ANALYTICAL EDITION
324
Vol. 3, No. 3
Rapid Volumetric Method for Determination of Potassiuml** Loyal Clarke and J. M. Davidson U.S. BUREAUOF MINES,NONMETALLIC MINERALS EXPERIMENT STATION, NEW BRUNSWICK, N. J.
D
URING the course of development work on the extraction of potash from polyhalite, it was found desirable to develop a rapid method for the determination of potassium in solutions containing potassium and magnesium sulfates. Such a method, depending upon the precipitation and titration of potassium acid tartrate, has been developed and found to be sufficiently accurate for many purposes. The entire procedure for a single analysis may be carried out in less than 30 minutes, and as many as six determinations may be completed in 90 minutes. The method is consequently especially suited for industrial chemical control work. A modification of the method as originally developed for the analysis of potassium in solutions of potassium and magnesium sulfates has also been applied to mixtures of sodium and potassium chlorides and to potassium nitrate. Acid Tartrate Methods Previously Used and Observations Leadiung t o New Procedures Several workers ( 1 4 , 5-9) have suggested acid tartrate methods for the determination of potassium. Of these, the procedures of Meurice (6) and Ajon (1) are the most promising. The method of Meurice depends upon the precipitation of potassium acid tartrate from a solution containing potassium salts by the addition of requisite amounts of a saturated solution of sodium acid tartrate and methanol. An overnight period of standing was necessary for complete precipitation of the potassium. After filtration and washing, Meurice dissolved the potassium acid tartrate in a measured volume of standard sodium hydroxide, and finally titrated the excess of this reagent with hydrochloric acid. From this titration and the amount of base used in neutralizing and dissolving the acid tartrate, the amount of potassium present was calculated. Meurice found his method to be moderately accurate for pure potassium sulfate, chloride, or nitrate and to be unaffected by moderate amounts of magnesium or calcium. Ajon had previously developed a somewhat similar method using ethyl alcohol instead of methanol. The authors have also adopted the use of ethyl alcohol. The method of the authors differs from previous methods (1) in the use of a mechanical stirrer to accelerate the formation of potassium acid tartrate, (2) in effecting the precipitation of the bulk of the potassium from aqueous solution before addition of alcohol which is then added with stirring a t a controlled rate, (3) in the use of tartaric acid in addition to sodium acid tartrate as the precipitant, and (4)in direct titration of the potassium acid tartrate with 0.12 N sodium hydroxide. About 40 pilot experiments were conducted to determine the best conditions for carrying out the analysis of potassium sulfate in solutions also containing magnesium sulfate. From this work it was found that too rapid addition of alcohol results in the preciphation of potassium sulfate which when once formed was only slowly converted to the much less soluble but slow-forming acid tartrate. Also, the best concentrations of alcohol, potassium sulfate, sodium acid tartrate, and tartaric acid to effect complete precipitation of pure potassium
* Received September 6. 1930. * Published by permission of the (Not subject t o copyright.)
Director, U. S. Bureau of Mines.
acid tartrate were determined. Too large an excess of sodium acid tartrate is undesirable because this salt is of limited solubility in alcoholic solutions (super-saturated solutions of the salt are readily formed, however, and are moderately stable). A considerable excess of the acid tartrate is necessary for complete precipitation of the potassium salt, especially in the presence of magnesium sulfate. The excess necessary is considerably reduced by the addition of tartaric acid. The above observations and other data obtained in the pilot experiments led to the adoption of a satisfactory set of conditions for the analysis of potassium and magnesium sulfate solutions. This procedure will for convenience be designated as procedure I. An attempt was made to apply the procedure to mixtures of sodium and potassium chloride. While the method was satisfactory for analysis of solutions of the latter salt, it was useless in the presence of appreciable amounts of sodium chloride, as the results were much too high. (The analysis of a single polyhalite extract which was high in salt, however, indicates that as much as 0.2 gram of sodium chloride may not interfere with the method.) Some of the sodium acid tartrate was replaced by the magnesium salt in order to obviate the difficulty. A satisfactory procedure, 11, was so developed. The results using these conditions, however, were consistently low, necessitating the use of an empirical factor in order to obtain correct results. On this account the method could not be recommended as a general method for potash, but the authors feel that the method or modifications of it should prove of considerable value for routine analytical work, The two procedures are outlined below. Procedure I was especially developed for analysis of solutions containing K+, Mg++, and SO4--, and procedure I1 for solutions containing K+, Na+, C1-, and NOa-. Procedure I A sample is diluted so that a 25-cc. aliquot (or a smaller aliquot later to be diluted to 25 cc.) contains 0.20 to 0.48 gram of potassium sulfate. If this sample contains 0.15 gram of magnesium sulfate or more, 20 cc. of a saturated sohtion of sodium acid tartrate and 5 cc. of a 20 per cent solution of tartaric acid are then added and mechanical agitation started, Within 2 or 3 minutes a crystalline precipitate of potassium acid tartrate will appear. If less than 0.15 gram of magnesium sulfate is present, more should be added, or better, only 15 cc. of saturated sodium acid tartrate and 60 cc. of alcohol should be used in the procedure, which is otherwise unaltered. Stirring should be continued for at least 2 minutes more before addition of alcohol. Then 65 * 2 cc. of U. S. P. 95 per cent alcohol are added from a buret or a dropping funnel a t a rate such that 6 to 8 minutes are required for this addition, The solution should be stirred continuously during the addition of the alcohol, and stirring should be continued 2 to 3 minutes after the addition is complete. The mixture is then immediately filtered and the crystalline potassium acid tartrate washed with not more than six small portions of wash solution prepared by mixing two parts of 95 per cent U. S. P. alcohol with one part of distilled water (four washings are sufficient to remove all SOI-- from the solid).
INDUSTRIAL AND ENGINEERING CHEUISTRY
July 15, 1931
Two of these washings are usually sufficient for transfer of the solid to the paper, as this transfer does not need to be quantitative. The time required for this filtration may be very materially reduced by using 9-cm. rapid filter paper fitted to a long narrow stem funnel by wetting with distilled water. The filter paper is freed from water by washing with two portions of alcohol wash solution before the filtration is begun. After filtration and washing, the potassium acid tartrate is dissolved in nearly boiling distilled water and titrated with 0.12 N sodium hydroxide. Procedure I1 aliquot later diluted to 25 cc.) contains 0.08 to 0.22 gram of potassium. If more than 0.40 gram of sodium chloride or nitrate is present, the method is not recommended. Twentyfive cubic centimeters of a solution containing 104 grams of MgC4H4064Hz0,95 grams of NaHC4H40~Hz0, and 264 grams of tartaric acid per liter are then added to the sample and mechanical agitation started. Subsequent steps are exactly the same as those outlined for procedure I. Comparison of New Method with Hicks Method Procedure I has been used extensively for over a year by this laboratory for analysis of water extracts of the mineral polyhalite. These extracts contain nearly equivalent amounts of potassium and magnesium sulfates and small amounts of sodium chloride and calcium sulfate. A comparison of results obtained by this method with analysis by a modification of the chloroplatinate method of Hicks (4) is given in Table I. Table I-Comparison of New Method a n d Hicks Method MAGNESIUM POTASSIUM SULFATE FOUND IN Hicks New DIFSULFATE SAMPLE method method FERENCE
SUBSTANCE ANALYZED
Polyhalite extract Polyhalite extract Polyhalite extract Polyhalite extract Polvhalite extract Polvhalite extract Polyhalite extract Rali magnesia Potassium sulfate
A comparison of the results of analysis of synthetic potassium chloride and potassium nitrate solutions by procedure I1 with analyses by the method of Hicks (4) is given in Tabla 11. In the case of the potassium chloride solution used, the chloride content was also determined by Mohr's method, and the titer calculated as potassium checked the chloroplatinate procedure to 0.1 per cent. Table 11-Comparison of Procedure I1 w i t h Hicks Method --POTASSIUX FOUND--FORMOF SODIUM PROCEDURE IT POTASSIUMCHLORIDE Calcd. by Calcd by IN IN Hicks theoretical empirical DIFFERENCE SAMPLE SAMPLE method factor factor"
Gram
A sample is taken so that a 25-cc. aliquot (or a smaller
Grom
Gram
Grom
0.20 0.24 0.24 0.27 0.24 0.28 0.32 0.17 0.01
0.3156 0.3358 0.3494 0.3912 0.3392 0.4118 0.4648 0.2368 0.2257
0.3136 0.3330 0.3510 0.3872 0.3372 0.4084 0.4644 0.2380 0.2260
Gram
Gram
KC1 None 0.1038 0.1015 KC1 None 0.1038 0.1022 KC1 None 0.1038 0.1023 KCI None 0.2076 0.2040 KCl None 0,2076 0,2027 KC1 0.40 0.1038 0.1023 KC1 0.40 0.1038 0.1016 KC1 0.40 0.2076 0.2040 KC1 0.40 0.2076 0,2050 KNOs None 0 1960 0 1933 KNOI None 0 1960 0 1933 0 0983 0 0974 KNOa None KNOa None 0 0983 0 0972 KNOs None 0 0983 0 0970 KNOa None 0 0983 0 0968 a Empirical factor used IS 1.016 times theoretical
Gram
%
0.1032 0.1038 0.1040 0.2074 0.2060 0.1040 0.1032 0.2074 0.2083 0 1964 0 1964 0 0990 0 0988 0 0986 0 0984 factor.
-0.6 -0.0 +0.2 -0 1 -0.8 +o 2
-0.6. -0.1 +0.3 $0 2 +O 2 +O 4 +O 3 +O 2 +O 1
Reference to this table shows that if the empirical factor of 1.016 times the theoretical is used, the results of the procedure agree fairly satisfactorily with the results of the chloroplatinate procedure. Acknowledgment The authors wish to acknowledge the valuable suggestions of Nathan Fragen, junior chemist, of this laboratory. Literature Cited (1) Ajon, A n n . slue. sper. d i ogrumicoltura c frutlicoltura di Acirsale, 3, B t
% -0.7 -0.8 $0.5 -1.0 -0.6 -0.8 -0.1 $0.5 +0.1 Mean - 0 . 3
325
(2) (3) (4) (5) (6) (7) (8) (9)
(1915). Bayer, Chem.-Ztg., 17, 686 (1893). Bokernuller, Chem. Zenlrolblatl, 1918, 11, 764. Hicks, J. IND.END.CHEM.,5, 650 (1913). Marshall, Chem.-Zlg., 88, 585 (1914). Meurice, Ann. chim. m a l . oPPl., 7 , 161 (1925); 8, 130 (1926). Okada, Mcm. Coll. Sci. Kyoto I m p . Uniu., 89 (1914). St. Minovici and Kolla, Bul. SOC. chim. Romdnio, 8 , 25 (1921). Touaritske and Slezak, Zhurnab Sokharnoi Prom., 2, 462 (1928).
Method for Increasing Sensitivity of Certain Chemical Test Reactions' Irwin Stone 360 WADSWORTR AvE., NEWYORK,N. Y.
H E general class of organic precipitant reagents used in qualitative inorganic analysis includes some of the most sensitive chemical tests for metals known. Some, such as the dimethylglyoxime test for nickel, are well known, whereas others, such as the p-nitrobenzeneazoresorcinol test for magnesium (4),or the p-dimethylaminobenzylidenerhodaninetest for silver (9) have not such widespread recognition. The usual reaction that takes place in this class of tests is one where the metal enters the molecule of the organic compound to form a very insoluble, highly colored precipitate. This precipitate, composed of a generally water-insoluble organic residue coupled to a metal, which is more or less repelled by organic solvents, forms a typical polar molecule. This polar precipitate, if shaken with a mixture of water and 1 Received
April 27, 1931.
some immiscible organic solvent, will tend to collect at:the water-solvent interface, the molecules probably orienting themselves similar to the polar molecules described by Harkina and his co-workers (1) and Langmuir (3), the organic portion jutting into the immiscible solvent phase while the metallic portion points toward the water. This fact may be applied to extending the range of sensitivity of these tests by collecting a t the interface the small amount of precipitate which would be undiscernible suspended in the large bulk of test liquor. It may also be utilized to make faint reactions more easily and surely recognizable. As a general method for performing this sensitization, one may conduct the test in the usual manner and when completed add a few cubic centimeters of ether and shake thoroughly. When the ether has separated, note any color or precipitate that has collected a t the water-ether interface.
