GRAVIMETRIC DETERMINATION OF POTASSIUM AND SODIUM IN LOW-MINERALIZED WATERS

M. E. Effendi, V. B. Sokolovich

(Presented by Prof. B. V. Tronov, Doctor of Chemical Sciences)

The determination of alkali metals, owing to the high solubility of their salts, presents considerable difficulties. In water analysis, sodium is determined gravimetrically by means of magnesium or zinc uranyl acetates at sodium concentrations of, respectively, not less than 10 mg/ml or 1 mg/ml [3, 5, 8]. Potassium is determined by the laborious chloroplatinate method [1, 3], and also with the use of nitro complexes [2, 4, 7, 10, 13] or tetraphenylboron sodium [15]. At potassium concentrations of not less than 1.3 mg/ml it is precipitated by sodium or magnesium dipicrylaminates [9, 12, 13, 14]; the latter is also used for photocolorimetric determination [6]. Experimental determination of alkali metals is often replaced by calculation, usually of the sum of milligram-equivalents of potassium and sodium [1, 3].

We set ourselves the objective of determining the applicability of magnesium uranyl acetate and sodium and magnesium dipicrylaminates for the gravimetric determination of sodium and potassium in low-mineralized waters.

Determination of Sodium

After determining the effect on the accuracy of sodium determination of the temperature and time of precipitation and drying of the precipitate, excess precipitant, and admixtures of sulfate ions, potassium, magnesium, and calcium ions, a procedure was developed, subsequently tested on a 0.0519 N sodium chloride solution (Table 1) and on river-water samples with additions of sodium chloride (Table 2). Magnesium uranyl acetate is used as the precipitant; to prepare it, 5.6 g of magnesium oxide is dissolved in 50 ml of heated 80-percent acetic acid solution, mixed with 6.4 g of uranyl acetate, 100 ml of ethyl alcohol, and 30 ml of water. The mixture is heated on a water bath for two hours, then cooled, diluted to 200 ml, and left overnight. After filtration of the precipitate that has separated, the solution is stored in a Jena-glass flask.

250 ml of water is evaporated more than 10-fold, freed from sulfate ion with 0.1 N barium chloride solution, and made up to 25 ml. Sodium is precipitated at 0° with periodic stirring of the reaction mixture. Into a Pyrex-glass beaker placed in a snow bath, 5 ml of water is introduced (with a sodium content of not less than 0.2 mg/ml); first 10 ml of cooled magnesium uranyl acetate solution is added, then after 20 minutes 5 ml more is added twice. After one hour the precipitate is filtered through glass filter No. 3; the remnants of the precipitate are extracted with a cooled solution of the precipitant saturated with the triple salt. The precipitate is washed first with small portions of alcohol saturated with the triple salt, then with acetone. It is dried—by a stream of air through the crucible for several minutes, then in a desiccator at room temperature. It is weighed after 2 hours. The weighing form is the triple acetate of sodium, magnesium, and uranium (1:1:3) with six molecules of water. The conversion factor to sodium is 0.01536.

Table 1

Table 2

Determination of potassium

Taking into account previously obtained data on the effect, on the completeness of potassium precipitation, of temperature, excess precipitant, artificial narrowing of crystallization, and also information from the literature on the effect of impurities [11], a procedure was drawn up for the determination of potassium, subsequently tested on a 0.0107 N solution of potassium chloride (Table 3) and on river-water samples with additions of potassium chloride (Table 4). As precipitant, both a 1% aqueous-alcoholic solution of sodium dipicrylaminate and a solution of magnesium dipicrylaminate [2, 12] are used.

From 250 ml of water, evaporated to 40 ml, the sum of sesquioxides is precipitated. The filtrate is acidified with hydrochloric acid and evaporated to dryness; the residue is ignited until ammonium ions and organic impurities have been removed. The ignited residue is dissolved in hot distilled water; after filtration the volume of the solution is brought to 25 ml. Into a small glass beaker, 5 ml of water (with a potassium content of about 0.4 mg/ml) is introduced and, rapidly with stirring, 5 ml of a 1% solution of sodium dipicrylaminate. The reaction mixture is kept for 15–20 minutes, with stirring, in a bath with snow; then precipitation is continued at room temperature for two hours. A glass filter No. 3 is used for filtration. The residues of the precipitate are extracted with a chilled sodium dipicrylaminate solution. The precipitate is washed with ether chilled in snow, dried first by a stream of air through the filter, and then in a drying oven at 90–80°. The weighing form is potassium dipicrylaminate. The conversion factor to potassium is 0.08191.

Table 3

Table 4

Determination of Sodium and Potassium

In the analysis of waters, the determination of alkalis is best carried out in a separate sample.

After evaporating 250 ml of water and removing the sum of the alkaline-earth oxides, sulfate ions, ammonium ions, and organic impurities, the volume is brought to 25 ml and potassium and sodium are determined from aliquot portions (5 ml) by the methods indicated above. If a low-mineralized water is evaporated not 10-fold but 50-fold (i.e., 1000 ml to 20 ml), then the determination of sodium and potassium with the indicated reagents can be carried out at room temperature.

In several samples of natural waters, the alkalis were determined both by the methods proposed by us and by calculation of the sum of milligram-equivalents from the data of a complete chemical analysis (Table 5).

Table 5

Conclusions

1. The possibility has been demonstrated of determining, in natural waters of low mineralization, sodium with the aid of magnesium uranyl acetate and potassium with the aid of sodium dipicrylaminate and magnesium dipicrylaminate.

Literature

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