SABCO GROUP

 

                                          TPRI Formulation and Pesticide residue staff members

pH measurement in Water samples

1. INTRODUCTION This is the measurement of the hydrogen-ion concentration in the water. Natural waters usually have pH values in the range of 4 to 9 and most are slightly basic because of the presence of bicarbonates and carbonates. A pH below 7 is acidic (the lower the number, the more acidic the water, with a decrease of one full unit representing an increase in acidity of ten times) and a pH above 7 (to a maximum of 14) is basic (the higher the number, the more basic the water). Natural fresh waters have a pH range from 4.0 to 10.0, High pH values tend to facilitate the solubilization of ammonia, heavy metals and salts. The precipitation of carbonate salts is encouraged when pH levels are high. Low pH levels tend to increase carbon dioxide and carbonic acid concentrations. Lethal effects of pH on aquatic life occur below pH 4.5 and above pH 9.5. Mining, agriculture, industrial effluents, acidic precipitation (derived from emissions to the atmosphere from cars and industry). 2. SCOPE This method is applicable for testing pH of water and waste water within a range of 4.5 to 9.5. 3. INTERFERENCES The glass electrode is relatively free from interference from colour, turbidity,oxidants,and reductants .Sodium error which normally interfere with electrode is minimized by using special low sodium error electrode 4. PRINCIPLE The basic principle of pH measurement is determination of the activity of the hydrogen ion by potentiometric measurement using a standard hydrogen electrode and a reference electrode or a combination pH electrode. 5. SAFETY Wear protective laboratory protective gears and read the Material Safety Data Sheet before commencing the analysis. Refer to part 1090 page 1-50 of Standard Methods for the Examinations of Water and Wastewater, 23rd edition, 2017, APHA. (CWQL/ED 004) 6. QUALITY CONTROL Refer Standard Methods for the Examinations of Water and Wastewater, 23rd edition, 2017, APHA 4020B Quality Control Practices, and EPHA 3020 Quality assurance/Quality control. (CWQL/ED 003) 7. SAMPLING AND SAMPLE STORAGE For collection of samples and preservation refer to Standard Methods for the Examinations of Water and Wastewater, 23th edition, 2017, APHA 1060B and 1060C Collection of samples,Storage and Preservation (CWQL/ED 005) 8. APPARATUS 8.1 pH meter consisting of potentiometer with temperature compensation probe 8.2 Combination pH electrode 8.3 Beakers - preferably use polyethylene / disposable cups 8.4 Magnetic stirrer, 8.5 TFE - coated stirring bar. 9. REAGENTS Three different value of Standard buffer solutions for pH calibration. 10. PROCEDURES 10.1. CALIBRATION As per equipment manufacturers operating manual. 10.2 pH Measurement of Sample 10.2.1 Rinse the electrode and temperature probe with distilled water. 10.2.2 Rinse the electrode, temperature probe and beaker with portion of sample. 10.2.3 Place a fresh portion of the sample in the beaker containing a magnetic stirring bar and place it on the magnetic stirrer plate, stir gently to minimise carbon dioxide entrainment. 10.2.4 Immerse the electrode and temperature probe into the sample and allow the meter reading to stabilise. Note: Store electrode in distilled water or a buffer solution between measurements. 11. CALCULATION AND REPORTING OF RESULTS Read direct and give pH results with corresponding temperature 12. REFERENCES Standard Methods for Examination of Water & Wastewater, 23rd Edition [2017], APHA, Washington DC, APHA Method 4500-H+ B.