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Dissolved oxygen is probably the outstanding influence in the corrosion of boilers and accessories. It must be reduced to a certain minimum, depending on conditions, in order to secure satisfactory service. Speller (1) states that the amount permissible depends upon the hydrogen ion concentration and the nature of the feed water, but, in some cases, e.g. large units operating at high pressure with evaporated water, the feed water should not contain over 0.05 cc. per liter of oxygen. The method commonly employed for the determination of dissolved oxygen is the one proposed by Winkler (2) or some modification of it. This method depends on the fact that bivalent manganese is oxidized to a higher state of valence in an alkaline solution in the presence of oxygen. The resulting products are then allowed to act on potassium iodide in acid medium and the liberated iodine is titrated with a standard solution of sodium thiosulfate using starch as an indicator. Several modifications have been suggested from time to time in order to take care of interfering substances such as "organic matter," nitrites and iron (3, 4, 5). Aside from the interference of such substances, neither the Winkler method nor the modifications suggested, are particularly satisfactory for the low concentrations of dissolved oxygen ordinarily encountered in boiler water control. Small variations in titrations may cause marked differences in the results secured, due to the fact that the amount of standard thiosulfate solution required is quite small. Sodium thiosulfate solutions are not particularly stable and require frequent checking. If this is not done, the variations introduced when such a weak solution, 0.025 N or less, is used may be quite large. In addition some form of laboratory with the necessary equipment is |
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