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73 <br />that if' we cannot see particulates in the water, then we <br />won't filter. We have tested this rule of thumb by taking <br />unfiltered samples and comparing them with samples that have <br />been filtered by various methods (35). The concentrations <br />of all species were the same to within 5% on all but two <br />cases. In one case, the water was slightly turbid and the <br />analysers were predictably different. In the other case, the <br />sample couldn't be filtered until 20 minutes after collection <br />and the results were different. So, if the water looks tur- <br />bid, filter and do it immediately. If not, filtering can oe <br />neglected. <br />If the water is clean, great care should be taken to <br />insure that the stre_am.is__not roiled up during collection <br />so that suspended material inadvertantly finds its way into <br />• the sample. Collection should be done at a pool so that as <br />much suspended material as possible has settled out. Also, <br />if a large sample bottle is placed in a stream, filling it <br />sometimes roils up the system. This is the reason for the <br />plastic beaker on the necessities list. Use it to sample <br />the water with little disturbance to the system. <br />Handling the Sample <br />The species in water are usually not in equilibrium <br />and precipitation reactions will occur if the water is left <br />to stand as is. Consequently, the water has to be stabilized. <br />For cations this is done by adding nitric acid to the sample. <br />The amount added is 5 ml of 1:1 HNO3 per liter of water. <br />• <br />