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7s <br />• Storing Samples <br />For the acidified water samples which will be used for <br />trace metal analyses, deterioration of the sample poses no <br />problem. In our laboratory, we have stored mine drainage <br />waters; for a year in polyethylene bottles with no change in <br />the concentration. For the unacidified water, one has to <br />act rapidly. The water will start to react to form precipi- <br />tates in the matter of an hour, especially if there is an <br />appreciable concentration of Fe in the sample. <br />pH MEASUREMENTS <br />:[n our experience, the measurement of pH is technically <br />difficult to do in the field. The primary cause for this <br />difficulty is the measuring concept involved in a glass mem- <br />brane electrode. Hydrogen ions have to flow through the mem- <br />brane to complete the electrical circuit. This is not easy <br />and consequently, a typical pH electronic circuit has an <br />impedance of at least 10,000,000 ohm (47). This high resis- <br />tance requires that the pH meter be a complex test instrument. <br />When such an instrument is taken in the field, it has to <br />be superbly constructed or otherwise the electronic components <br />will be damaged. Even with a good instrument, the following <br />problems have occurred to us more than once: <br />1. Moisture on the electrode jack causes intermittent <br />shorting of the circuit. <br />2. Oil or rust on the,electrode jack causes poor <br />connection in the circuit. <br />i <br />