonia solution (29ml concentrated ammonia in 50ml water) Hair shampoo Beaker Retort Stand Burette Clamp