2011
V O L U M E 2 8 , NO. 1 2 , D E C E M B E R 1 9 5 6 hlellor, J. W., “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” vol. 9, p. 739, vol. 12, p. 747, Longmans, Green, London, 1933. Miller, G. L., “Metallurgy of Rarer Metals,” vol. 2, “Zirconium,” p. 253, Academic Press, Kew York, 1954. Pigott, E. C., “Ferrous Analysis, Modern Practice & Theory,” p. 342, Chapman and Hall, London, 1953. Read, E. B., Zopatti, L. P., “Determination of Oxygen in Zirconium,” presented at Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Feb. 1950. Schneider, E., Pitman-Dunn Laboratories, Philadelphia, Pa., oral communication. Seybolt, A. V., Sumsion, H. T., J . Metals 5 , 292 (1953). Short, H. G., Analyst 75, 335 (1950). Sidgwick, N. V., “Chemical Elements and Their Compounds,” pp. 638, 652, 816, 819, 826, 1012, 1332, 1369, Oxford Univ. Press, London, 1950.
Singer, L., IND. EXG.CHEM.,~ ~ N A LED. . 12, 127 (1940). Sloman, H. A., J . 1 s t . Metals 71, 391 (1945). Sloman. H. A.. Trans. Am. SOC.Metals 37. 331 (1946). Sloman; H. A.,’Harvey, C. A., Kubaschewiki, O.‘,J . inst. Metals 80, 391 (1952).
Stanley, J. K., Hoehne, J. von, Weiner, G., ANAL. CHEJI.23, 337 (1951).
Sully, A. H., “Metallurgy of the Rarer Metals,” vol. 1 , “Chromium,” pp. 33, 42, 52, 240, ilcademic Press, New York, 1954. Thompson, J. G., Vacher, H. C., Bright, H. A., J . Research Xatl. Bur. Standards 18, 259 (1937).
Thorpe, J. F., Whitely, 51. A., “Thorpe’s Dictionary of Applied Chemistry,” vol. 2 , p. 115, Longmans, Green, London, 1938. RECEIVED for review March 31, 1956. .4ccepted August 11. 1956. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February 27, 1956.
Determination of Potassium as the Metaperiodate RALPH E. JENTOFT’ and REX J. ROBINSON Departments o f Chemistry and Oceanography, University
An analytical procedure for the determination of potassium with periodic acid is described, w-hich is very sensitive, has wide range, and is free from the empiricism usually associated with the determination of this element.
A
FEW years ago Willard and Boyle (IS)developed a new pro-
cedure for the determination of potassium, in which the potassium was weighed as potassium metaperiodate or estimated iodometrically through reduction of the potassium metaperiodate to iodate with iodide a t a p H of 7.5. T o minimize the relatively large solubility error of potassium metaperiodate in water, a rather large volume of 1 to 1 ethyl alcohol-ethyl acetate was added and the temperature of the solution was lowered to about 0 ” C. Because the procedure of Willard and Boyle did not yield the accuracy desired for a particular application, it has been investigated thoroughly by the present authors. It was ascertained that the chief factors limiting the accuracy of the method were: the relatively great solubility of the potassium metaperiodate due to the dilution effect of the organic solvents, an insufficient common-ion effect, and the deposition of difficultly soluble sodium metaperiodate by surface evaporation of the volatile solvent. T o reduce the loss of potassium metaperiodate through solubility to its barest minimum, the following steps have been taken by the authors in their proposed method. The volume of water used for solution of the potassium sample was reduced to 4 ml. from 7 to 8 ml. as specified by Willard and Boyle. A 33% (by volume) ethyl alcohol solvent medium was used instead of the 90% ethyl alcohol-ethyl acetate medium of Willard and Boyle. The former medium n a s shown by the authors to be preferable because of its simplicity, lesser volatility, and lesser solvent effect. The p H of the solution was buffered to 3 5 with lithium acetate to give maximum common-ion effect. The temperature of the solution was lonered to 0” C. as was done by Willard and Boyle. These measures are not without their disadvantages. Using a small volume of water for solution limits the amount of sample which can be dissolved. It also increases the ionic strength of the solution, which increases the solubility of potassium metaperiodate as shown by Jones and Heckman (9). The use of an added organic solvent produces a dilution effect and also decreases the 1
Present address, California Research Corp., Richmond, Calif.
o f Washington, Seattle 5, Wash. activity of the common-ion. The adjustment of p H involves the use of a buffering agent, which also adds to the ionic strength of the solution. Cooling the solution to 0’ C. reduces the common-ion effect of the excess metaperiodate as indicated by Crouthamel, Hayes, and Martin ( 2 ) . Finally, these measures also suppress the solubilities of the periodate salts of other ions which may be present in the solution. I n spite of these limitations, all the foregoing measures to suppress solubility may be utilized to net advantage. Ammonium metaperiodate and the alkali metaperiodates have low solubilities and may interfere in the determination of potassium. Because rubidium and cesium are rarely present in significant quantities and ammonium salts are easily volatilized by ignition, sodium is the chief source of interference. An excessive concentration of sodium is conveniently reduced to an allowable amount by a single separation of potassium as the insoluble potassium-sodium cobaltinitrite. Cobaltous ion is formed in the solution during subsequent destruction of the cohaltinitrite to render the potassium soluble again. This caused serious difficulties in the present investigation, until it was realized by the authors (7‘) that a redox reaction occurred between cobaltous ion and periodic acid. Fortunately, cobaltous ion may be rendered passive in the presence of periodic acid by converting it t o the insoluble cobaltous oxalate with oxalic acid. The sodium present is precipitated as sodium acid oxalate at the same time. This suggested the possibility of initially removing sodium and the alkaline earths, if present, by precipitating as the oxalates rather than by some other separation procedure. However, because of difficulties in washing large bulks of insoluble oxalate, this procedure is feasible only in removing small amounts of these cations. Many other aspects of the chemistry of periodic acid and its salts v,-ere involved in the development of this procedure ( 5 ) . SELECTIOIV OF ORGANIC SOLVENT MEDIUM
The organic solvent medium for this procedure was selected only after careful consideration of a number of factors, the most important of which were the solubility of potassium metaperiodate and the effect of the solvent medium on the common ion. The solubility of potassium metaperiodate was measured in a number of organic solvent-water mixtures cooled to 0” i 0.1” C. in a chest type of refrigerator. I n order t o simulate as nearly as possible the solubility of potassium metaperiodate under the actual conditions of its precipitation in the analytical procedure
ANALYTICAL CHEMISTRY
2012 for potassium, the solubility measurements were made bj- precipitating an excess of a definite amount of potassium metaperiodate from a known volume of solution, 0.lM in periodic acid and buffered to pH 3.5 v i t h lithium acetate. The organic solvent concentration was adjusted and the solution was agitated at 0" C. for a t least 24 hours'to allow for solubility equilibration. The precipitated potassium metaperiodate \vas filtered from the solution, washed n-ith cold ethyl alcohol, and finally determined iodonietrically. The difference between the amount of potassium taken and that found was considered to he the amount soluble as the metaperiodate. Complete details of the experimental procedure are given by Jentoft ( b ) . The data for the various organic solvent-water mixtures are presented in Table I, arranged in the order of decreasing efficiency of the solvent as solubility suppressors. The dielectric constants of the organic solvents have also been listed in Table I because of the correlation of dielectric constant with the common-ion effect. The initial study Tyas made with et,hgl alcohol as t,he solvent over the entire range of concentrations. The authors (6) have shown by a graphical method that the minimum solubility for potassium metaperiodate occurred at an ethyl alcohol concentration of 3570 by volume. A feiv pertinent data for ethyl alcohol have been presented in Table I for comparison with the other organic solvents. Preliminary solubility measurements Tvith two or three appropriate concentrations of other organic solvenwater mixtures did not reveal a solvent which v a s considered more suitable than ethyl alcohol; consequently detailed solubility studies similar t o that made for ethyl alcohol were not made for the other solvent systems. On the basis of the solubility data shown in Table I, ethyl alcohol and isopropyl alcohol are almost comparable in efficiencies. However ethyl alcohol was selected over isopropyl because of its ready availability, lesser cost, and higher dielectric constant which favors commonion activity. .\ solution of high dielectric strength is preferable to keep the relative solubility of sodium metaperiodate high.
Table I. Solubility Data for Potassium Metaperiodate in Various Organic Solrent-Water Mixtures, 0.1M in Periodic Acid at pH 3.5 Solubility % KIO4, LIg. Organic Dielectrica Solvent K /lo0 111. Solvent Constant 0 2 82 22 0 (200 e.) Ethyl alcohol
Isopropyl alcohol
15 7 (20' C.)
1 to 1 ethyl alcoholisoprog) 1 alcohol
tert-But? 1 alcohol
24
32 6 6 (19' C.j
llethanol
33 1 (200 C.)
n-Propyl alcohol
12 3 (20' C.)
5 t o 1 ethyl alcohol-
ethyl acetate Ethyl Cellosol\-e
16 24 32 36 16 24 32 16
6
(20' C.)
16
24 32 16 24 32 16 24 32 24
24
0 86
0 0 0 0 0 0
0
0 0 0 0 0 1 0 0 0
0
0 0
57 43 37 80 50 48 80 53 43 77 55 58 00 65 53 93 70 65 73
1 10 1 18 1 95 Acetone 21 2 (200 C.) 2 05 16 1 88 Butanone 18 l ( 1 7 ' C . j 24 2 20 0 All dielectric constants measured at radio-frequency by Drude (3).
32 16 24
The solubility of potassium metaperiodate was determined t o be equivalent to 0,009 mg. of potassium per 4 ml. of water under these conditions of determination. Since sodium commonly occurs with potassium, it seemed desirable to knopv the solubility of sodium metaperiodate. Under the conditions used for the determination of potassium a solubility value of 2 . i my. of sodium per 4 ml. of water was obtained. REAGENTS AND SOLUTIONS
Reagent grade chemicals \yere used throughout t'hia investigation unless otherwise stated. PERIODICACID. The periodic acid, prepared by the G. F. Smith Chemical Co., vas checked for purity as described b>Willard and Boyle ( 1 3 ) and found t o be free of iodic acid. A reagent solution, containing 0.5 gram of HjIOs per nil., \vas prepared as needed, as it was found to deteriorate on standing. This salt was purified by rePoTassIuar ~IETAPERIODATE. crystallization, dried first a t 110' C. and then a t 180" C. t o constant TT-eight according t o Jones (8). Lithium carbonate was suspended in LITHIur CARBONATE. distilled water and dissolved by bubbling carbon dioxide. Solution of the gas T ~ aided S by cooling the reaction vessel with an ice pack. The carbon dioxide was first washed through concentrated sulfuric acid, then through saturated sodium bicarbonate solution, and finally through distilled water. The lithium bicarbonate solution was filtered throiigh a fritted-glass filter funnel and lithirim carbonate n-as precipitated 1))- heating the solution until evolution of carbon dioside was complete. During this operation vigorous stirring of the suspended lithium carbonate \vas necessary to avoid bumping. The hot solution was decanted and the lithium carbonate was washed several times with boiling water. This procedure was repeated on the recovered product. The second crystalline precipitate of lithium carbonate n-as dried a t 120" C. for 24 hours. The over-all yield as 65:;. LITHIUMACETATE. -12.5-V lithium acetate solution was prepared by dissolving 18.5 grams of lithium carbonate in 30 ml. of glacial acetic acid and diluting t o 200 ml. OXALIC ACID. Two solutions of osalic acid were prepared; one had a concentration of 100 grams of osalic acid dihydrate per liter, the other 10 grams per liter. O.lyo solution was made up in n-ater ~ , ~ - D I N T R O P H E XA OL . wit,h the aid of an equivalent amount of lithium carbonate. The potassium iodate was purified by POTASSIUM IODATE. recrystallization, and dried a t 100" C. and then a t 180" C. to constant weight according to Scott ( 1 2 ) . A series of standard solutions of appropriate concentrations was prepared on which to standardize all the sodium thiosulfate solutions except the 0.35S solution. Equivalent concentrations of potassium bromate Rere made according to Kolthoff and Sandell (10). Solutions \yere prepared which were SODIEMTHIOSULFATE. 0.35, 0.12, 0.04, 0.014, and 0 . O O i S . These solutions ivere stabilized with sodium furoate, according t o Platon (11). The sodium thiosulfate solutions \!-ere standardized against potassium iodate or potassium bromate. When standardized against potassium bromate, 4 or 5 drops of 370 ammonium molybdate solution were added t o catalyze the oxidation of iodide as recommended by Kolthoff and Sandell (10). The 0.35.\- sodium thiosulfate solution was standardized with weighed amounts of solid pot or potassium metaperiodate. PoTAssImf S I T R A T E . The potassium nitrate was purified by recrystallization from water, and dried at, 100" C. and then at 210" C. t o constant weight according to Scott (12). Standard solut'ions were prepared t o contam approximately 3, 1, 0.4, 0.12, 0.06, and 0.02 mg. of potassium per ml. ORGANIC SOLVEXTS.All the organic solvents were redistilled, using those portions lvhich distilled over within a range of 1' to 2' C. A 2 to 1 ethyl alcohol-isopropyl alcohol wash liquor was also prepared. ASBESTOS.ilsbestos of good grade was digested with concent,rated nitric acid for 4 hours, filtered, and washed with distilled water. This treatment was necessary t o Drevent retention of periodic acid which could not be washed free from the asbestos, causing high results. ANALYTICAL PROCEDURE
The sample containing no more than 75 mg. of potassium is put into a 100-ml. beaker. Oxidizing and reducing agents may not be present. Halides are removed by evaporation just t o dryness with nitric acid. Only lithium, sodium, and the alkaline earth cations (except barium) may be present. With only small amounts of the alkaline earth cations and less than 2.5 mg. of sodium present, the potassium may be precipitated directly with periodic acid. Moderate amounts of these
V O L U M E 28, NO. 12, D E C E M B E R 1 9 5 6
2013
cations (less than 1.5 times the equivalents of potassium) may be prevented from int,erfering by precipitation as the oxalates. A solution of oxalic acid in excess of that required t o convert the cations to oxalates is added, together with 3 ml. of concentrated nitric acid. The solution is taken to dryness on the steam bath. Sublimed oxalic acid on the sides of the beaker is evidence that an excess has been added. With large amounts of sodium, lithium, or alkaline earths present (except barium which forms insoluble cobaltinitrite), the potassium is separated by precipitation as potassium-sodium cobaltinitrite. The solution is adjusted to 25 ml. and 0.2 ml. of O.lCr, gelatin solution is added. One gram of sodium cobaltinitrite is added and dissolved. The solution is cooled to 0" C., 25 ml. of 95y0 ethyl alcohol are added, and the solution is refrigerated at 0" C. for 8 hours. The solution is filtered and the precipitate and containers are washed once with 95% ethyl alcohol cooled to 0' C. The alcohol is evaporated completelJ- on the steam bath and the precipitak is dissolved in 10 ml. of hot 5-1- nitric acid. The sodium is precipitated by adding oxalic acid in t'he ratio of 10 mg. of oxalic acid dihydrate per milligram of potassium in the sample. The solution is taken to dryness on the steam bath in preparation for precipitation ivith periodic acid. The potas3ium is dissolved in 2 nil. of water and precipitated slo\r-ly by adding 1 ml. of periodic acid, containing 0.5 gram, in rich a manner to attain maximum crystal size. Dropwise addition of the reagent over a period up to 30 minutes is required for maximum amounts of potassium. The solution must be stirred rapidly whenever the reagent is added and occasionally for a time thereaft,er, About 10 minutes after the last addition of periodic acid, the p H of the solution is adj mted by adding 0.4 ml. of . 2 . 5 S lithium acetate dropwise with st'irring. .%bout 10 minutes later a drop (0.03 ml.) of 2,B-dinitrophenol indicator is added and the dropu-ise addition of lithium acetat,e is continued until the indicator turns a dark yellow, indicating a pH of 3.0 to 3.5. Usually a total volume of about 1 ml. of lithium acetate is required. Then 2 ml. of 95Ck ethyl alcohol are added. The beaker and contents are cooled for 30 to 40 minutes in an ice-bath or refrigerator, at 0" C. The solution is decanted through a 10-ml. Gooch crucible (with an asbestos pad made from finely shredded fibers) cooled previously to 0" C. The precipitate and cont'ainers are washed 10 to 12 times with minimum amounts of 2 to 1 ethyl alcohol-isopropyl alcohol a t 0" C. The potassium periodate is dissolved in 10 ml. of 4 5 sulfuric acid and 25 ml. of water. One to 5 grams of potassium iodide are added depending upon the amount of the precipitate. The liberat'ed iodine is titrated with standard sodium thiosulfate solution of appropriate concentration, starch being used as the indicator.
the maximum concentration of metaperiodate ion is attained at p H 5 for a temperature of 0" C., it is undesirable to adjust the p H to this value because of the danger of precipitating Li2HJ06. For this reason it seemed best to adjust the pH of the solution to between 3.0 and 3.5.
DISCUSSION OF PROCEDURE
The sodium cobaltinitrite XT-as added as a solid as suggested by Burkhart (1) to encourage immediate precipitation of potassium-sodium cobaltinitrite. This technique was permissible because the stoichiometric composition of the precipitate was of less importance than the complete separation of the potassium. This also accounts for the fact that the precipitate does not need to be washed free of the excess reagent. Gelatin is precipitated upon the addition of the ethyl alcohol and sweeps doivn the fine particles of the precipitate. This assists in the completeness of the precipitation. Special care must be given to dissolve the evaporated sample clompletely in only 2 ml. of m-ater. The oxalic acid which mag have sublimed onto the sides of the beaker may be disregarded. .1light green color is often observed in the solution R hen potassium is precipitated a s potassium metaperiodate in the presence of cobaltous oxalate. I t is probably due to a cobaltic oxalate complex and causes no difficulty. Gmelin ( 4 ) reports that potassium and sodium cobaltic oxalate salts are dark green and very soluble. The evapoiate from the solution of potassium-sodium cobaltinitrite in nitric acid should not be heated above 100" C., as cobaltous oxalate may decompose to cobalt sesquioxide If formed, this compound would later oxidize iodide to free iodine and invalidate the final determination of potassium. The fundamental reaction for the volumetric determination of potassium as described indicates an equivalent -iveight of one eighth its atomic weight.
The available lithium carbonate alviays contained some potassium as an impurity, which necessitated purification before it was suitable for use in the preparation of the lithium acetate reagent. Sitric acid must be present \Then the solution of oxalates is evaporated to dryness in order to deposit potassium as the soluble nitrate rather than the difficultly soluble potassium acid oxalate. The authors have shoir-n (6) that the minimum solubility loss of potassium periodate is esperienced when a (by volume) ethyl alcohol solution is used. I n the previously outlined procedure the dcohol concentration is about 33y0 because it is impractical to adjust the concentration t o 35%. Under the conditions of this experimental xork, no evidence n-tts ever noted of oxidation of ethyl alcohol to aldehyde by periodic acid with subsequent reduction of potassium periodate. Consequently, the addition of ethyl acetate to retard the osidation of ethyl alcohol x a s not required, as had been done bjWillard and Boyle ( I S ) when using a medium of higher ethyl alcohol concentration. I t is important to evaporate the ethyl alcohol wash solution completely before attempting t o dissolve the potassium-sodium cohaltinitrite precipitate in hot nitric acid; otherwise there may be a reaction of near explosive violence between the hot nitric acid and the ethyl alcohol. 2,B-Dinitrophenol was chosen as the indicator both for its transition interval and for its resistance to oxidation by periodic acid. Although the data of Crouthamel and co-ivorkers (2) show that
Table 11. Analyses of Potassium h-itrate Samples by Periodate Method K So. lIean of Error Group Standard Taken, Average, D ~ \ i a t l o n . of K Found, for Set, llg. Samp1r.s lIg, llg. llg. Ng. 74 92 T4 i 7 -0 15 3 -0 12 - 0 13 74 83 74 95 = D 027 3 44 88 44 95 6 -0 07 44 90 44 9-1 - 0 06 3 -0 04 0 028 24 69 6 24 60 -0 09 3 24 63 -0 06 3 24 68 24 66 -0 02 4 24 71 - 0 06 -0 06 24 65 0 026 6 14 79 -0 02 14 81 14 7 8 3 - 0 03 -0 02 0 014 8 967 6 8 951 - 0 016 8 962 8 957 - 0 005 6 2 9 228 - 0 019 9 247 9 241 2 9 228 - 0 013 3 9 243 9 232 -0 011 9 368 9 336 -0 032 3 -0 017 -0 016 9 352 9 335 3 0 012 5 378 6 5 372 -0 006 6 5 369 5 375 -0 006 -0 006 0 003 3 157 6 3 I54 -0 003 3 160 +O 003 3 729 3 751 - 0 021 -0 007 0 009 1 513 1 511 6 + o 002 1 501 -0 010 3 -0 004 0 003 6 0 908 +o 002 0 906 3 0 893 -0 013 -0 006 0 004 6 0 512 0 507 +o 005 6 0 507 0 000 +O 003 0 005 2 0 298 -0 006 0 304 -0 006 0 008 6 0 204 +o 001 0 203 +o 001 0 007 0.098 8 - 0 003 -0 003 0 101 0 009 ?
i
KIOi
+ ;I- + 8 H + = K T + 412 + 4H20
ANALYTICAL CHEMISTRY
2014 By the method of Willard and Boyle the equivalent weight of potassium is one half its atomic weight. KIOi
+ 21- + 2 H +
=
Kf + I? + IO;
+ H20
Thus the proposed method has a sensitivity four times that of the Willard and Boyle method, making it particularly suitable for the estimation of small amounts of potassium. For amounts of potassium greater than 25 mg. the potassium metaperiodate may be determined by arsenite titration as described by Willard and Boyle (IS). I n their method the potassium metaperiodate is reduced to iodate with iodide in a biiflered solution at p H 7.5 and the liberated iodine is titrated with standard arsenite solution. This obviates the use of a high noimality sodium thiosulfate solution. By preference sodium thiosulfate solutions were used throughout the present study. EXPERIMENTAL
Conditions of Measurement. The analytical weights were calibrated against Class AI weights calibrated by the Nations 1 Bureau of Standards. All weighings involved in the various analyses were corrected for air buoyancy effects. Calibrated Normax volumetric equipment was used throughout. Standard solutions were calculated to 20" C. I n order to minimize eriors in the calculated results, the normalities of the titrants were adjusted so that the titration volumes fell between 40 and 50 ml. whenever possible. Each titrant was always standardized in respect to two independent standard solutions and check values were demanded which differed by no more than 0.1%. Proof of the Method. The reliability of the method previousl) presented was verified by analysis of known amounts of potassium. This was accomplished in two stages, because it was necessary to prove the method of determining the potassium Rith periodic acid before proving the method of separating the potassium with sodium cobaltinitrite. To test the recovery of potassium by the periodate procedure, known amounts of potassium nitrate were analyzed over a range of 0.1 to 75 mg. of potassium. The results of 139 analyses are presented in Table 11. For verification of the entire procedure, knon-n amounts of potassium nitrate yere first precipitated as potassium-sodium cobaltinitrite and then estimated as the metaperiodate according to the prescribed procedure. The results of 56 analyses are given 111 Table 111. No attempt \\-as made to separate potassium from other than a solution of sodium cobaltinitrite, because the precipitation of potassium-sodium cobaltinitrite under various experimental conditions has been well treated in the literature. Discussion of Results. Because of the large number of data involved, it is impractical to list the individual data for the anal? sis of the various amounts of pure potassium nitrate taken. Rather the mean of each set of analyses has been calculated as well as its deviation from the known amount of potassium taken for analysis, and these values are presented in Tables I1 and 111. For a particular amount of potassium the average of these deviations has been taken as a measure of the error in the method foi this amount. The deviation of each set has been given equal weight in this calculation because the results for a particular set are dependent upon the same experimental conditions. The standard deviation of the method for a particular amount of potassium has been calculated from all the result6 for that amount by treating the deviation of an individual result from the mean of the set. These values give a measure of the precision of the method. These results for the proof of the periodate separation and estimation are recorded in Table I1 and have been plotted on a semilogarithmic scale in Figure 1. The standard deviation of the method has been plotted for amounts of potassium ranging from 0.1 to 75 mg. The error in the method as defined above also has been plotted for the same range. Taking into account that a semilogarithmic plot has been used,
Table 111. Analyses of Potassium Nitrate Samples with Cobaltinitrite and Metaperiodate Precipitations K No. Mean of Error Group Standard Taken, of K Found, for Set, Average, Deviation, 1zIg. Samples bIg. RZg. hfg. hk -0 12 -0 12 zk0 050 74 99 4 74 87 3 44 86 - 0 08 44 94 4 44 95 -0 03 -0 05 0 046 44 98 3 24 61 -0 07 -0 07 0 010 24 68 -0 02 14 80 3 14 78 -0 04 -0 03 0 017 14 84 3 14 80 8 969 4 8 944 -0 025 3 8 956 -0 011 8 967 9 36'1 3 9 336 -0 032 5 9 342 -0 027 9 369 -0 023 -0 023 0 009 9 377 3 9 354 5 5 614 -0 010 - 0 010 0 002 5 6-04 4 1 487 - 0 004 -0 004 0 005 1 491 -0.002 0 507 2 0 605 0 508 4 0.504 -0,004 -0.003 0 003 0.203 3 0.201 -0.002 -0.002 0 002
:I -010
- 0 I5
1
01
1
05
l 1 l l l
Figurell.
I
IO
MG
OF
,
I
I
, 1 1 1 1
5 POTASSIUM
10
,
1
,
50
TAKEN
Analysis of results on potassium nitrate samples
it is seen that both the standard deviation and the error graduallj increase for larger amounts of potassium. For amounts of potassium greater than about 3 mg. the results are low by less than 0.2% on the average. For amounts of potassium between 0 1 and 3 mg. the results are low by 0.005 mg. of potassium or less. The low results are attributed to the solubility error. The standard deviation for amounts of potassium greater thnn 3 mg. is about =kO.l% on the average, being someffhat greatrr for small amounts and gomewhat less for larger amounts. T h r standard deviation for amounts of potassium betn-een 0.1 acd 3 mg. is about 10.005 mg. on the average, increasing somewhat for the smallest amounts. The data for the analysis of various amounts of potassium nitrate after the preliminary separation of the potassium as the cobaltinitrite are presented in Table 111. These results have been treated and presented in the manner described above. From Figure 2 it is seen that the data present much the same picture as Figure 1. However, the solubility error is somewh:,t greater for these data for amounts of potassium greater than 3 nig. It varies from about 0.3% for 3-mg. amounts to less than 0.2% for 75-mg. amounts. The standard deviation of these data for the largest amounts of potassium is a little greater than that s h o r n by the data in Figure 1. However, it is still less than = k O . l % on the average for the amounts greater than 3 mg. Undoubtedly there has been compensation of the solubility error to a small extent in almost all the analyses. The 10v solu-
V O L U M E 2 8 , NO. 1 2 , D E C E M B E R 1 9 5 6
2015 that coprecipitation may be controlled so as to compensate for the solubility error if desired. This could be accomplished by adding the ethyl alcohol before the complete neutralization of the periodic acid with lithium acetate. This measure may be of the greatest advantage, should the method be adapted to microtechniques. Although this possibility is noted here in the light of experience with the method, there is no real need to use such a technique in macro work. The solubility error may be corrected for by means of the experimental curves, in which case the uncertainty in the results will be of the order of z t O . l % for amounts of potassium greater than 3 mg. Even if no correction is applied, the uncertainty in this range would probably not e\ceed -0.3y0 and in most cases would appear to be less than this
- 3 131
-0151 01
I
1
,
95
/
,
I
I
80
MG
OF
5 POTASSIUM
0 TAKEN
50
J
Figure 2. Analysis of results on potassium nitrate sample with cobaltinitrite and metaperiodate precipitations
bility error for small amounts of potassium is probably due t o this. Since the precipitation of small amounts of potassium as thc metaperiodate is slow and incomplete in aqueous solution, most of it precipitates upon addition of the ethyl alcohol, with resultant coprecipitation of periodate. The faster rate of precipitation inherent with t h e largest amounts of potassium taken for analysis noald also he likely t o cause some coprecipitation of periodate. The results for both sets of data diverge from the curves in Figure$ 1 and 2 for 25-mg. amounts of potassium. TI is is probably due to practically no compensation by coprecipitation for this amount, because the rate of precipitation t~:isrelatively slower than for the largest amounts. Although it was recognized that there x a s a h a y s somr coprecipitat ion, efforts were directed to mininizing it in order to evaluate the real magnitude of the solubility error as nearly :IS possible. On the othrr hnnd, there is good reason to believe
LITERATURE CITED
Burkhart, L., Plant Physiol. 16, 411 (1941). Crouthamel, C. E., Hayes, A. AI., Martin, D. S . , J . Am. C h e w . SOC.73, 82 (1951). Drude, P., 2.physik. Chem. 23, 267 (1897). “Gmelins Handbuch der anorganiachen Chemie. Kobalt,” Pdi t A, Sect. 2, 8th ed., pp. 405, 420, 422, Verlag Chemie G. i n b. H., Berlin, 1932. Jentoft, R. E., “Study of Determination of Potassium as t h e Rletaperiodate,” thesis, University of Washington, 1952. Jentoft, R. E., Robinson, R. J., ANAL.CHEJI.26, 1156 (1954). Jentoft, R. E., Robinson, R. J., J . -4m. Chem. SOC.75, 4083 (1953). Jones, J. H., I b i d . , 68,240 (1946). Jones, J. H., Heckman, N., Ibad., 69, 536 (1947). Kolthoff, I. M.,Sandell, E. B., “Textbook of Quantitative Inorganic -4nalysis,” 3rd ed., pp. 594, 606, Rlacmillan, Ken York. 1952. (11) Platow,’ -1.M., Chemist-Analyst 28, 30 (1939). (12) Scott, W.W., “Standard Alethods of Chemical Analysis,” 5 t h ed., pp. 1200, 1209, Van Nostrand, Sew York, 1939. (13) Willard, H. H., Boyle, 9.J., IND. ENG.CHEM.,ANAL.ED. 13, 137 (1941). RECEIVED for review February 3, 1956. Accepted August 6. 1886. Work Office of Naval Research Contract 5 8 onr-BYO/III with t h e U n i v r s i t y of Washington.
supported b y
Coprecipitation of Rare Earth Iodates with Thorium Iodate Precipitated from Homogeneous Solution KENNETH J. SHAVER’ M o u n d Laboratory, Monsanto Chemical Co., Miamisburg, O h i o
T h e separation of thoriuni from rare earths by piecipitation of thorinm iodate from homogeneous solrition is evaluated. The extent of coprecipitation is given when thorium is precipitated in the presence of the rare earths; lanthanum, praseodyiniuni, promethium, and europiuni; and the related elements, yttrium and scandium. The extent of coprecipitation is determined by the use of radioactive tracers of these elements. The distribution of rare earths with thorium follows the logarithmic or heterogeneous distribution law. I t is concluded that trivalent rare earths coprecipitate with thorium iodate by a type of isomorphous replacement within the thorium iodate lattice. A correlation is given between ionic radii and extent of coprecipitation.
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HE precipitation of thorium iodate from a nitric acid sohtion has long been used for the qualitative separation of thoiium from rare earths This separation depends on the solubility of rare earth iodates in a moderately concentrated nitric
acid solution. As a quantitative method, hoxever, the separation is riot satisfactory because rare earths are coprecipitated to an appreciable extent. Recently, this situation has been improved through the application of the technique of precipitation from homogeneous solution. Stine and Gordon (8) a t Syracuse University applied this technique to the precipitation of iodates in a unique manner. I n this method, iodate ion is produced in solution from the reduction of periodic acid with ethylene glycol. The rate-controlling reaction is the production of glycol from the hydrolysis of 0-hydroxj-ethyl acetate. Thorium iodate slon-ly precipitatra as iodate ion is produced homogeneously throughout the solution. The degree of separation of thorium from rare earth 11-as determined by Stine and Gordon by precipitation of thorium iodate in the presence of trivalent cerium. The present work is a detailed investigation of this separation. The separation efficiency is given in terms of the degree of coprecipitation of a number of rare earths and related elements. 1 Present address, Inorganic Chemicals Division, hlonsanto Chemical Co., Everett Station, Boston 49, Mass.
