Title: Determination of pH
- Objective: To determine the pH of samples to represents the acidity or alkalinity of an aqueous solution of the sample.
- Principle: The acidic or basic property of substances is measured in terms of pH. It is a measurement of the hydrogen ion concentration. pH is defined as the negative logarithm (base 10) of hydrogen ion concentration.
If the hydrogen ion concentration is very high, the pH value is very low. This is determined using a scale ranging from 0-14 called the pH scale. Substances with pH lower than 7 are acidic, those with pH equal to 7 are neutral and those with pH greater than 7 are basic in nature.
3. Procedure:
The pH determination is carried out at a temperature of 25°C ± 2°C, unless otherwise specified in the individual monograph.
- Apparatus:
The pH value of a solution is determined potentiometrically by means of a glass electrode, a reference electrode and a pH Meter either of the digital or analogue type.
Operate the pH meter according to the manufacturer’s instructions. Calibrate the apparatus using buffer solution D as the primary standard, adjusting the meter to read the appropriate pH value given in Table 1, corresponding to the temperature of the solution. To set the scale, use a second reference buffer solution, either buffer solution A, buffer solution E or buffer solution G or buffer solution I and carry out a check with a third buffer solution of intermediate pH . The pH reading of the intermediate solution must not differ by more than 0.05 from the corresponding value indicated in Table 1.
- Reference Buffer Solutions:
Note: Prepare the following buffer solutions using Purified water. Buffer solutions should be stored in bottles made of alkali-free glass, and must not be used later than 3 month after preparation or avail readymade buffer solution from the chemical supplier with COA.
Buffer A: A 1.271% w/v solution of Potassium Tetraoxalate. (Dissolve 1.271 g of Potassium Tetraoxalate in sufficient water and dilute to 100 ml with water.)
Buffer B: A freshly prepared solution, saturated at 25°C, of Potassium Dihydrogen Tartrate. Dissolve Potassium Dihydrogen Tartrate in about 50 ml of water until it no longer dissolves creates saturated solution of Potassium Dihydrogen Tartrate.
Buffer C: A freshly prepared 1.151% w/v solution of Potassium Dihydrogen Citrate. (Dissolve 1.151 g of Potassium Dihydrogen Citrate in sufficient water and dilute to 100 ml with water.)
Buffer D: A 1.021 % w/v solution of Potassium Hydrogen Phthalate, previously dried at 110°C to 135°C for 2 hours. (Dissolve 1.021 g of Potassium Hydrogen Phthalate in sufficient water and dilute to 100 ml with water.)
Buffer E: A mixture containing 0.348% w/v of Potassium Dihydrogen Phosphate (Dissolve 0.348 g of Potassium Dihydrogen Phosphate in sufficient water and dilute to 100 ml with water.) and 0.355% w/v of anhydrous Disodium Hydrogen Phosphate (Dissolve 0.355 g of anhydrous Disodium Hydrogen Phosphate in sufficient water and dilute to 100 ml with water.), both previously dried at 110°C to 130°C for 2 hours.
Buffer F: A mixture containing 0.1184% w/v of Potassium Dihydrogen Phosphate (Dissolve 0.1184 g of Potassium Dihydrogen Phosphate in sufficient water and dilute to 100 ml with water.) and 0.4303% w/v of anhydrous Disodium Hydrogen Phosphate (Dissolve 0.4303 g of anhydrous Disodium Hydrogen Phosphate in sufficient water and dilute to 100 ml with water.), both previously dried at 110°C to 130°C for 2 hours.
Buffer G: A 0.3814% w/v solution of Sodium Tetraborate (Dissolve 0.3814 g of Sodium Tetraborate in sufficient water and dilute to 100 ml with water.) stored protected from carbon dioxide.
Buffer H: A mixture containing 0.2649% w/v of Sodium Carbonate (Dissolve 0.2649 g of Sodium Carbonate in sufficient water and dilute to 100 ml with water.) and 0.210% w/v of Sodium Bicarbonate (Dissolve 0.210 g of Sodium Bicarbonate in sufficient water and dilute to 100 ml with water.).
Buffer I: Shake excess of Calcium Hydroxide with Purified water and decant at 25°C.
- Method:
Immerse the electrodes in the solution being examined and measure the pH at the same temperature as for the standard solutions. At the end of a set of measurements, record the pH of the solution used to standardise the meter and electrodes. If the difference between this reading and the original value is greater than 0.05, the set of measurements must be repeated.
When measuring pH values above 10.0 ensure that the glass electrode is suitable for use under alkaline conditions and apply any correction that is necessary.
Table 1 – pH of Reference buffer solutions at various temperature
| Temperature
in °C |
Reference Buffer Solution | ||||||||
| A | B | C | D | E | F | G | H | I | |
| 15
20 25 30 35 |
1.67
1.68 1.68 1.68 1.69 |
—
— 3.56 3.55 3.55 |
3.80
3.79 3.78 3.77 3.76 |
4.00
4.00 4.01 4.02 4.02 |
6.90
6.88 6.87 6.85 6.84 |
7.45
7.43 7.41 7.40 7.39 |
9.28
9.23 9.18 9.14 9.10 |
10.12
10.06 10.01 9.97 9.93 |
12.81
12.63 12.45 12.29 12.13 |