SOP FOR Determination of pH

Principle

Electrometric Method

The pH value is determined by measuring the electromotive force of a cell that is comprised of a test solution-immersed indicator electrode (a hydrogen-ion-responsive electrode, such as a glass electrode) and a reference electrode (typically a mercury/calomel electrode), which is contacted with the test solution through a liquid junction that forms part of the reference electrode. The electromotive force is measured using a pH meter, which is a high-impedance voltmeter calibrated in terms of pH.

Although the hydrogen gas electrode is recognized as the primary standard for electrometric pH determination, the combination of a glass electrode and a calomel electrode with the reference potential provided by a saturated calomel electrode is generally used. Several types of electrodes have been suggested for this purpose. The glass electrode system is based on the fact that a change of 1Ph unit produces an electrical change of 59.1 mV at 25°C. The membrane form a partition between the two sides of the membrane which is proportional to the difference in pH between the liquids.

Interference

1. Above pH value of 10, high sodium concentration interfere with the measurement, correction for the sodium error may be made by consulting the chart supplied by the manufacturer of electrodes being used. Sodium errors at pH value levels greater than 10 can be reduced or eliminated by using a low sodium error electrode.
2. Oil and grease may interfere by coating the pH electrode and causing a sluggish response. These coatings Can usually be removed by gentle wiping or detergent washing, followed by distilled water rinsing. An additional treatment with hydrochloric acid (1%) may be necessary to remove any remaining film.
3. Temperature affects the pH values in two ways. The first is covered by the change in electrode output at various temperatures. This interference can be controlled with instruments having temperature compensation or by calibrating the electrode-instrument system at the temperature of the sample. The second source is the change of pH inherent in the sample at various temperatures. This error is sample-dependent ant cannot be controlled. Therefore, the temperature at the time of analysis should be reported.

Apparatus :-

1. pH meter– With glass and reference electrode (saturated calomel), preferably with temperature compensation.
2. Magnetic stirrer- With polytetrafluoroethylene coated stirring barr.
3. Thermometer- With least count of 0.5

Reagents

Standard pH buffer solution from available tablets or powder, or know amount of chemicals may be used for the preparation. Procedures for the preparation of some standard pH buffer solution are given below and show the pH value of these buffers at different temperatures.

1. Borax buffer- 0.01 M solution, pH value 9.18 at 25°C. Dissolve 3.814g borax. (Na₂B₄O₇.10H₂O) in deionised or distilled water and dilute to 1lit. Fresh borax may be used or it may be recrystallized but it should not be over dried. For preparation of dilution water freshly boil and cool deionised or distilled water to expel carbon dioxide gas. Specific conductance of dilution water should be less than 2µS at 25°C and pH value 5.6 to 6.0 for preparation of all standard solution.
2. phosphate buffer- 1:1 solution, pH value 6.865 at 25°C. For preparing 0.025M potassium dihydrogen phosphate and 0.025M disodium hydrogen phosphate, dry potassium dihydrogen phosphate and sodium dihydrogen phosphate, in an oven at 130°C for 2 hours and cool in a desiccator. Dissolve 3.388 g potassium dihydrogen phosphate and 3.533g sodium dihydrogen phosphate in deionized or distilled water and make up to 1 liter.
3. Tartrate buffer- 0.034M solution, pH value 3.56 at 25°C. Prepare a saturated solution of potassium hydrogen tartrate in deionised or distilled water.
4. Phthalate buffer– 0.05M solution, pH value 4.008 at 25°C. Dissolve 10.12g potassium hydrogen phthalate min deionised water and dilute to 1 litter.
5. Tetraoxalate buffer- 0.05M solution, pH value 1.68 at 25°C. Dissolve 12.51g potassium tetetraoxalate dehydrate in deionised water and dilute to 1 litre.
6. Calcium hydroxide buffer- 0.0203M solution, pH value 12.45 at 25°C. Ignite well-washed calcium carbonate (CaCO₃) of low alkali grade in a platinum dish at 1000°C for 1 hour. Hydrate the cooled calcium oxide by adding slowly, with stirring, distilled or deionised water and heat to boiling. Filter the cooled suspension and collect the solid calcium hydroxide on a fritted glass filter of medium porosity, Dry the collected calcium hydroxide in an oven at 110°C, cool and pulverise to uniformly fine granules. Vigorously shake an excess amount of this product in polyethylene bottle with distilled or demineralised water. Allow the gross excess to settle and filter by suction through a fritted glass funnel. Keep the bottle securely stoppered to prevent ingress of carbon dioxide.

Samples Handling and Preservation

1. Samples should be analysed as soon as possible, preferably in the field at the time of sampling.
2. High purity waters and waters not at equilibrium with the atmosphere (ground water or lake waters collected at depth) are subject to changes when exposed to the atmosphere. Therefore the sample containers should be filled completely and kept sealed prior to analysis.

Procedure

• Follow the manufacturer’s instruction for operation of pH meter. After required warm-up period, standardize the instrument with a buffer solution of pH near that of the sample and check electrode against at least one additional buffer of different pH value.
• Measure the temperature of the water and if temperature compensation is available in the instrument adjust it accordingly.
• Rinse and gently wipe the electrodes with solution, if field measurements are being made, the electrodes may be immersed directly in the sample stream to an adequate depth and moved in a manner to ensure sufficient sample movement across the electrode sensing element as indicated by drift free readings. (<0.1pH unit).
• If necessary, immerse them into the sample beaker or sample stream and stir at a constant rate to provide homogeneity and suspension of solids. Rate of stirring should minimize the air transfer rate at the air-water interface of the sample.
• Note and record sample pH and temperature. However, if there is a continuous drift, take a second reading with the fresh aliquot of sample without stirring and report it as the pH value.

Calculations

Report pH to the nearest coefficient or 0.01 unit (if instrument ready up to 2 decimal places) and temperature to the nearest °C.

Safety Guideline

Proper safety & health precaution are taken while doing the analysis by using safety goggles, apron, safety shoes & using fume chamber for fume generating chemicals.

Reference

IS 3025 Part-11 1983 RA: 2017 and Manufacturer Manual