ANALYTICAL EDITION
September, 1945
acid are added in rapid succession in that order, This sample is allowed to stand while these reagente are added to all of a series of 20 to 30 samples before titrating the excess ferrous sulfate. The range and accuracy of the method were determined aa follows:
Table 1.
Range and Accuracy Found hfillkwamr of NoCIOI
Taken
1.30 2.02 2.52 3.04 3.79 12.46
1.25 2.00 2.60 3.00
3.76 12.60 Teble
Error 0.00 Jr0.05
0.00
0.00
f0.02 +o. 02 +0.04 +0.04 -0.04
11. Precision under Best Conditions NaClOi Found, 0.066 0.066
0.068 0.066
O.Oo0 +0.001 -0.001
5%
1 2 3
0.067
4
5
6 7
8
9
10 AV.
Standard deviation of group Standard deviation of indnite group L U I of method
Deviation,
% O.OO0 -0.001 -0.001
T u t No.
0.067 0.067
0.000
0.067 0.067 0.066
0.000 0.000
-0.001 0.067 0.00071 *0.00076 r0.0023
sulfuric acid concentration of 3.5 N waa required for quantitative reduction in 5 minutea when using 2.2-milliequivalent excem of ferrous sulfate (Figure 1, curve 2). Thus, the presence of the chloride ion reduces somewhat the acid concentration required to give a fixed rate of reduction. I t was found that 2.2-milliequivalent excess of ferrous sulfate gave quantitative reduction in less than 1 minute if the sulfuric acid concentration was increased to 6.3 N . Reduction required 5 minutea for completion a t this higher acid concentration when the exce9s of ferrous sulfate waa reduced to 0.7 milliequivalent. In practice the phosphoric acid, ferrous sulfate, and sulfuric
An
To 25-ml. portions of 100-gram-per-liter eodium hydroxide were added measured volumes of sodium chlorate solution. There were then added 25 ml. of 1 to 1 phosphoric acid, 25 ml. of 0.1 N ferrous sulfate, and 50 ml. of 12 N sulfuric acid in that order. The excess ferrous sulfate waa titrated after 10 minutes or longer. The m&uimum error observed in the ran e up to 12.50 mg. of 80d i m chlorate is 0.05 mg. (Table I). b h e n calculated on the basis of a 5-gram sample of 50% sodium hydroxide this is equivelent to a range of 0.25% and a,maximum error of 0,001~. This error is well within the precision of the method aa gven in Table I1 and indicatos that the method 1s free from constant errors. The precision of the recommended method waa determined by making replicate analyses of a sample of commercial 50% caustic soda according to the procedure outlined by Moran (a). The limit of uncertainty under the best conditions (LUi) waa *0.0023% for a sample analyzing 0.067% (Table 11). The limit of uncertainty under routine conditions was calculated fiom 24 analyses made by seven analysts over a 12-month period. The average value and precision under these conditions were 0.067 *
0.0046%. ACKNOWLEDGMENT
The assistance of C. C. Meeker and G. F. Foy in performing most of the experimental work and of R. F. Moran and members of the Control Laboratory staff in supplying part of the precision data is gratefully acknowledged. LITERATURE UTED (1) Boyle, A.'J., Hughey, V. V., and Casto, C. C., IND.ENG.CHEM., ANAL.ED., 16, 370-1 (1944). (2) Moran, R.F., Zbid., 15, 361-4 (1943).
lodometric Method
DWIGHT WILLIAMS Commercial rayon-grade 50% caustic soda, which contains only a few parts per million of chlorate, is analyzed b y boiling the acidified semple in the presence of the iodide ion, collecting the die tilled iodine in e dilute solution of potassium iodide, end titrating with sodium thiosulfate solution. The error due to the oxidation of the iodide ion b y air on boiling the acidified solution i s reduced to a minimum by keeping the concentration of both the iodide ion and the hydrosen ion at a minimum. The interference of iron is stoichiometric, permitting e correction for this impurity. A constant correction is applied for manganese. The recommended method i s subject to a constent error of -4% of the amount of chlorate present, necessitating the use of an empirical factor. The limit of uncertainty of the method under the best conditions (LU,) is *0.46 p.p.m. of sodium chlorate for a sample analyzing 7.60 p.p.m.
A
535
LOW concentration of sodium chlorate is desired in caustic soda which is to be used in the manufacture of rayon. Reduction with ferrous sulfate rn described in the preceding paper (1) is not sufficiently sensitive for this application. After consideration of other methods it was decided to investigate an iodometric procedure. The reaction between hydrochloric acid and chloric acid waa tested first but waa found to be too slow. The reaction between hydrobromic acid and chloric acid was little better, but that between hydriodic acid and chloric acid at the boiling point proved to be sufficiently rapid.
AND
C. C. MEEKER
The oxidation of hydriodic acid by the air proved to be a serious source of error, which was not eliminated by sweeping the air out with an inert gas. However, it waa reduced to a minimum by reducing the concentrations of the hydrogen and iodide ions to a minimum. The conditions which were chosen for the analysis were those giving the best precision, but they resulted in a constant error of -4% of the amount of chlorate present. This constant error can be eliminated by increasing the concentration of the iodide ion but this also results in larger blanks and poorer precision. Sodium hydroxide readily dissolves many compounds which reduce chlorate and prevent its quantitative recovery. Contact with rubber stoppers waa found to be an especially eerious source of error. Storage of samples in bottles closed with screw caps with which the eodium hydroxide does not react is recommended. REAGENTS
Potassium iodide, 50 grams per liter. Dissolve 50 grams of of U.S.P.or C.P. potassium iodide in sufficient water to make 1 liter of solution. If a yellow color develops, add 0.01 N sodium thiosulfate solution dropwise until colorless. Starch solution, 10 grams per liter. Store in sterile bottles or prepaie fresh daily. Hydrochloric acid, c.P., specific gravity 1.19. Sodium thiosulfate, 0.01 N , containing 0.4 gram per liter of sodium carbonate. Lubricant. All ground-glass joints and stopcocks are lubri-
INDUSTRIAL AND ENGINEERING CHEMISTRY
536
cated with pure white Vaseline, Dow-Corning stopcock grease, or other lubricant which has been ahown to cause no error in the analysis. PROCEDURE
Samples of caustic soda which are to be used for this analysis must not be collected or stored in rubber-stoppered bottles. Harshaw Scientific, Cincinnati, Ohio, NoSol-Vit screw-capped bottles are satisfactory for storqe of samples. Set up the apparatus shown in Figure 1. Weigh 25 * 0.3 grams of 50% caustic soda into the reaction flask. Add 1 ml. of 50-gram-per-liter potassium iodide solution, 50 ml. of water, and one or two glass beads to prevent bumping. Connect the reaction flask to the droppin funnel. Introduce 50 ml. of 50-gram-perliter potassium iodicfe solution and 5 ml. of starch solution into the receiver. Support the receiver on a wooden block so as to make a liquid seal a t the outlet of the condenser. Introduce 32 ml. (5-ml. excess) of hydrochloric acid, specific gravity 1.19, into the separatory funnel. Open the sto cock to allow the. acid to run into the reaction flask. Close t i e stopcock and light the burner. Adjust the flams so that the solution boils vigorously. Titrate the iodine as it is liberated with 0.01 N sodium thiosulfate solution. Continue boiling until no more iodine is liberated during a 2-minute interval. The total boiling time will be about 5 minutes if boiling is vigorous. Prepare a reagent blank by introducing 100 ml. of water, 1 ml. of potassium iodide solution, and one or two glass beads into the reactor. Acidify with 5 ml. of hydfochloric acid and distill into a mixture of 50 ml. of potassium iodide solution and 5 ml. of starch solution. The iodine distills more slowly in this case and the distillation should be continued until half of the water distills over. After each analysis rinse the reactor with water, wipe the ground-glass surfaces clean; and apply fresh lubricant.
- ml. of Na&Os
blank) N NazSzOa X 17,700 X 1.04 grams of sample apparent p.p.m. of NaClOs
(MI. of NalSzOa sample
Vol. 17, No. 9
with hydrochloric acid was tried unsuccessfully as a means of preventing oxidation by air. Consistently low blanks were finally obtained by reducing the excesses of potassium iodide and hydrochloric acid to minimum values, as was shown by the following experiments: A measured volume of otassium iodide solution waa introduced into the reaction flas! and diluted to about 200 ml. Natural gas was then forced into the train through a tube which passed through a rubber stop er fitted to the top of the separatory funnel. The gas bubbled &rough the solution in the reactor and escaped by bubbling through the water seal in the receiver (Figure 1). Gas waa passed through the system for 5 minutes at a rate of 300 ml. per minute. The total volume of gas was sufficient to displace all the air in the system ten to fifteen times. A measured volume of hydrochloric acid was then introduced throu h the separatory funnel and the analysis waa completed as descri%ed above. In another series of tests the gas sweep waa omitted. The data in Table I show that high blanks are obtained regardless of the gas sweep if the concentration of the hydrogen or iodide ion is high. The blank may be reduced to a negligible valuethe results are reported only to the nearest 10 micrograms-by keeping the concentrations of hydrogen and iodide ions low, even though the gas sweep is omitted. The use of 0.05 gram of potassium iodide (1 ml. of 50-gram-per-liter solution) and 60 milliequivalents of hydrochloric acid (5 ml., specific gravity 1.19) gave no detectable blank. Any iron which is present in sodium hydroxide may be assumed to be present in the ferric state at the time of analysis, since ferrous iron is rapidly oxidized to ferric iron in this medium. The effect of ferric iron was determined aa follows:
3
Corrections must be applied for iron and manganese. A constant correction of 0.2 p.p.m. is applied for manganese. The iron content is determined colorimetrically by the o-phenrtnthroline method.
+
Apparent p.p.m. of NaClOs- (0.318 X p.p.m. of Fe 0.2) = p.p.m. of NaClOs
Table
To 25-grm portions of 50% sodium hydroxide were added measured quantities of water solutions of C.P. sodium chlorate and C.P. ferric chloride. These mixtures were analyzed by the
0 f3
I. Effed of Concentration of Potassium Iodide, Hydrochloric Acid, and Gas Sweep on Blank
Gas Sweep Yea Y 08 Yea YeS Yea YeS
KI
HCl
NaClOa Found
Qrama
Miliicquiaalcnts 12
Mierograma
60
240 480
YeS
300 300 300
No No No No No
60 60 60 60 60
Yea
300
10 10 80
200
70
240
570 500 0 0 10 20 40
EXPERIMENTAL
In the initial experiments large excews of hydrochloric acid and potassium iodide were'used. High and erratic blanks were obtained under these conditions, owing to the reaction of the acidified iodide solution with the oxygen of the air. An attempt was made to exclude air by adding sodium carbonate to the sample prior to acidification. When sodium carbonate was added in sufficient quantities so that the resulting carbon dioxide swept all the air from the flask, recoveries of sodium chlorate were low, apparently because iodine was swept through the system by the carbon dioxide without coming in contact with the potassium iodide solution in the receiver. Sweeping the reaction flask with natural gas prior to acidifying
A. REACTION FLASK, 2 5 0 ML. B. DROPPING FUNNEL C. R E C E I V E R , 2 5 0 ML D. W O O D E N BLOCK
Figure 1.
Diagram of Apparatus
A N A L Y T I C A L EDITION
September, 1945
$--
I . F E R R I C IRON 2. M A N G A N E S E
a
Q'
z I5
b' W
-I
ss IO
0 W
w a 5 0: I-
0
-I
I
0
IO Figure 9.
20 30 40 M E T A L L I C ION, FFM.
50
Effect of Mmganere and Ferric Iron on Recovery of Chlorate
recommended procedure described previously. The apparent sodium chlorate content with no iron added was subtracted from that which waa found when iron was added. The difference is the chlorate equivalent of the iron and WBS lotted against the amount of ferric ion added (Figure 2, curve 17. The slope of this curve is 0.328 p.p.m. of sodium chlorate per p.p.m. of iron. This agrees with the theoretical slope of 0.318 within the precision of the data. The effect of manganese was determined in a similar manner. The observed slope is 0.390 while theory predicts a slope of 0.310 for a valence change from 3 to 2 (Figure 2, curve 2). Thus, the valence change for manganese appears to be slightly greater than from 3 to 2. Experience indicates that a constant correction of 0.2 p.p.m. of sodium chlorate can be applied for the manganese content of rayon-grade sodium h droxide. Where the manganese content is unknown, it may {e determined by the method of Williams and Andes (2). Ammonia is known to react with free halogen under certain conditions. Because ammonia is likely to be present in caustic soda which has been purified by the liquid ammonia extraction process, the effect of this impurity waa determined. No detectable error was observed in the recovery of chlorate when 0.04% of ammonia was present. During the course of this work a number of observations indicated that some stopcock lubricants cause low recoveries of chlorates. T o determine whether this apparent error waa indeed real, the magnitude of the possible error was increased by adding a 2-gram portion of lubricant t o the reaction mixture. Cello-grease, the laboratory lubricant in use at the time, caused negative errors of 10 to 20 p.p.m. Petrolatum, commonly sold under the trade-name Vaseline, caused no significant error. Another similar product, Parma, manufactured by Standard Oil Company of N. J., was found to cause no significant error. DowCorning stopcock grease is dso without action. Another lubricant which waa found to be without action waa a paste made by mixing phosphorus pentoxide with phosphoric acid. Any of these lubricants is satisfactory. It is common practice t o take samples of commercial 50% caustic soda in rubber-stoppered bottles. Experimentation showed that added chlorate could not be recovered quantitatively from sodium hydroxide which had been stored in contact with rubber. T o determine the magnitude of this error, a new, size 4, gray rubber stop er was immersed in several successive 100-ml. portions of 5 0 8 sodium hydroxide at the boiling oint for 1-minute intervals. The bloom waa removed from tge surface of the stopper by the first portion of sodium hydroxide. No chlorate was recovered from the 6rst portion when 32 .p.m. were added and errors of -5 and 7 6 p.p.m. were observdon the second and third ortione, respectively. The same stopper was then immersef in a fourth portion of boiling sodium hydroxide for 5
537
minutes. The recovery ot sodium chlorate was 23 p.p rn. ION when 32 p.p.m. were added to this portion. Following this, the stopper was returned to the fourth portion of sodium hydroxide and allowed to stand for 16 hours at room temperature. N o chlorate was recovered from this portion after this treatment. I n another test a bottle of 50% sodium hydroxide was closed with a gray rubber stopper which had been treated with boiling 50% sodium hydroxide for 10 minutes, to remove the bloom. After shaking for 64 hours at room temperature to ensure contact between the sodium hydroxide and the stopper, the chlorate recovery was 6 p.p.m. low. Red rubber stoppers and neoprene stoppers, neither of which are covered with bloom, were treated with boiling sodium hydroxide for 1-minute intervals. Recovery of chlorate added to these or tions of sodium hydroxide was low by as much as 7 p.p.m. $he polystyrene cap of a No-Sol-Vit bottle was treated with boiling sodium hydroxide for 5 minutes; the recovery of chlorate was 2 p.p.m. low. No error was observed in the recovery of chlorate from sodium hydroxide which had been agitated for 64 hours in a No-Sol-Vit bottle. These data show that gray rubber stoppers must not come in contact with sodium hydroxide which is to be used for the determination of Chlorates, even though the stoppers have been treated to remove bloom. Further, it appears desirable to avoid contact between the sodium hydroxide and any type of rubber stopper, The only part of a No-Sol-Vit bottle cap which comes in contact with the sodium hydroxide is the wax liner and the above data show that this liner has no significant reducing action under the conditions of this analysis. The accuracy of the method was determined using 0.05 and 0.25 gram of potassium iodide. The recovery a t each potassium iodide level was calculated by subtracting the apparent sodium chlorate content of the sodium hydroxide, with no sodium chlorate added, from the total sodium chlorate found. This sample blank was 2.8 p.p.m. for 0.05 gram of potassium iodide and 3.4 p.p.m. for 0.25 gram of potassium iodide. The recovery averaged 96% of the amount taken at the lower potassium iodide level. Accurate results may be obtained a t this potassium iodide level by multiplying by the empirical factor, 1.04. Results at the higher potassium iodide level are within the precision of the method (Tables I1 and 111).
Table 11. Rrnyr and Accuracy of Method NaClOa NaClOl NaClOi KI Added Recovered Error Recovery Qram P.p.m. P.p.m. P.p.m. % -0.1 98 0.05 5.0 4.9 -0.4 96 0.05 10.0 9.6 92 -1.6 20.0 18.4 0.05 40.0 60.0 1 .o 2.0 4.0 8.0 16.0 32.0 64.0
0.05
0.05 0.25 0.21 0.26 0.25 0.25 0.25 0.25
Table
38.4 57.9 1.0 2.3 4.2 7.7 16.0 32.7 63.8
Ill.
KI
1 2 3 4 5 6
7 8 9 10 Av.
standarddeviationof group LU of method Reawnt blank 0.0 p.p.m.
0.0 +0.3 +0.2 -0.3 0.0 +0.7 -0.2
96 97 100 115 105 96 100 102 100
Precision of Method 0.05 Gram of
No.
-1.6 -2.1
Toth Found' 7.5 7.8 7.8 8.0 8.0 8.1 8.0 7.8 7.9 8.0 7.90 h0.14 *0.46
0.25 Gram of KI Total Reagent Net found blank found Parla per Million 8.1 9.0 0.9 9.4 1.6 7.8 7.2 9.3 2.1 9.2 7.6 1.6 9.0 2.0 7.0 7.7 9.5 1.8 8.2 9.5 1.3 9.5 8.6 0.9 1.3 7.4 8.7 7.0 1.7 8.7 9.18 1.52 7.66 a0.40 -0.51 a0.30 a1.28 al.64 10.98
538
INDUSTRIAL A N D ENGINEERING CHEMISTRY
The precision of the method was determined by making 10 analyses at each potassium iodide level. No correction was made for the iron on manganese contents of the sodium hydroxide which was used for the determination of the precision. Thus, the results represent the precision of the determination of the total oxidizing power ot the sodium hydroxide, calculated as sodium chlorate. The reagent blank was equivalent to 0.0 p.p.m. of sodium chlorate at the lower potassium iodide level. Thus, the total chlorate found at this level is also the net chlorate found. The limit of uncertainty (LU)was *0.46 p.p.m. and the average analysis, after multiplying by the empirical factor 1.04, was 7.90 p.p.m. Since a sustantial blank titration was expected at the higher potsssium iodide level, a blank was run on the reagents after completing each analysis and the net chlorate was calculated by subtracting the corresponding blank from the sample titration. The limit of uncertainty obtained in this manner was h1.64 p.p.m. and the aversge analysis 7.66 p.p.m. This average analysis, while slightly lower than that which was obtained when using only 0.05 gram of potaasium iodide, agrees with the latter within the precision of the method. The precision at this potassium iodide level was calculated for the total chlorate and
A
Vol. 17, No. 9
for the blank, cm well as for the total minus the blank, or net chlorate. The precision observed for the blank was somewhat poorer than for the total chlorate found, the LU values being b1.28 and *0.98, respectively (Table 111). It is thus evident that the decrease in precision when the larger amount of potassium iodide is used is associated with the higher blank. The use of 0.05 gram (1 ml. of 50-gram-per-liter solution) of potassium iodide is recommended to providc the highcst precision. The slightly low recovery which is obtained under this condition is compensated for by the use of the empirical factor. ACKNOWLEDGMENT
This work was done in cooperation with the Control Laboratory of the American Viscose Corporation in connection with a study of improvements in the quality of rayongrade caustic soda. The method has proved very valuable for analyzing caustic soda shipments to the rayon trade. LITERATURE CITED
(I) Williams, Dwight, IND.ENO.CHEM.,ANAL.ED.,17,533 (1945). (2) Williams, D., and Andea, R. V..Ibid., 17,28-31 (1945).
Colorimetric Method
DWIGHT WILLIAMS AND GEORGE S. HAINES The use of o-tolidine, the conventional reagent for determining free chlorine, has been applied to the determination of small amounb of chlorates in caustic soda. A stable yellow color i s formed in a rtrongly acid solution, permitting the colorimetric determination of as much as 300 micrograms of.aodium chlorate in a 10-gram sample of 50% sodium hydroxide. A single determination requires about 15 minutes, but ten determinations can be made in 80 minutes or less. The limit of uncertainty under the best conditions (LUJ was found to be hO.90 p.p.m. in a sample containing 5.00 p.p.m. of sodium chlorate. Iror) and manganese cause positive errors proportional to their concentration, permitting conectionr for these impurities.
THE
iodometric determination of low concentrations of chlorate in high-purity, rayon-grade caustic soda as described in the preceding paper (16)is only one of a number of reactions which might be utilized for carrying out this analysis. The well-known advantages of colorimetric methods for determining small amounts of impurities indicated the desirability of investigating this type of procedure. One possible colorimetric method involves an adaptation of the ferrous sulfate reduction procedure which wm described in the first paper of this series (14). Either the amount of ferric ion formed by reaction with chlorate, or the excese ferrous ion could be measured. The latter procedure was investigated briefly, using the 0-phenanthroline method for estimation of the excese ferrous ion. Quantitative reduction of chlocates with an amount of iron which could be estimated colorimetrically was obtained at room temperature in 6 N hydrochloric acid. However, since this is a “by-difference” method and the solution must be buffered prior to development of the o-phenanthroline coIor, the procedure was abandoned in favor of a more direct procedure. A brief survey of the literature indicated the paucity of colorimetric methods for the determination of small amounts of chlorate. Siiell (13) describes a method for chlorate based upon the yellow color produced by the action of chlorate on thiocyanate test papers, originally described by Offord (10). Although this method is indicated to be sensitive, the necessity of drying the test papers and the subjective nature of the measurements made it appear unpromising. Hunt (6, 6) reports that very m a l l
mounts of chlorate may be detected by measuring the time required to decolorize a solution of indigo-carmine. The nature of the measurement in this test does not make it seem conducive to good precision. Mellor (9) describes colors obtained when chlorates react with brucine, resorcein (sic), and indigo. Several colorimetric methods have been described which utilize the colors developed when chlorates react with amines. The colored compounds are oxidation products of amines and in feneral amines may be used as reagents for chlorates as well as or other oxidizing agents. Lesnicenko (7) reports !hat the reaction of chlorates with aniline may be made the basis of a sensitive method. The authors obtained a stable blue color with aniline in hydrochloric acid having a concentration of 4.5 N or greater. A spectrophotometric curve of the color showed absorption below 450 and abqve 650 mil,limicrons, but the method did not appear to be sufficiently sensitive for thew application. Ro (11) utilized the color developed by the reaction of. chlorates wit$ pyridine in concentrated sulfuric acid. The necewity for developing the color in concentrated sulfuric acid made the method appear un romising and e mentation showed that the color could not \e developed in)%elute sulfuric acid. Sa (18) reports that chlorate produces colors with phenyl-8-naphthylamine, di-8naphthylamine, and phenyldihydrodibensacridine which will distinguish it from nitrates and nitrites. While these reagents were not available in this laboratoy at the time this work was d y e , they su gested that other amines be tried. Mellor (9) describes colors ottained when chlorates react with diphenylamine and a mixture of aniline and o-tolidine. A solution of m-phenylenediamine in 6 N hydrochloric acid was found to give a faint pink color with 2 micrograms of sodium chlorate per milliliter. A similar solution of benzidine gave a faint yellow color with as little as 0.2 microgram of sodium chlorate per milliliter. It is possible that either of these reagents might be made the basis Of a colorimetric method. &Tolidine, although widely used for the determination of free chlorine, has ap arently never been applied to the quantitative determination ofchlorate. Ellms and Hauser (1, 9 ) report that this reagent is sensitive to 0.005 p.p.m. of free chlorine, that it will react with oxidizing agents in general, and that chloride ion does not interfere. These observations made it appear very attractive for the authors’ application, and experimentation indicated that it was probably the most sensitive of the reagents which were tested. The effect of the hydrochloric acid concentration on the rate of color development and on the stability of the color has been determined. Ellms and Hauser (1,,2) and Forsborg (3) report that iron interferes with the determlnstion of oxidizing agents using o-tolidine. Hopkins ( 4 ) found that manganese interferes and Mellan (8) reported that copper reacts with otolidine. The interference of iron and manganese was found to be pro ortional to the amount present and copper did not interfere un&r the authors’ conditions.
