Bubble measurement precautions

Measurements on molten metals. The maximum bubble pressure method has proved one of the most satisfactory, but sessile drops, and drop-volumes have also been used with success.2 The principal difficulty lies in the proneness of metals to form skins of oxides, or other compounds, on their surfaces and these are sure to reduce the surface tension. Unless work is conducted in a very high vacuum, a freshly formed surface is almost a necessity if the sessile bubble method is used, the course of formation of a surface layer may, if great precautions are taken, be traced by the alteration in surface tension. Another difficulty lies in the high contact angles formed by liquid metals with almost all non-metallic surfaces, which are due to the very high cohesion of metals compared with their adhesion to other substances. 

The determination of density (specific gravity) (ASTM D-287, ASTM D-891, ASTM D-941,ASTM D-1217, ASTM D-1298,ASTM D-1555, ASTM D-1657, ASTM D-2935, ASTM D-4052, ASTM D-5002, IP 160, IP 235, IP 365) provides a check on the uniformity of the gasoline, and it permits calculation of the weight per gallon. The temperature at which the determination is carried out and for which the calculations are to be made should also be known (ASTM D-1086). However, the methods are subject to vapor pressure constraints and are used with appropriate precautions to prevent vapor loss during sample handling and density measurement. In addition, some test methods should not be appUed if the samples are so dark in color that the absence of air bubbles in the sample cell cannot be established with certainty. The presence of such bubbles can have serious consequences for the reUabiUty of the test data. 

Special precautions are needed for the determination of the sulfide ion. At very low sulfide ion concentrations 1 M NaOH solution should be used. This insures that the sulfur present exists exclusively as the S ion. In such solutions, however, the danger of oxidation by oxygen from the air is very great. The sample solution should be kept for just a short period of time, and in a well-stoppered flask without much air space above the solution. During the measurement the solution should not be stirred, and no inert gas should be introduced, since bubbling the solution will result in loss of gaseous H2S. The electrode should be dipped as deeply as possible into the test solution, and allowed to stand undisturbed during the slow establishment of po-... 

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