I N D US TR I A L A N D EN G I N EER I N G CH E M I STR Y
November 15,1934
Place an amount of e. P. antimony powder, such that 30 or 40 cc. of bromate will be required (0.02 t o 0.03 gram), in a 50-CC. Erlenmeyer flask, add 4 cc. of concentrated sulfuric acid, and heat carefully over a small flame until solution is complete, removing at once to prevent oxidation. After cooling, transfer the solution to a 200-cc. Erlenmeyer flask, and add water and 20 cc. of concentrated hydrochloric acid, so that the final volume shall be 60 to 70 cc. Heat the solution t o a temperature of about 70" C., add a drop of methyl orange indicator, and carry on the titration to the disappearance of the pink color. The indicator is prepared by dissolving 0.1 ram of methyl orange powder in 100 cc. of water, and filtering t f e solution. From the final buret reading 0.25 cc. is deducted. This correction factor has been found to be uniformly recurrent in a number of tests performed on blanks containing the amounts of acids and water designated above. In the routine of actual white metal analysis where sulfuric acid is not present, the end-point correction is 0.15 cc. This value was evolved in the same way as above, determining the end-point correction on blank solutions containing no sulfuric acid. I n the titrations to determine the value of the end-point corrections and in the titrations of white metal analyses a 10-cc. buret was used in which the smallest graduations are 0.05 eo. The presence of copper would prevent the use of this method, but it may be used in the presence of as much as 25 to 50 per cent of iron, The only modification required would be the addition of 15 to 20 cc. of phosphoric acid, sp. gr. 1.37 (1); a complex ion is formed from the phosphoric
457
acid and ferric iron, which in a concentrated hydrochloric acid solution is very stable against the reducing agent, sodium sulfite, From this stage the process can be continued as presented.
REFERENCE STANDARDS It was difficult to obtain metals or solders to be used as ultimate standards of reference for the development of this method, As far as the writer was able to determine, they were not procurable from the Bureau of Standards in Washington. It was therefore decided to use as a basis for the work Bureau of Standards pure tin sample No. 42b which was submitted to this laboratory as a standard for melting point determinations with no information as to the antimony content. Upon analyzing this sample by the present method an average of three determinations gave 0.02 per cent of antimony. From another laboratory there was obtained a sample of 50-50 solder designated to contain 0.04 per cent of antimony by the permanganate method. The average of five determinations by the bromate method gave 0.024 per cent. With these two samples as standards, the attached experimental analyses were performed.
LITERATURE CITED (1) Anderson, C. W., IND.ENG.CHEM.,Anal. Ed.,5, 52 (1933). (2) Rowell, H.W., J. SOC.Chem. Ind., 25, 1181 (1906). RECEITBDAugust 22, 1933.
Iodometric Determination of Phosgene MARYANP. MATUSZAK,~ U. S. Bureau of Mines, Pittsburgh, Pa. OW a n d erratic r e s u l t s have led to an investigation of the iodometric or so-called acetone m e t h o d o f
L
Data are presented {hat indicale the nature c o of side reactions causing low and erratic results fieation and Mohr determination by the iodometric rnefhod of determining phos&! t ' $ $ ! O gene. A stmpk modifzcation is suggested for by adding an excess of iodate and iodometrically determining all the iodine present $ ? ~ ~ ~ ~ ' ' ~ $ f in the form of free iodine, hydrogen iodide, and ing the acid gave a result, in the thTrd column, 13 per cent too iodoacetone. low on immediate titration and
d e t e r m i n i n g phosgene (1, 7), which c o n s i s t s of a b s o r b i n g the phosgene, free f r o m a c i d gases, in 2 per c e n t s o d i u m iodide in anhydrous a c e t o n e and titrating w i t h a q u e o u s 0.01 N thiosulfate to disappearance of the yellow color: COClz
+ 2NaI---tCO + 2NaCl + I,
Since sodium iodide is markedly hygroscopic and can become alkaline by loss of iodine on drying (6), potassium iodide in saturated acetone solution (about 1.9 per cent) is a better reagent. Beside presenting the danger of hydrolyzing phosgene COClz
+ HzO+COg
+ 2HC1
wi!gg~
11 per cent too low on delayed titration. Assuming formation by hydrolysis of phosgene gave a result, in the fourth column, 5 per cent too low for both determinations; the agreement indicated that the additional iodine liberated on standing was accompanied by disappearance of an equivalent amount of acid. Assuming acid formation by iodination gave a result, in the last column, 3 per cent too high on immediate titration and 0.5 per cent too high on delayed titration. The agreement between the last figure and the correct one is fortuitous, as there is no criterion for selecting the correct length of the pre-titration period. TABLEI. RESULTSBY DIFFERENT METHODS MINUTES BEFORE
moisture promotes the acid-catalyzed iodination of acetone (2) by shifting the equilibrium in favor of iodoacetone: CHaCOCHaeCHaC(0H) :CH, CHIC(OH):CHz 12@CHaCI(OH)CHJ CHaCI(OH)CHJ@CHJCOCH21 HI
+
+
Although this effect cannot be avoided during titration, it should be minimized by avoiding any prior addition of water. Reasonable precautions did not prevent the appearance of acid in the solution. Typical results are given in Table I. I
Present address, 301 South Creek Ave., Bartlesville, Okla.
TITRATION 6 06
PHOSQENE MOHR METHOD
-1ODOMETRIC
Iodine
P,p.m.
P,p.m.
1970
1714 1751
+
MITHOD-
Iodine acid P.p.m. 1869 1865
+
Iodine 2 acid P . p . m. 2025 1979
The observed gradual replacement of acid by equivalent iodine, more clearly shown by the first three determinations of Table 11, obtained by titrating 10-cc. portions of acetoneiodide solution through which phosgene-containing air had been passed, is incompatible with the theory of origination of acid by iodination, which, as illustrated by the last two determinations in the table, causes acid to increase with simultaneous consumption of two equivalents of iodine.
~ [ ~ ~ ~
ANALYTICAL EDITION
458
TABLE11. EFFECTOF TIME AND WATER WATER ADDED
cc.
HOURS BBFORE~ TITRATION 0
None
0.01 N THIOSULFATE Iodine Acid Total
cc .
cc *
cc .
5.50
0.52
6.02
5.m
0.2s
Vol. 6,No. 6
minutes: the precipitate formed by adding acetone-hydrogen chloride to acetone-iodide and letting stand overnight required 9.72 cc., whereas direct titration required 9.62 cc.; no further precipitation occurred during 10 days.
TABLEIV. PRECIPITATION OF POTASSIUM CHLORIDEFROM ACETONE-IODIDE BY ACETONE-PHOSGENE The conclusion that the acid did not originate primarily by iodination can be reached independently. Others (6, 10) have observed that iodination of anhydrcius acetone can be prevented by excess iodide. The smallest possible molecular ratio of iodide to iodine in the experiments of Tables I and I1 was 6 and 30, respectively. Table 111, confirming the finding of Professor Olsen that iodination does not occur unless this ratio is less than unity ( B ) , shows that no iodination could have occurred prior to addition of water.
SILWRNIT RAT^
cc.
1 ''
3 6 10
8.87 9.82 10.55 11.03
The foregoing data indicate that the following changes occur when the iodometric method is used: Most of the phosgene liberates iodine. A little reacts instead with acetone, giving hydrogen chloride and isopropenyl chloroformate. The chlorine in these products is replaced by iodine, rapidly in the former and very slowly in the latter; TABLE111. EFFECTOF EXCESSIODIDE ON IODINATION OF potassium chloride is precipitated simultaneously. (An ANHYDROUS ACETONE alternative reaction for the chloroformate is the formation RATIO HOURS BEFORE 0.00684 N THIOSULFATE KI/II) TITRATION Iodine Acid Iodine + 2 acid of diisopropenyl carbonate by reacting with the enol form cc . cc. cc. of acetone-a reaction typical of alcohols and chloroformates 12.21 12.21 ... Blank 10.91 2:il (8)-the by-product hydrogen chloride then precipitating 5.29 0.00 1.5 11.82 2.49 6.84 0.20 1.0 potassium chloride.) The hydrogen iodide is slowly but 11.95 1.98 7.99 0.40 1.25 11.98 9.46 1.16 0.60 1.25 quantitatively oxidized to iodine, apparently by atmos12.07 10.83 0.62 1.25 0.80 11.92 11.54 0.19 pheric oxygen. Iodination of acetone occurs if the molecular 0.93 17 12.34 12.34 0.00 1.86 17 ratio of iodide to iodine falls below unity, equilibrium soon 12.23 12.23 0.00 9.30 1.6 12.21 12.21 0.00 23.2 1.5 being reached. As oxidation of hydrogen iodide proceeds, the equilibrium shifts in favor of more iodoacetone. When Likewise, formation of acid cannot be explained by as- water is present, iodination, catalytically initiated by the suming hydrolysis of unreacted phosgene upon titrating acid from the action of phosgene on acetone, takes place with the aqueous thiosulfate solution, for small amounts regardless of excess iodide. of phosgene cannot exist in the presence of acetone. A fresh Since the net effect of oxidation of hydrogen iodide, when dilute solution of phosgene in acetone does not have the two only free iodine and acid are determined, is a loss of iodine specific characteristics of phosgene, its unmistakable odor as iodoacetone, the method can be made quantitatively and its ability to form diphenyl urea with aniline, showing accurate only if iodoacetone is also determined. Fortunately that any phosgene that escaped reacting with iodide would this can be done in an exceedingly simple manner because react with acetone (4): of the general reaction between sodium thiosulfate and halogen-substituted alkyl groups to form compounds that COClz CHsC(0H) :CHz+HCl CHaC(OCOC1):CH, do not react with iodine (chloroacetone, for example, reacts This reaction accounts for the primary formation of acid. rapidly and quantitatively, 9) : Although the amount of acid formed is small, it strongly CHaCOCH21 N ~ ~ S Z O ~ + C B ~ C O C H Z N ~ SNaI ~O~ catalyzes the iodination of acetone when water is added; This reaction was confirmed quantitatively in the course hence, it may lead to an error much larger than itself. Many determinations of phosgene by the iodometric method have of the study of the effect of excess iodide. At the end of four convinced the writer that prevention of this side reaction of the determinations given by Table 111, 5.00 cc. of thiosulfate were added to each of the solutions. After standing is practically impossible. I n harmony with this reaction, the progressive liberation for half an hour, they were diluted with enough water to of iodine in the determination of the acid indicated that the permit the use of starch as indicator and then titrated to a acid was not all in the form of hydrogen iodide (any hydrogen distinct blue with 0.01 N iodine. Finally, the blue color was chloride would react immediately with iodide to give hy- discharged with thiosulfate. As Table V shows, the total drogen iodide) but that some was present that reacted as thiosulfate used for free iodine, acid, and iodoacetone agreed a moderately strong acid (3). On titrating a solution made well with that required for the quantity of iodine used. The by adding acetone-phosgene solution to water and then excess agreement was substantially better than for the values in iodide and excess iodate, 10.61 cc. of thiosulfate were required the last column of Table 111,since more thiosulfate was used immediately and a total of 4.73 cc. during the next 3 hours. to react with iodoacetone than with acid, due to loss of On similar treatment, an acetone solution of hydrogen hydrogen iodide by oxidation. chloride containing slightly more chloride, as shown by TABLE V. DETERMINATION OF IODOACETONE comparative Mohr determinations, required 16.36 cc. of RATIO, THIOEULFAT~, USFiD FOR thiosulfate on immediate titration and only 0.48 cc. during KI/Ii Iodine Acid Iodoacetone Total the next 3 hours. Progressive hydrolysis of the ester accc. cc. cc . cc. counts readily for this difference. Also in harmony with this reaction, precipitation of potassium chloride on adding acetone-phosgene solution to acetoneiodide required several days for completion (Table N). In 10 days the equivalent of 11.03 cc. of 0.00732 N silver SUGGESTED PROCEDURE nitrate was precipitated; by direct titration an equal volume Although extraneous circumstances have prevented further required 11.17 cc. On the other hand, precipitation by hydrogen chloride was probably complete within ~l few experimental work, the evidence is sufficient to indicate t h a t
+
+
+
r
+
Y
November 15,1934
I N D U S T R I A L A N D E N G IN EE R I N G CHE MISTR Y
the reaction between iodoacetone and thiosulfate can be used to raise the iodometric method t o quantitative accuracy. The stoichiometric relations are fortunately such that errors due to the discussed side reactions, occurring because of presence of acetone, presence of moisture, or local deficiency of iodide, are corrected automatically. Thus, it is immaterial whether the acid is formed by hydrolysis of phosgene, by iodination of acetone, or by the action of phosgene on acetone. The following may prove helpful as a suggested procedure: Absorb the phosgene, freed from acid gases, in a saturated acetone solution of potassium iodide containing at least several times as much iodide as iodine to be liberated. Add an excess of iodate and then a measured excess of 0.01 N thiosulfate. Let stand for half an hour or more. Make sure that the solution contains more water than acetone, add several drops of starch indicator, and titrate back with 0.01 N iodine t o a distinct coloration. (Since excess iodate is present, 0.01 N hydrochloric acid probably could be used instead of iodine.) Finally, a t once discharge the iodine color with thiosulfate. The total
459
thiosulfate minus the equivalent of the added iodine then represents the phosgene exactly as if the side reactions did not occur. (1)
LITERATURECITED Anonymous, Jahresber. chem.-techn. Reichsanstalt, 5,
11-20
(1926).
(2)
Dawson et al., a large number of articles beginning with J.
Chem. SOC.,95, 1860-70 (1909). (3) Kolthoff and Furman, “Volumetric Analysis,” Vol. 11, pp. 38992, John Wiley & Sons, N. Y., 1929. (4) Matuszak, J . Am. Chem. SOC.,56, 2007 (1934). ( 5 ) Morley and Muir, “Watt’s Dictionary of Chemistry,” Vol. IV, p. 481, Longmans, Green and Co., N. Y . , 1894. (6) Olsen, J. C., Brooklyn Polytechnic Institute; private communication. (7) Olsen, Ferguson, Sabetta, and Scheflan, IND.ENQ.CHEM., Anal. Ed., 3, 189-91 (1931). (8) Rose, Ann., 205, 230-47 (1880). (9) Slator and Twiss, J. Chem. Soc., 95, 95 (1909). (10) Wallauer, Chemisoh-technischen Reichsanstalt; private com-
munication.
RECDIVPD April 21, 1934. Published by permission of the Director, U. S. Bureau of Mines. (Not subject t o copyright.)
A Volumetric Method for Determination of Cobalt and Nickel J. T. DOBBINS AND J. P. SANDERS, University of North Carolina, Chapel Hill, N. C . HE reaction between pyridine, the thiocyanate ion,
T
and several metal ions (cobalt, copper, cadmium, manganese, nickel, and zinc) which results in the formation of complexes of the general formula M(Py)d(CnS)z has been the basis for quantitative determinations of these ions developed by Spacu. Obviously the method is not applicable for one ion in the presence of any other. As all the methods of Spacu were gravimetric, it occurred to the authors that these reactions would also serve for indirect volumetric methods. As there is no reliable volumetric method for either cobalt or nickel, they were chosen to test the method. Since completion of the work, Spacu (1) has published a variation of the method in which he titrates the excess thiocyanate in the whole filtrate. This method did not give satisfactory results for tho authors, as it was not possible to wash all the excess thiocyanate out without dissolving some of the complex salt. Spacu (2) has also published a potentiometric method for nickel for which he claims an error of less than 0.5 per cent.
SOLUTIONS STANDARD AMMONIUMTHIOCYANATE. An approximately 0.1 N solution of ammonium thiocyanate was repared and standardized by titrating it against a standard sogtion of silver nitrate. Potassium thiocyanate may be used in place of the ammonium salt, as later work has shown. STANDARD SILVERNITRATE. A 0.1 N solution of silver nitrate was prepared and standardized against c. P. sodium chloride. INDICATOR SOLUTION.This solution was prepared by dissolvin~10 crams of ferric alum in a mixture of 80 cc. of water and 20 cc. ;f 6 “N nitric acid.
PROCEDURE The sample, which should contain from 0.05 to 0.1 gram of cobalt as some cobaltous salt, is dissolved in about 150 cc. of water in a 250-cc. volumetric flask. The solution is made just acid to litmus with nitric acid, 3 cc. of pyridine are added, and an excess of standard ammonium thiocyanate is run in. Upon the addition of the thiocyanate, the cobalt is precipitated as pink C O ( P ~ ) ~ ( Cwhich ~ S ) settles ~ rapidly and filters very easily. The solution is diluted to the mark and mixed thoroughly. A portion of the solution is filtered through a dry filter and the first few
cubic centimeters of the filtrate are discarded. An aliquot of 50 cc. of the filtrate is transferred to a beaker and diluted to 100 cc., 1 cc. of concentrated nitric acid is added, and an excess of standard nitrate is immediately run in. Five cubic centimeters of indicator are added and the titration is completed in the usual way. The error introduced by the presence of the precipitate in the solution is negligible. The weight of cobalt may be calculated from the formula: g =
[(cc. of NHdCnSXN)
- 5 (cc. of AgNOtXN)]0.02947
In order to test the precision of the method, three solutions of cobalt sulfate were made and 25-cc. portions were analyzed by this volumetric method and also by the electrolytic method of Brophy. The results are given in Table I. TABLE I. DETERMINATION OF COBALT SOLUTION I1 Gram
SOLUTION I Gram
SOLUTION I11 Gram
VOLUMIOTRIC METLIOD
0.1059 0.1062 0.1057 0.1056 0.1063 0.1058 0.1059
0.1328 0.1330 0.1326 0.1329 0.1326 0.1330 Av 0.1328
0.1596 0.1598 0.1594 0.1595 0.1593 0.1594 0.I595
DLDCTROLYTIC M l T H O D
Av. 0.1329
0.1596
0.1060
The same procedure may be used for the determination of nickel. A comparison of the results of the method and the dimethylglyoxime method is seen in Table 11. TABLE 11. DETERMINATION OF NICKEL SOLUTION I Gram
SOLUTION I1 Gram
VOLUMDTRIC METHOD
0.0128 0.0125 0.0126 0.0123 0.0127 0.0126 Av. 0.0126
0.0505 0.0507 0.0510 0.0510 0.0503 0.0504 0.05065
D I X D T H Y L G L Y O X I Y E METHOD
Av. 0.0125
0.0506
