Hygroscopic substances weighing

Weigh the substance in a small open platinum boat weigh hygroscopic substances in a small weighing bottle. 

The final product must have a known, stable composition. A hygroscopic substance is one that picks up water from the air and is therefore difficult to weigh accurately. Many precipitates contain a variable quantity of water and must be dried under conditions that give a known (possibly zero) stoichiometry of H20. 

Alter, H. K. Hygroscopic Substances in Microanalysis. Part I. New Techniques for Drying and Weighing Solids. Mikrochem. 25, 47 (1938). 

The substance should be unaltered in air during weighing this condition implies that it should not be hygroscopic, oxidised by air, or affected by carbon dioxide. The standard should maintain an unchanged composition during storage. 

In general, liquid substances of B.P. above 170° may be weighed directly into the boat. If the substance is hygroscopic the boat must be enclosed and weighed in a dry stoppered tube. Volatile... 

The chromium(VI) oxide is hygroscopic weigh it quickly in the hood and do not leave the bottle unstoppered. The substance dissolves very slowly in acetic acid-water mixtures, and solutions are prepared by adding the acetic acid only after the substance has been completely dissolved in water. 

These highly purified chemicals may be weighed out directly for the preparation of solutions of selected concentration or for the calibration of solutions of unknown strength. They are supplied with a certificate of analysis for each lot. These chemicals must be stable substances of definite composition that can be dried, preferably at 104 to 110 C, without a change in composition. They must not be hygroscopic, so that water is not absorbed during weighing. 

Drying is always necessary as a preliminary to weighing, whether the substance is hygroscopic or not. It is usually effected, in water-ovens (Fig. 55), if a temperature of 100 (213° F.) be sufficient or in air-ovens, somewhat similarly constructed, if a higher temperature be desired. As a substance can never be accurately weighed while it is warm, it is removed from the oven and placed in the desiccator (Fig. 56), over HjSO or CaCh, until it has cooled. 

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