Second Edition 1979 Third Edition 1989

Glassware and other containers All glass apparatus and containers used in analytical work must be carefully selected to meet the requirements of their particular use. Although several types of special-purpose glasses are available, borosilicate thermal-resistant types, such as Pyrex or Kimax, are generally satisfactory for all ordinary labor-atory purposes in water analysis. Borosilicate glass is especially suitable for storage of neutral or acid solutions, for volumetric glassware, and for conducting reactions. Because borosilicate glass is not entirely resistant to attack by strongly alkaline solutions, bottles of polyethyl-ene or Teflon need to be used for storage of standard solutions of silica, boron, and alkali-metals hydroxides. All volumetric glassware, such as burets, pipets, and volumetric flasks, must be of boro-3 silicate glass and must contain or deliver vol-umes within the tolerances of the method. In addition if such glassware is frequently used to measure strongly alkaline solutions, it needs to be recalibrated at frequent intervals. Directions for such calibration and testing of volumetric glassware are given by the National Bureau of Standards (1959) and in standard texts of quan-titative analysis. Evaporations may be carried out in glass, por-celain, zirconium, or platinum dishes. Platinum is preferred if the weight of the residue needs to be determined accurately, because the weight of platinum vessels is relatively constant. Although platinum is one of the most resist-ant metals, it is not completely inert and is subject to embrittlement. The following precau-tions are recommended: Never put solutions containing tin, mercury, or lead in a reducing environment in platinum; if the free metal should be formed it will alloy with the platinum, especially if heated. Do not heat mixtures of hydrochloric acid with oxidizing substances, such as nitrate or manganese dioxide; ferric chloride in hydrochloric acid attacks platinum appreciably. Place hot platinum vessels on