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74 <br />This amount of acid is sufficient to prevent precipitation <br />of hydroxides, to kill most microbes, and to prevent adsorp- <br />tion onto the bottle walls. <br />The nitric acid is diluted because concentrated acid <br />attacks plastic bottles. No matter how much care is taken, <br />the acid bottle invariably leaks or spills so keep the bottle <br />and graduate wrapped tightly in a plastic bag so leaks can <br />be contained. <br />For pH measurements, bicarbonate analyses, and dissolved <br />solids measurements, unacidified water is needed. Usually <br />lZ of sample will be sufficient for analyses on the unacidified <br />water. For the acidified sample, collect 500 ml of water. <br />When. taking the sample the bottle you put,it_in may_be_ <br />dirty even though it is new. Rinse the bottle twice with a <br />• sufficient amount of the sample water to make sure any con- <br />taminants have been removed or replaced by what is in the <br />water. <br />Bottle s <br />Glass bottles are heavy and adsorb most trace metals <br />so the), are not good for inorganic analyses of water. Some <br />plastic: polymers such as polycarbonates or polyacrylics also <br />adsorb trace metals. The best bottles to use are made of <br />conventional polyethylene. This material is free from most <br />inorganic contaminants and also doesn't adsorb trace metals <br />as extensively as glass (46). <br />I -I <br />UA