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B.10.4. Interferences <br /> Factors that can effect pH are; temperature, atmospheric contamination, and ionic strength. <br /> The pH measurement is relatively free from interference from color, turbidity, colloidal matter, <br /> oxidants, or reductants. <br /> The temperature compensation on a pH meter only permits adjustment of the electrode slope. <br /> It does not compensate for changes in the potential of the reference electrode, the asymmetry <br /> potential of the glass electrode, or the liquid-junction potential. Nor does it compensate for <br /> changes in pH due to temperature. Thus, the temperature of the buffer and the unknown shall <br /> be recorded at the time of measurement. Ideally,their temperatures shall be within ±10°C. <br /> Atmospheric contamination can be a significant problem for groundwater samples. Dissolved <br /> oxygen (DO) and carbon dioxide can be evolved or dissolved when the sample is exposed to air, <br /> and a considerable change in pH may result. In situ measurements should be taken where <br /> possible, but for groundwater that must be pumped, the use of a flow cell gives the best <br /> results. <br /> Because of errors due to ionic strength (which are not worth correcting in the field), pH <br /> measurements shall be accompanied by a measurement of the conductivity. The pH is a <br /> measurement of the hydrogen-ion activity. An ideal solution is assumed in which other ions do <br /> not affect the hydrogen activity. This assumption deteriorates if the ionic strength is too high. <br /> Similarly, samples with low ionic strength will cause difficulties because the resistance of the <br /> sample approaches that of the glass electrode. For best results, samples with very low ionic <br /> strength should be stirred for a few seconds prior to the reading. Even then, it may require <br /> several minutes for the reading to stabilize. <br /> High sodium and alkalinity may also produce errors in the pH measurement. For example, for a <br /> solution of pH 9 with a sodium concentration of 10 moles per L (approx. 230 g/L), a special <br /> electrode is needed. Similarly, any pH value that is less than 1 or greater than 9 will have a <br /> greater uncertainty associated with it because the electrode response is non-Nernstian in those <br /> regions. <br /> B.10.5. Apparatus <br /> • pH meter with a temperature-compensating device, slope adjustment, and capable of <br /> reading pH to±1/100 units <br /> • A flow cell for continuous-flow measurements <br /> • Standard pH-buffer solutions of 4, 7, and/or 10 <br /> • Combination pH electrode (pH, conductivity, and temperature) <br /> • Temperature-measuring device capable of reading temperatures to ±1/10°C <br /> • Distilled or deionized water in a squeeze wash bottle <br /> Daub &Associates, Inc. Page B-26 Natural Soda LLC 2022 SAP <br />