Exposure of the crucible and of the upper section of the furnace tube to the atmosphere during the time (about 1.5 minutes) required to load the sample was found to have little effect on the overall blank of the procedure. Atmospheric contamination increased the final gravimetric measurement by only about 0.2 mg., an amount which is small compared to the quantities of about 100 mg. usually measured for each sample.Most of the gravimetric measurements in this investigation were made with weighing tubes having 12/6 ground ball joints for connection to the analytical train. Near the end of the work, however, these were replaced by ball joints of the same size but with rubber O-rings embedded in the center of the face of the male section, and with the female section smooth (unground). As a result of this change, an improvement in the precision of weight data was noted. Probably there had been minor misfits in the ungreased, ground ball joints that had connected the regular-type weighing tubes to the analytical train. The O-rings eliminated this potential source of error.Although a general procedure is applicable to the analysis of a variety of oxides, minor modifications to the procedure may be necessary to give the best results for specific oxides. The ultimate degree of accuracy and precision attainable by this method clearly depends upon the art of the analyst in successfully carrying out the oxidereduction step without the loss of sample.
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The effect of bicarbonate on the formation of the chromium(III)-thiocyanate complex is an example of coordination catalysis by a secondary ligand and may be a vehicle for the study of elementary solution kinetics.
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