Determination of Potassium by Sodium Cobaltinitrite J. E. SCHUELER AND R. P. THOMAS Department of Agronomy, University of Maryland, College Park, Md.
V
ARIOUS methods have been proposed from time to
time for the determination of small amounts of potassium by the use of sodium cobaltinitrite. Although many workers have reported their methods as rapid, economical, and accurate, none seems to have been extensively used, probably because of unsatisfactory results. Since good and poor results obtained by the same methods have been reported by various workers, it was thought that perhaps certain steps or points in the piocedure had been overlooked or insufficiently evaluated. A study of all the factors which it was thought could influence the recovery of potassium was made, and this paper presents an outline of the procedure evolved from this study with certain valuable points in technic. After a review of procedures proposed for the use of sodium cobaltinitrite, the method given by Peng (6)was selected as a basis for this work, as being most satisfactory and suitable for existing conditions. A large number of potassium determinations on known and unknown solutions under varying conditions were made, and the following procedure was evolved. Its advantages are primarily in the temperature of precipitation, the precipitating medium, the time of precipitation, the washing of the precipitate, and the titration. It has given excellent results for many workers on different kinds of extracJs and water solutions of potassium during the last two years.
best, with very little difference between 5" and 8" C. Temperature also affected the amount of alcohol required for complete precipitation of the potassium. It was possible to bring about almost complete precipitation a t 6" C. without any alcohol, but the crystals were too fine to be retained in filtration. The size of the crystals increased as the amounts of alcohol increased. A 25 per cent alcoholic solution (by volume) gave crystals of sufficient size to be retained easily on the filter and not large enough to cause excessive occlusion. The temperature of the precipitating (sodium cobaltinitrite) and potassium solutions a t the time of combining affected the recovery. When the solutions were mixed a t room temperature the amount regained was low, probably because of the formation of small crystals and subsequent difficulty in filtering. No such difficulty was experienced when the solutions had been previously cooled to 6 O C. TABLEI. TITRATION VALUE AND POTASSIUM RECOVERED FROM KNOWN POTASSIUM SOLUTION TEMPERATURE OF SAMPLEPRECIPITATION O
DISCUSSION The manner in which the procedure is carried out is important. The proper preparation of the different solutions is also essential. It was found that the conditions under which precipitation was made influenced considerably the recovery of potassium. As the method of Peng (6) recommended lower temperatures of precipitation than other methods, a number of determinations were made a t various temperatures. Low temperatures gave a much better recovery of potassium, 5" C. being
c.
0.1 N KMnOa
6 6 6
REQUIRED M1. 6.98 7.13 8.09 7.89 8.14 8.09 8.09 8.22 8.19 15.97 15.97 8.80 8.85 8.83
JWQ. 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 10.0 10.0 5.0 5.0 5.0
Room Room Room Room Room Room Room Room Room Room Room
1 2 3 4 5 6 7 8 9 10 11 12 13 14
PROCEDURE Place in a 250-ml. beaker a 25-ml. or less aliquot of the unknown potassium solution. Make just alkaline to phenolphthalein with sodium hydroxide and add 4 drops of acetic acid. After adding 10 ml. of 95 per cent ethyl alcohol, bring total volume of solution to 35 ml. Cool this solution to 5 " to 6' C. and then add 5 ml. of a similarly cooled sodium cobaltinitrite solution slowly down the sides of the beaker with shaking. After the precipitate and solution have stood overnight at 5"to 6" C., filter on a freshly prepared asbestos pad and wash beaker and pad thoroughly with the cold wash solution. Usually 10 t o 20 washings of 5 to 10 ml. each will be sufficient. After completing the washing, transfer the pad and precipitate t o the ori inal beaker with a stream of hot water. If a Kirsch funnel, wfich is very convenient, is used to retain the asbestos, it is necessary to pour the water through it to rinse out the inside. Bring the content of the beaker t o about 150 ml. with the hot water and titrate immediately with standard potassium permanganate. Simultaneously with the starting of titration add from 5 to 10 ml. of concentrated nitric acid with rapid stirring. Always keep an excess of permanganate present until nearing the end point. The pink color should remain for about 1 minute. With this end point 1ml. of a 0.1 N potassium permanganate solution is equivalent to 0.0005G72 gram of potassium.
POTASsIUM IN SOLUTIOX
POTASSIUM RECOVERBD Me. 3,913 4.04 4.59 4.47 4.62 4.59 4.59 4.70 4.64 9.08 9.06 4.99 5.02 5.01
TABLE11. EFFECTOF ACIDITY AND ALKALINITYUPON RECOVERY OF POTASSIUM FROM STANDARD POTASSIUM CHLORIDE SOLUTION(PRECIPITATED AT 6 ' C.) CONCD.HC1 0.1 N 10% NaOH 0.1 N A D D ~TOD K KMnO4 POTASSIUM ADDED TO K KMnOr POTASSIUM SOLUTIONREQUIREDRECOVEREDSOLUTIONREQUIR~D HECOV~DRED Drops MI. MO. Drovs M1. IMO.
5 5 5 5 7 7 7 10 10 10 10
8.74 8.80 8.85 8.44 8.44 8.49 8.44
4.96 4.99 5.02 4.79 4.79 4.82 4.79
8.29 8.59 8.44 8.29 7.73 7.89 8.29 6.17 6.82 7.28 7.08
4.70 4.87 4.79 4.70 4.39 4.47 4.70 3.50 3.78 4.13 4.01
5 5 6 10 10 10 20 20 20 20 20
8.90 8.96 8.49 8.90 8.90 9.00 9.20
5.05 5.07 4.82 5.05 5.05 5.10 5.22
9.15 9.00 8.80 8.90 9.00 8.95 8.69 8.69 8.80 8.75 8.69
5.19 5.10 4.99 5.05 5.10 5.07 4.93 4.93 4.99 4.96 4.93
When the precipitate was permitted to stand for a longer time before filtering the amount of potassium recovered was also greater, as previously noted by Van Rysselberge (6). A maximum was reached after the precipitate had stood overnight a t 6" C. The reaction of the unknown solution also influenced the recovery of potassium (Table 11). Peng controlled this point indirectly and partially when he suggested adding acetic acid until the phenolphthalein color disappeared. In the procedure as outlined the solution is
163 I
ANALYTICAL EDITION
164
made slightly acid with acetic acid, which supplies the necessary acetate ion in addition to aiding in the control of the reaction. The stabilizing influence of the acetate ion is shown in Table 111. It was found that it could be supplied by a salt as well as by acetic acid, although the latter was preferred as it helped adjust the reaction. TABLE111. EFFECTOF ACETATE 10s UPON RECOVERY OF POTASSIUM (PRECIPITATED AT 6' C.)
HCETATBIONIADDED
0 . 1 N KMnO4 REQUIRED
M1. None None None None None ACETICACID 4 drops 4 drops 4 drops 1 ml. 1 ml. 1 ml. 5 ml. 5 ml. 6 ml. SODIUM ACETATE 0.1 gram 0.1 gram 0.1 @ram 2.5 ;rams 2.5 grams 2.5 grams
POTAEEIUM RECOVERED Mu. 5.62
8.85 8: 49 8.64 8.80 8.69
4.82 4.90 4.99 4.93
8.80 8.82 8.84 8.14 8.39 8.14 7.19 7.89 7.59
4.99 5.00 5.02 4.62 4.76 4.62 4.07 4.47 4.30
8.80 8.84 8.77 9.10 9.05 9.15
4.99 5.02 4.97 5.16 5.13 5.19
Vol. 5, No. 3
cipitate is soluble in the wash solution, some of it may be lost in the numerous washings required. As none of the previous wash solutions proved satisfactory, trials with different wash solutions were made. It was found that distilled water saturated with dipotassium sodium cobaltinitrite and kept a t 6' C. was very satisfactory. Washing with this solution removed the alcohol and sodium cobaltinitrite and did not dissolve any detectable amounts of the precipitate. The solution is prepared by adding a small amount of dipotassium sodium cobaltinitrite or a water suspension of the salt to distilIed water previously cooled to 6' C. The water must be cooled before it is saturated with this salt and kept cold afterwards, or the salt decomposes and leaves the wash solution unsatisfactory. The colloidal nature of dipotassium sodium cobaltinitrite makes it difficult t o obtain a saturated solution quickly, and it is therefore better to prepare the wash solution a day or two before it is needed. As this salt cannot be conveniently obtained commercially, it was prepared from a potassium chloride and sodium cobaltinitrite solution and purified by decanting. The solution will remain satisfactory for a long time if kept as a water suspension in a dark cool place. Only a few drops of this salt suspension are necessary t o prepare a liter of the wash soh-
tion.
The asbestos pad and precipitate should be washed into the beaker in which the precipitate was made by means of hot water (preferably water which has just been boiled) immediately upon the completion of the washing. This Many previous investigators (1-5, 7) had noted the ne- high temperature gives a sharper end point in titration. cessity for an excess of sodium over potassium ions, ob- It is very desirable to work rapidly from the beginning of tained by precipitating in the presence of some sodium salt. filtration until the end of the titration, which normally should I n the authors' work sodium cobaltinitrite seemed to give not take more than 3 to 4 minutes. It is not necessary to the best results, as is shown in Table 111 where the large dissolve the precipitate completely before starting the titraamount of sodium acetate was used. When the amount of tion, as the stirring in titration will help bring it into solution. sodium cobaltinitrite was less than 150 times that of potas- The mineral acid added liberates the nitrite rapidly, causing sium, low results were obtained. Excessive amounts, up to a loss of nitrous acid if the addition of the acid and perman1000 times the potassium content, gave satisfactory results. ganate is not properly regulated. After a few titrations this The data in Table IV show the influence of varying the sodium can be easily controlled. Either sulfuric or nitric acid can be and potassium ion ratio upon the recovery of the potassium. used, but nitric acid seemed to give a sharper end point. Ordinary c. P. nitric acid was used by the authors, although it TABLEIV. INFLVENCE OF VARYING RATIOOFTPOTASSIUM should be aerated to remove any nitrous acid that may be present TO SODIUM COBALTINITRITE UPON RECOVERY OF POTASSIUM (PRECIPITATED AT 6" C.) It is important for the worker to reach the same end point 0.1 N KMnOi POTASEIUM POTAESIIJM PRECIPIT.4TINQ each time. A blank determination should be run along with REQUIRED RECOVER~D REAGENT ADDED ADDED some of the other determinations. This is generally from Mu. MI * M1. Mg. 0.1 to 0.3 ml. of 0.1 N potassium permanganate, depending 2.92 5.15 5.0 3 3.01 5.30 5.0 3 upon the end point. 2.98 5.25 5.0 3 Although the end point in titration will vary with the 5 2.0 worker, it only slightly affects the potassium equivalent of 2.0 5 2.0 5 1 ml. of 0.1 N potassium permanganate. The variations in 2.0 5 2.0 5 values reported by different investigators indicate that the 5 5.0 procedure followed will greatly influence this potassium 5 5.0 5 5.0 factor. These values were generally lower than the theoreti5 5.0 5 5.0 cal ones, regardless of the consideration given to the final 5 5.0 valence of the cobalt. 17.30 9.81 10.0 10 This method of titration is different from any other the 10.04 17.70 10.0 10 17.65 9.96 10.0 10 writers have seen, as other methods require two titrations 9.70 17.10 5.0 10 with permanganate and one with an oxalate solution. The 9.64 17.00 10 5.0 9.59 16.90 10 6.0 single titration is much preferred as it is simpler, easier, 14.75 26.00 15 15.0 14.49 25.56 15 15.0 quicker, and just as accurate, if the temperature is high 14.46 25.50 15 15.0 enough and the addition of the acid and permanganate is properly regulated. The factor of 0.0005672 gram of potasThe sodium cobaltinitrite solution was prepared by dissolving 30 grams in 100 ml. of distilled water. After this solution has sium for each milliliter of permanganate required is lower than cooled at 6" C. for at least an hour it must be filtered through an any other seen. This is believed to result from the length of asbestos pad on a Hirsch funnel similar t o that used for filtering time the precipitate is allowed to stand before filtering. the precipitate. It is best to make up this solution fresh when The equilibrium which is reached may influence the arrangeneeded. ment of the double salt crystals or the amount of occlusion. As the precipitation is made in an alcoholic sodium cobalti- It is always preferable to check the potassium factor by nitrite solution, which is oxidized by potassium permanganate, making some determinations on a known potassium solution, it is necessary that the precipitate be washed free of these containing somewhere near the same amount of potassium as substances, If the dipotassium sodium cobaltinitrite pre- the unknown. I
May 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
This method has been found very accurate when the unknown contains between 3 and 10 mg. of potassium, but less than 1 mg. and more than 50 mg. have been satisfactorily determined. For small amounts a 0.05 N potassium permanganate solution was used. I n the case of large amounts only 0.1 N permanganate was used, but it was standardized against a similar amount of known potassium. Ammonium salts have been the only salts which have interfered, because ammonia also forms an insoluble precipitate with sodium cobaltinitrite. Organic matter must not be present in the unknown solution in such a form that it will be occluded in the precipitate or retained on the filter pad. The method is rapid, as one worker can easily make from 20 to 40 determinations a day. By proper precautions the asbestos can be used over and over, and since the sodium cobaltinitrite and alcohol
165
are not exceedingly costly substances the method is not very expensive. This method has been used on many kinds of ash, soil extracts, and total potassium determinations in soils, with very satisfactory results. LITERATURE CITED (1) Adie and Wood, J. Chem. Soc.. 77, 1076 (1900). (2) Bonneau, L.,Bull. SOC. chim., 45,798 (1929). (3) Christensen, H. R., and Feilberg, N., Landw. Veers. Sta., 97,!27 (1920). (4) Milne, G., J. Agr. Sci., 19,541 (1929). (5) Peng, Chein, private communication, 1928. (6) Van Rysselberge, P.J., IND.ENO.CHEM.,Anal. Ed., 3, 3 (1931). (7) Van Rysselberge, P. J., and McBain, J. M., J. Am. Chem. SOC., 52,2336(1930). RECEIVIDD November 8, 1932.
Experimental Determination of Void Content of Close-Packed Mineral Powders ‘Effect of Particle Size, Shape, and Texture R. N. TRAXLER, L. A. H. BAUM,AND C . U. PITTMAN, The Barber Asphalt Company, Maurer, N. J.
T
’
give greater stability to the mixHE fabrication of bitumiI n industrial processes the value of a mineral ture than do equal weights of nous, cement, ceramic, filler depends chiefly on its physical properties, powders with low void contents, paint, plastic, r u b b e r , of which the void content is one of the most Some theoretical and experiand numerous other products important. Several commercial mineral powmental work has been done on often requires the use of fine ders were divided inlojive or six definite size fracpacking and the void contents of powders. I n some processes the powders, sands, and crushed agp a r t i c l e s lose their identity, tions and the void contents of the unfractionated gregates, but most of the mawhile in others they retain their powders and the various fractions determined by terials used have been coarser origin~al structure and function three methods-dry compaction, liquid absorption, and less homogeneous with reas fillers, imparting strength, and briquetting. For most of the powders the spect to particle size than the toughness, durability, w e i g h t , lowest void content values were obtained by the powders with which this paper stability, and weather resistance deals. Norton and Hodgdon (11) to the material to which they briquetting method. T h e data for limestone, investigated the void content and are added. When a powder is silica, marble, and black slate powders indicate particle spacing for a number of used as a filler its physical propthat particle size has no appreciable effect on the clays and “nonplastic” mineral erties are usually of primary imvoid content, but the results for trap rock and soappowders. Roller (IS)compacted portance, though adsorption a t stone apparently do not agree with this generalizathe solid surface, the adhesion dry, carefully size-graded microtension between the solid partiscopic powders by hand tapping tion. The void content values of the various size cles and interstitial m a t e r i a l , and found that the bulkiness infractions of powdered Tripoli and argillaceous and polymerization caused by creased with decrease in particle silica, minerals which possess peculiar charsurface catalysis may affect to size below 15 to 30 microns in acteristics, are discussed. Although texture has diameter, and Steiner (14) Obsome extent the characteristics a n efect on void content, regularity in particle of the resulting mixture. t a i n e d t h e same result with cement. From a study of pigThe physical properties of a shape is probably of greater importance. powder which may affect its ments, Klumpp (6, 7) concluded value as a filler are the shape, that liauid absomtion deDended texture, size, and size distribuiion of the particles, the surface mainly on the void content of the ;lose-packed powdk, and area presented by unit weight or volume of the material, and not so much on the size and surface area of the particles as the percentage of voids in the closely packed dust. Although on their shape. Manegold, Hofman, and Solf (8, 9) made the significanceof these different properties may vary with the an extensive study of the packing of granular materials. The material and the purpose for which it is to be used, the void void contents which they calculated for various types of packcontent, bulkiness, or weight per unit volume of the closely ing of spheres may be of interest. packed powder is almost always of importance. Consider, for POINTS OF CONTACT BETWIDENPARTICLES P ~ CENT R VOIDS example, bituminous products which contain high percentages 4 66.0 of mineral matter. Other factors being equal, powders which 6 47.6 8 39.6 in a close-packed condition possess high void contents raise 10 30.2 the softening point, increase the yield value, and in general 12 26.9
