Sodium sulphate heptahydrate, 5.29

Sodium Sulphate Heptahydrate. Heat the sodium sulphate solution in the second flask to 32 °C in a water bath. When the liquid becomes transparent and contains no crystals, close the mouth of the flask with cotton wool and carefully, without shaking the contents, put it into a cooling mixture (ice with a small amount of sodium chloride). In 10 or 15 minutes, carefully extract the flask with the sodium sulphate solution. Note the shape of the crystals separated in the given case. 

With regard to sodium sulphate heptahydrate, the same considerations will hold as in the case of the decahydrate. Since at 24 the four phases heptahydrate, anhydrous salt, solution, vapour can coexist, the vapour-pressure curves of the systems hydrate—anhydrous salt— vapour (curve EB) and hydrate—solution— vapour (curve FB) must cut the pressure curve of the saturated solution of the anhydrous salt at the above temperature, as represented in Fig. 75 by the point B. This constitutes, therefore, a second quadruple point, which is, however, metastable. 

The elements present in a host of pharmaceutical substances are determined quantitatively by atomic absorption spectroscopy, for example Pd in carbenicillin sodium Cu, Pb and Zn in activated charcoal Fe in ascorbic acid Ag in cisplatin Ph and Zn in copper sulphate Zn in glucogen Zn in insulin Pb in oxprenolol hydrochloride Ni in prazosin hydrochloride Zn in sodium sulphite heptahydrate, and Cd and Pb in zinc oxide. 

The properties of the alkali sulphates.—Lithium sulphate can be prepared as the anhydrous and hydrated as monohydrated lithium sulphate, Li2S04.H20 and sodium sulphate as the anhydrous salt, as heptahydrated sodium sulphate, NaS04-7H20 and decahydrated sodium sulphate, Na2S04-10H20. Mono- and trihydrated sodium salts have been reported—the former by J. Thomsen,22 the latter by H. Rose—but L. G. de Goppet has questioned the two last-named hydrates. 

Sodium sulphate readily forms a supersaturated solution in water.9 When a solution is cooled to about 5° C., the heptahydrate crystallizes out. The crystals of the decahydrate weather in air, owing to loss of water of crystallization. 

The solubility relations of sodium sulphate illustrate very clearly the importance of the solid phase for the definition of saturation and supersaturation. Since the solubility curve of the anhydrous salt has been followed backwards to a temperature of about 18 , it is readily seen, from Fig. 73, that at a temperature of, say, 20 , three different saturated solutions of sodium sulphate are possible, according as the anhydrous salt, the heptahydrate or the decahydrate, is present as the solid phase. Two of these solutions, however, would be metastable and supersaturated with respect to the decahydrate. 

On heating a mixture of sodium sulphate decahydrate and magnesium sulphate heptahydrate, it is found that at 22° partial liquefaction occurs with formation of astracanite. At this temperature, therefore, there can coexist the five phases... 

In the early part of the 19th century several workers made the experimental observation that some aqueous solutions of inorganic salts, when cooled rapidly, first deposited crystals of a less stable form than that which normally crystallizes. A frequently quoted example is that of sodium sulphate solution which can precipitate heptahydrate crystals at around room temperature before the thermodynamically stable decahydrate appears. Another is the crystallization of an unstable polymorph of potassium nitrate in advance of the more stable rhombic form. 

Calcium-free magnesium carbonate. Dissolve 24 6 g of analytical reagent grade magnesium sulphate (heptahydrate) in 200 ml of water and add 20 per cent sodium hydroxide solution until a precipitate just forms. Heat the mixture to 80° and add, while stirring, a hot solution of 10 6 g of anhydrous sodium carbonate in 200 ml of water. Allow to cool, filter off the precipitate, wash thoroughly with water and dry at 110°. 

Reflux 1 to 1 5 g of the sample with 25 ml of 0 5N ethanolic potassium hydroxide for one hour, evaporate the ethanol, add 50 ml of water and warm until the residue is diffused throughout the liquid. Cool, add 80 ml of water and a solution of 1 5 g of magnesium sulphate heptahydrate in 50 ml of water, mix thoroughly and allow to stand for ten minutes. Filter, wash the residue with 20 ml of water and acidify the combined filtrate and washings with concentrated hydrochloric acid. Extract with four 40-ml quantities of ether, discard the aqueous liquid and extract the combined ether extracts with 20, 20, 10, 10 and 10 ml of 5 per cent sodium bicarbonate solution, washing each extract with the same 20 ml... 

Dissolve 25 g of analytical reagent quality sodium chloride and 8 g of magnesium sulphate heptahydrate, MgSO -THjO, in each liter of distilled water. 

Dissolve 25 g of analytical reagent quality sodium chloride and 8 g of magnesium sulphate heptahydrate, MgSO VHgO, in each liter of distilled water. The water is equivalent, for analytical purposes, to sea water of salinity 28%a. This solution is best made up in 5- to 20-liter quantities at a time and must be stored in a polyethylene container. The silicon content of this solution should not exceed 1 or 2 /Ltg-at/liter. 

Because of salt effects standards should be made with surface sea water of low nitrate content or with a synthetic sea water containing 620 g sodium chloride, 200 g of magnesium sulphate heptahydrate, and 5 g of sodium bicarbonate monohydrate per 20 liters of distilled water. The amount of nitrate in this sea water is generally too great to be neglected and a blank should be run on it when standardizing. 

---