Rapid Estimation of Chlorate Ion Employing Catalysis A. J. BOYLE', V. V. HUGHEY, AND CLYDE C. CAST0 Las Vegas, Nev.
Technical Service Laboratories, Basic Magnesium, Incorporated,
A method for estimating chlorate in cell liquor which i s produced during the manufacture of chlorine consists of reducing the chlorate ion in 40% hydrochloric acid b y volume with a standard ferrous ammonium sulfate solution. A few drops of a 10% solution of ammonium molybdete are employed as catalyst. The excess standard ferrous ammonium sulfate is then titrated with standard potassium dichromate, using diphenylamine sulfonate as the redox indicator.
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EVERAL methods for the determination of chlorate ion are described in the literature. Bacho (I) reduced the chlorate ion with excess sodium arsenite in hydrochloric acid solution, and then used the potassium bromate method of Gyory (4) in determining the excess arsenite. Peters and Deutschlander (6) employed an arsenite-bromide mixture in a strong hydrochloric acid solution. Osmium tetroxide was suggested as a catalyst by Gleu (3). Bolge and Troberg (9) reduced the chlorate ion with excess cuprous chloride a t 80" C. and finally titrated the excess with standard potassium dichromate. Harvey (6) used a ferrous sulfate-potassium iodide system, titrating the liberated iodine with standard sodium thiosulfate. Essentially two methods are employed a t Basic Magnesium, Incorporated, for the determination of sodium chlorate in Hooker cell liquor from the chlorine plant. Method 11, described in this paper, is a modified Bacho procedure, which differs from the original method in that refluxing of the sodium arsenitesodium chlorate mixture is omitted; iodine monochloride is employed as a catalyst in the titration of the excess sodium arsenite by potassium bromate, thus sharpening the end point and making the titration possible a t lower than boiling temperatures. Smith ( 7 ) advocates use of this catalyst in the potentiometric estimation of arsenite by potassium bromate. A newer procedure for the estimation of chlorate ion, Method I, is being used currently in this laboratory. It consists of reducing the chlorate m-ith an excess of ferrous ammonium sulfate in a strong hydrochloric acid solution, using ammonium molybdate as a catalyst. The excess ferrous ammonium sulfate IS titrated with potassium dichromate, using diphenylamine s d fonate as the internal redox indicator. Method 1 is preferred because of its high degree of accuracy and precision. It is less sensitive to variable conditions than the sodium arsenite-potassium bromate procedure, in which arsenic is lost if sufficient care is not exercised. METHOD I
REAGENTS.Ferrous ammonium sulfate, c.P. (0.25 N ) . Potassium dichromate, A.R. (0.1 N ) . Ammonium molybdate, C.P. (1Oyo solution). Sodium acetate-phosphoric acid buffer. Add 250 ml. of concentrated phosphoric acid, c.P.,to 1 liter of 4 molar sodium acetate, C.P. Diphenylamine sulfonate indicator. Dissolve 0.30 gram of the barium salt of diphenylamine sulfonic acid in 100 ml. of water, add 0.5 gram of sodium sulfate, and filter off the precipitate of barium sulfate. Concentrated hydrochloric acid, C.P. Hydrochloric acid d u t i o n (1 N ) . Phenolphthalein indicator ( 1 W solution). PROCEDURE. Pipet a 10-ml. sample of cell liquor into a 500-ml. Erlenmeyer flask, add 2 drops of phenolphthalein indicator, and t.itrate with 1 N hydrochloric acid. An estimation of total alkalinity sufficiently accurate for cell liquor chemical control may be made a t this point. .4dd 10 ml. of 0.25 N ferrous ammonium sulfate solution, 3 drops of ammonium molybdate mtalynt, and 40 ml. of concentrated hydrochloric acid. -4110~ __-
. Present address, Wayne University. College of Medicine, Detroit, 3lich.
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the mixture to stand 1 minute for complete reaction, then atid 20 ml. of phosphoric acid-sodium acetate b d e r reagent. Dilute to 200 ml. with distilled water, add 3 drops of diphenylamine sulfonate redox indicator, and titrate with 0.1 N potassium dichromate to the purple end point. A correction of 0.05 mI. of dichromate is made for each 6 drops of indicator solution (8). METHOD It
REAGENTS. Sodium arsenite, A.R. (0.1 N solution). I'otassium bromate, C.P. (0.1 N solution). Iodine monochloride. Dissolve 0.279 gram of pure potassium iodide and 0.178 gram of pure potassium iodate in 250 ml. of water. Add a t one time 250 ml. of concentrated hydrochloric acid (sp. gr. 1.19). The resulting solution is 0.005 M in iodine monochloride. Concentrated hydrochloric acid, C.P. Phenolphthalein indicator (lYO solution). Methyl orange indicator (0.1% solution). PROCEDURE. Pipet a 10-ml. sample of cell liquor into a 500-ml. Erlenmeyer flask, add 1 or 2 drops of phenolphthalein, and titrate the sample with 1 N hydrochloric acid. An estimation of total alhlinity sufficiently accurate for cell liquor chemi(sal control may be made a t this point. Add 30 ml. of standard arsenite solution and 20 ml. of concentrated hydrochloric acid, and dilute to 100 ml. with distilled water. Cover the flask with a small watch glass and bring the sample to a gentle boil on a hot plate, removing it after 8 to 10 minutes. \ h i l e the sample 1s still warm (40" to 70" C.), rinse down the watch glass, add 5 ml. of iodine monochloride and 5 drops of methyl orange indicator, and titrate the excess sodium arsenite with 0.1 potassium bromate. During the titration, the red color of methyl orange gradually fades to a yellow, until just a few drops before the end point a bright pink color develops. The further addition of potassium bromate will completely destroy the indicator which is considered to be the end point.
Table
I. Effect of Catalyst and A c i d Concentration on Reduction of Chlorate Ion
Concentration of HCI by Volume
(Reaction time, 1 minute) Chlorate Reduced, Chlorate Reduced, Catalyst Present Catalyst Abaent
%
%
%
18 31 36 40 50
34.99 91.22 98.40 99.70 99.70
32.90 82.45 94.61 98.80 98.80
DISCUSSION
Table I illustrates the importance of acid concentration in the determination of chlorate ion, using an excess of standard ferrous ammonium sulfate with and without ammonium molybdate as a catalyst. The standard 0.1 N solution of sodium chlorate was prepared from Merck reagent quality sodium chlorate crystals; 10 ml. of this solution were employed. The ferrous ammonium sulfate solution was standardized against the standard potassium dichromate solution in both the presence and the absence of the catalyst. The results indicated no interference by the molybdate. Table I1 gives a comparison of Methods I and 11. The values, represent the sodium chlorate content of the samples expressed as milliliters of 0.1 N solution. In general, the methods are in close agreement, accounting for about 99% of the chlorate ion present. In routine analyses for control purposes this accuracy is entirely adequate. Hypochlorite in cell liquor is not considered in this discussion since it is present only in microquantities. The hot alkaline liquor of the Hooker cell promotes the formation of chlorate ion.
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ANALYTICAL EDITION
June, 1944
Table 11.
Comparison of Methods
>aiiiple No.
I and I1 on Hooker Cell Liquor
Method I
J I e t h o d I1
2.82, 2.84 6.45, 6 . 4 1 16.77, 16.84
2 77, 2 7 1 6 32, 6 22 16 77, 10 84
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of thc amount of chlorate present. I t permits greater precision than the sodium arsenite-potassium bromat,e method. The sample of cell liquor for the sodium chlorate estimation may also serve for a total alkalinity deterniination if standard hydrochloric acid is used in the initial neutralization process.. LITERATURE CITED
SUMMARY
;liter corisiderahle iuvestigation, it is helieved that the most r:ipi(l, Liccurate method for the determination of sodium chlorate i n (*ellliquor produced in chlorine manufacture is t,he reduction of 11ic ctiloratc with excess ferrous mimonium sulfate in 40% Ii~-ti~~ochloric acid hy volume. Ammonium molylidate is used : j i t l i c c:ir:ily,
