Potassium-Sodium Cobaltinitrite Precipitate REX J. ROBINSON AND JAMES D. HAUSCHILDT University of Washington, Seattle, Washington
I
N THE determination of potassium as cobaltinitrite most investigators have assumed that the composition of the potassium-sodium cobaltinitrite precipitate produced under ideal conditions corresponds to the formula K2NaCO(h'02)6.H20,though a few have maintained that it is a mixture of compounds. Van Rysselberge (8) accounted for the nonstoichiomet'ric composition of precipitate which he obtained on the basis that i t was a mixture of the mono- and trihydrates of dipotassiumsodium cobaltinitrite. I n gravimetric estimations variation in hydration causes erratic results (9),whereas in colorimetric or volumetric work it has no effect. To explain nonstoichiometric precipitates in such cases there must be a variation ot'her than hydration. The most reasonable explanation is that there is a mixture of KNa2CO(SO2)6and KzNaCo(NO&,. I n support of this idea Sobel and Kramer ( 7 ) found in the precipitate a potassium-cobalt ratio of 2 t o 1.2, which would indicate such a mixture. The results of Hubbard (4) could have been due to such a mixture, though he thought it more likely that the precipitate was pure KlSaCo(KOz)6 and that the deviations resulted from experimental errors. Scheuler and Thomas (6) obtained a precipitate, formed a t a low temperature in a n alcoholic solution! which corresponded to the formula K1.S;Nal.43Co(K02)6. Preliminary to the analysis of sea water samples for their potassium content, a thorough study of the cobaltinitrite method was made in all its phases. Particular emphasis was placed on the application of the procedure of Scheuler and Thomas to the analysis of sea water. It was found that the composition of the precipitate could vary, depending upon the condition of precipitation and the method of handling the precipitate. As the literature on this point is incomplete, scattered, and controversial and is not fully appreciated, the following experimental results are presented.
Experimental Results REAGENTS AND METHOD OF ESTIMATION. In general the condi-
tions of determination were as specified by Scheuler and Thomas.
Except as specifically noted, the only variation in the procedure was in the manner of titration of the potassium-sodium cobaltinitrite with permanganate solution. When the approximate potassium content was known, the nitric acid was not added until just before the end point. In this manner an oxidizing agent (Mn02.zH20)was present when the solution was acidified and there was little opportunity for loss of oxides of nitrogen. Moreover, the titration was made with the solution at a temperature of almost 100' C., thus facilitating the reaction with the nitrite without fear of decomposition of permanganic or nitrous acids. The reagents were prepared as recommended by Scheuler and Thomas. A synthetic sea water was prepared by mixing the appropriate solutions. When 27.9 grams of sodium chloride, 4.5 grams of sodium sulfate, 5.8 grams of magnesium chloride, and 1.45 grams of calcium chloride were dissolved and diluted to 1 liter, the solution contained 22 parts of chlorine as chloride per 1000 parts of solution-i. e., 22 o / o ~ of chloride. Waters with less chloride per kilogram were prepared by appropriate dilution.
EFFECT OF FOREIGN TURBIDITY. Even when the cobaltinitrite precipitate is to be estimated by volumetric tit'ration or b y colorimetric comparison, the precipitating reagent should be free from suspended material. Otherwise these particles act as nuclei for the formation of the potassium-sodium cobaltinitrite crystals to give abnormal and irreproducible results. CobiposrTros OF THE REAGEST.It has been the custom to prepare the sodium cobaltinitrite reagent from sodium nitrite
and a cobaltous salt in the presence of acetic acid. Consequently the precipitation of the potassium-sodium cobaltinitrite has been from a solution containing acetic acid. As shown by Bowser ( 2 ) , the presence of acetic acid is beneficial in preventing the adherence of the precipitate to the walls of the container. Scheuler and Thomas prepared the reagent by dissolving sodium cobaltinitrite in water. This enabled them t o make the precipitation in a solution that was just acid with acetic acid, since they had found that less precipitate was obtained from either acidic or basic solutions. Experiments of the authors confirmed this conclusion. Although reproducible results were obtained in a solution containing acetic acid, i t seemed desirable to reduce the solubility of the precipitate as much as possible by working with a nearly neutral solution. Consequently a reagent of pure sodium cobaltinitrite Tyas used throughout the work. With a reduced temperature of precipitation and in an alcoholic solution there n-as never any difficulty with the precipitate's adhering to the beaker walls even in the absence of acetic acid. ALCOHOLIC PREcIPITATIOS REAGEST. The precipitation of potassium-sodium cobaltinitrite, for the most part, has been from aqueous solutions. Scheuler and Thomas precipitated from 25 per cent (by volume) ethyl alcohol a t a temperature of 6" C. According to them the beneficial effect of the alcohol was in the formation of crystals of larger particle size which facilitated their retention during filtration. A series of samples, each containing 8.79 mg. of potassium, \\as analyzed to determine the effect of alcoholic concentration upon the recovery of potassium (Table I). The alcoholic concentrations were expressed as per cent of total volume, R-hich, including the 5 ml. of reagent added to each sample, was 40 ml. One series of samples was prepared in distilled water and another in synthetic sea water m-here chloride equaled 22 O/oo. The precipitation was made at 6' C. Table I clearly shows the importance of careful regulation of the alcohol concentration, as the quantity of precipitate recovered was a function of the alcohol concentration. The increase in the quantity of precipitate was too much to be attributed to decreased solubility. I t seemed improbable that the increasing recovery of precipitate could be explained merely by the formation of larger crystals in the alcoholic solutions, as more erratic results would be expected if part of the precipitate was passing through the filter. Rather it is thought that the precipitate from the alcohol solutions contained a larger proportion of sodium. The effect of the alcohol was more pronounced in the distilled mater than in the TABLE I. EFFECTOF ETHYLALCOHOLCOKCENTRATIONY UPON
PRECIPITATE
--Distilled 0.1000 'V
WaterLfg. of
111.of K X n O l
--
0.1000
Sea Water-
s
KXInOh
%
MI.
0
12.32 12,41 12.42
0.709
13.63 1 3 . 59
0.643
13.28 13.39
0 . 660
11.21 14.24
0.618
3
13
13.65 13.79
25
14.28 14.28
KXnOi
hIg. of K
Alcohol
LII. of KlInO4
Mi.
14.35
0.640
14.39
0.611
0.616
14.59 14.69
o . m
NOVEMBER 15. 1940
ANALYTICAL EDITIOK
TABLE11. EFFECTOF TEMPERATCRE OF PRECIPITATION CPON AMOUNTOF PRECIPITATE At 23' C.0.1000 N Mg. of KMnOl M1. of KhlnOa
7 -
Alcohol
%
MI.
0
12.31 12.32 12,52 12.63
25
0,710 0.698
A -t 0.1000 N KhlnO, MI. 13.00 12.85 14.28 14.28
6' C -. Mg. of M1. of KMnOd
0.680 0.616
sea water. More precipitate was obtained in sea water than in distilled water a t the same concentration of alcohol, probably because of the additional sodium ion present, but the difference became less a t the higher concentrations of alcohol. I n distilled vater with no alcohol present the precipitate was essentially K2KaCo(S02)6,which would have had a factor of 0.71 mg. of potassium per ml. of 0.1 N potassium permanganate. As the alcohol concentration was increased, more and more precipitate came down, until a t a concentration of 25 per cent the composition approached that found by Scheuler and Thomas. TEMPERATURE OF PRECIPITATIOS. Reduced temperature of precipitation in aqueous solution has been used by Peng and in alcohol solutions by Scheuler and Thomas to increase the yield of precipitate. The effectiveness of such a procedure was investigated for distilled water solutions both with and without alcohol. Again the potassium content was held constant a t 8.79 mg. K i t h no alcohol present, lowering the temperature of precipitation increased but slightly the quantity of precipitate. The addition of alcohol with precipitation a t room temperature likewise gave but slightly more precipitate. I n contrast, precipitation from alcohol solutions a t a reduced temperature gave considerably more precipitate. SODIUM-POTASSIUM RATIO. Cunningham and Perkin (3) thought t h a t by having an excess of sodium ion during pre-cipitation more constant results were obtained. T o achieve this, hIaw and Miller (5) saturated the solution with sodium chloride. Bonneau (1) found that when the mole ratio of sodium to potassium was 25 or greater, K*T\TaCo(NO2)6was thrown down whereas if the ratio was less than 25, K3Co(S02)6 was also precipitated. TTorking with a solution containing ethyl alcohol, Scheuler and Thomas regulated the sodiumpotassium ratio by adding a large excess of reagent. The fact that the precipitate which they obtained had a formula of Kl.s,xal.a3Co(K02)6(assuming all the potassium to be precipitated) rather than K2KaCo(K02)6suggested that in an alcohol solution the influence of the sodium-potassium ratio possibly is different from that noted by Bonneau. Various experiments were performed to determine the effect of the sodium-potassium ratio, Sodium ion may be present in the sample or may be furnished entirely by the reagent. Table I11 shows the effect of variation of the quantity of the reagent for these two conditions. All precipitations were made a t 6" C. with 8.79 mg. of potassium present. The amount of the reagent apparently had little effect upon the composition of the precipitate except in the case of the distilled water solution with 2 ml. of reagent, which gave a sodium-potassium ratio of only 20 I n the case of the sea water, where the original sodium-potassium ratio was large, the same amount of precipitate was obtained regardless of the amount of reagent used. Numerous experiments were made in m-liich the volume of reagent was maintained constant a t 5 ml. per sample and the sodium-potassium ratio varied by diluting the sea water, so that chlorinites over a range of 13 to 22 O/Oo were obtained.
677
Within the range of experimental error there was no significant change in the potassium factor caused by dilution, Again, the original sodium-potassium ratios were so large and the relative change in sodium concentration so small that such a result is not surprising. The sodium-potassium ratio was also varied by changing the potassium concentration, while maintaining the amount of reagent constant at 5 ml. per sample. Amounts of 7.03, 8.79, and 10.55 mg. of potassium per sample were selected because these concentrations covered the normal potassium range in ocean waters. For comparison the same concentrations in distilled water were investigated. A small increase in the potassium factor per milliliter of potassium permanganate was observed with increase in potassium concentration. However, as the variations were usually small and within the limits of experimental error, no great importance is attached to this factor. This conclusion seems reasonable because even in distilled water solutions the sodium-potassium ratio was never less than 40
TABLE111. EFFECTO F AMOUSTO F Reagent M1.
Excess Reagent
lox
2.0 3.5 5.0
lix 25x
2.0 3.5 5.0
lox 17x 25x
Na/IC Distilled Water 20 36 50
ILEAGENT
0.1000 .v
Mg. of I
