Although a variety of methods have been employed for the determination of solubility, only a few are suitable for the first year laboratory. Those described in this journal include the recovery of solute after evaporation of solution(1-5), loss of soluted during saturation (2,3) , titration of a species in solution(5-8), colorimetric determination of the concentration of a species in solution(9-11), determination of the pH of the solution(12), determination of the activity of a radio - labeled solution(13) , gravimetric analysis of cation concentration in solution(14) , and observation of the crystallization point during cooling (15). We describe here a method that is relatively fast and requires only a Buchner-type filter funnel, 500-mL round-bottom flask, and analytical balance. Because it involves only two weighings and no transfer of precipitate it is also potentially quite accurate. In our method, the compound whose solubility is to be determined is weighed directly into a clean, dry filter crucible. The compound is then dried for an hour in a 110° oven directly in the filter crucible. The crucible is then inserted into the round bottom flask and a given volume of distilled water, usually either 100 or 1000 mL, is then passed through the compound using aspiration. The compound remaining in the filter crucible is then dried in a 110° oven for two hours and then reweighed. The loss of weight is the mass of the compound dissolved in the given volume of water. Table 1 gives the average of two to five determinations of the solubility for a series of compounds of different solubilities at a temperature of 23 ± 2 °C.
|Compound||Exp Sol/g L-1||Lit Sol/g L-1|
A comparison of the experimental and tabulated solubilities indicates that the agreement is in general quite good. With crystalline compounds like Ba(NO3)2, accuracy was greatly increased by powdering the compound prior to weighing into the funnel, and by gently stirring the compound with a glass stirring rod during the suction extraction. Obviously with compounds as insoluble as BaSO4, the technique produces larger errors. Of course, the porosity of the filter crucible must be adjusted for the particle size of the compound. We have found that a D porosity works fine for most compounds and gives reasonably rapid filtration, but E porosity can increase accuracy and precision slightly for insoluble, finely divided compounds like Ba3(PO4)2. The average amount of time required for filtration using one liter of water in a D porosity filter is generally less than 10 minutes.
The hazards associated with this procedure are those related to the compounds chosen for solubility determination. Information about the handling of these compounds can be found on its MSDS.
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