lodometric Determination of Iodates, Bromates, or Permanganates in the Presence of Copper Determination of Copper in the Presence of Oxidizing Agents DAVID N. HUME AND 1. M. KOLTHOFF, School of Chemistry, University of Minnesota, Minnedpolir, Minn. of sodium citrate and then just 1 ml. in excess. It is usually advantageous to add a little more starch a t the end point. Although the citric acid present tends to sharpen the end point somewhat, the addition of 3 grams of solid potassium thiocyanate just before the end of the titration is strongly to be recommended.
K
APUR and Verma ( 1 ) have described a method for the iodometric determination of iodate in the presence of copper, based upon the formation of an unreactive copper pyrophosphate complex. The method has the disadvantage that, a t the high pH necessary for the formation of the complex, the reaction between iodate and iodide ions is rather slow. Swift and Lee (3) have recently shown that iodate, bromate, or permanganate may be determined in the presence of copper by titration to the iodine chloride end point with standard potassium iodide. The copper present can be determined indirectly by comparing the first result with the total oxidizing power as found by thiosulfate titration of all the iodine liberated from excess iodide in acid solution. The authors have developed methods by which all the above constituents can be determined, using only a single standard solution (sodium thiosulfate) and the familiar starch-iodine end point. A mixture of the strong oxidant and cupric salt is treated with excess iodide and acid until the reaction is complete. Sodium citrate is then added, forming a stable complex with copper ions, and the copper-iodide reaction reverses quantitatively. The iodine equivalent to the strong oxidant is titrated in the usual manner. After the end point, excess mineral acid may be added, decomposing the copper citrate complex and quantitatively liberating iodine equivalent to the copper. This iodine is titrated, giving a direct determination of both constituents in a single sample and with but one standard solution.
Typical results taken from 15 determinations are shown in Table I. Swift and Lee have shown that the sum of copper and iodate may be determined very accurately by iodometric titration. This may be used as an alternative method for the indirect determination of copper. -
0.1 iV KIO: Ml. 25.00 25.00 10.00 10.00
... ...
... ...
1.
Titration of Iodine Liberated Equivalent to (or Bromate) and Copper, in Mixtures" NaaSpOa 0.1 N NsiSZOa 0.1 N CuSO4 Required KBrOa Required Error Taken for Cu MZ. M2. % MI. ML. ... 29.32 5.00 +0.03 4.88 29.32 5.00 +0.03 4.87 ... 24.31 11.69 25.00 ... -0.26 24.31 11.70 25.00 -0.17 25.28 5.00 -0.04 2i:oo 5.08 25.29 5.00 0.00 25.00 5.09 25.27 25.00 25.00 -0.08 25.49 25.31 25.00 25.00 25.47 +0.08
Table
Iodate
ErPor
% 0.00
+0.2 +0.04 +0.04 -0.2 0.0
+0.08
0.00 Different thiosulfate solutions were used in first and last four erperimenta. 0
BROMATE AND COPPER IN ADMIXTURE
EXPERIMENTAL
The procedure for bromate is the same as for the determination of iodate except that 6 N hydrochloric acid is used instead of acetic acid. It is necessary to add 2 ml. for each 30 ml. of the original sample in order to obtain accurate results. Copper is determined as before; however, the hydrochloric acid added for the bromate reaction must be subtracted from the amount to be added to decompose the copper complex. Typical results are shown in Table I. It was found that in a phthalate buffer of pH 5 , copper reacts quantitatively with iodide without interference from bromate present. This permits a very simple determination of copper in the presence of bromate. To 20 to 50 ml. of the neutral sample solution, add 20 ml. of a 0.1 M pH 5 phthalate buffer, and 3 grams of potassium iodide. Titrate with sodium thiosulfate to a starch end point, adding 3 grams of potassium thiocyanate just before the color change. If free acid is present in the original solution, remove it by dropwise addition of ammonia until precipitation of copper hydroxide just begins.
Preliminary experiments showed that the cupric-iodide reaction was quantitatively reversed by excess of neutral oxalates, tartrates, and citrates. A slightly soluble complex cupric salt separates from oxalate solutions. Neither oxalate nor tartrate gave clean-cut reversal of the copper-iodide reaction on the addition of acid. Citrate was found to be the most satisfactory reagent. Approximately 0.1 N stock solutions of potassium iodate, potassium permanganate, potassium bromate, copper sulfate, and sodium thiosulfate were used in the investigation. Titrations were performed by adding nearly the equivalent volume of reactant by pipet and the last milliliter or two from a microburet. This technique permitted estimation of the titration volume to within about 0.002 to 0.003 ml. The procedures which follow were adopted after much experimentation. IODATE AND COPPER IN ADMIXTURE
To 20 to 50 ml. of a solution DETERMINATION OF IODATE. containing 0.5 to 3 milliequivalents of each constituent add 2 ml. of 6 N acetic acid and 6 grams of potassium iodide. Let stand 3 minutes, add 20 ml. of 1.0 M sodium citrate, and swirl until the solution becomes clear. This quantity of citrate is sufficient for 3 millimoles of copper. If the solution does not clear completely of cu rous iodide, more must be added. If more than 1 milliequivsnt of mineral acid is present in the original solution, it should be neutralized to incipient precipitation of copper hydroxide, and acetic acid added as in the regular procedure. The citrate should be tested, as some samples were found to use up iodine. Dilute to 200 ml. and titrate with 0.1 N sodium thiosulfate, adding starch at the end point. The color change is a sharp, easily seen transition from a deep murky blue to a clear light blue. DETERMINATION OF COPPER. T o the solution from the above titration, add 12 ml. of 8.0 N sulfuric or hydrochloric acid, wait 2 minutes, and titrate with thiosulfate. Insufficient acid results in an incomplete reaction. Too large an excess causes serious air-oxidation of iodide (catalyzed by copper). Good results are obtained on adding 1 ml. of 6.0 N acid for every 2 ml.
The procedure is accurate-for example, three samples of 20 ml. of approximately 0.1 N copper sulfate in the presence of 10 ml. of 0.2 N potassium bromate gave titration values of 19.01, 19.00, and 19.00 ml., the value being 19.00 in the absence of bromate. The authors have verified the observation of Swift and Lee that the sum of bromate and copper may accurately be determined by iodometric titration in acid medium. PERMANGANATE AND COPPER IN ADMIXTURE
The same procedures may be used as in the iodate and copper determinations, except that the original solution is acidified with sulfuric acid and the presence of this acid must be taken into account in the copper determination. The results are, however, 0.3 to 0.5% low for permanganate and correspondingly high for copper-for example, two mixtures of 20 ml. of 0.1 N permanganate and 5 ml. of 0.1 N ropper gave
103
INDUSTRIAL AND ENGINEERING CHEMISTRY
104
titration values of 20.79 and 20.76 ml. (20.84 theoretical) for the permanganate and 4.93 and 4.93 ml. (4.86 theoretical) for the copper, respectively. The sum of the two, determined as above or directly, is in good agreement with the theoretical. The authors have investigated the source of the error and found that, while neither manganous nor citrate ions affect the titer of an iodine solution, the two together cause a noticeable decrease. A small amount of the manganous citrate complex is evidently oxidized by iodine to the manganic complex, which is unstable in the presence of mineral acids. Attempts a t substantially increasing the accuracy of the method by variation of the conditions were unsuccessful. Therefore, the authors recommend that the sum of the two oxidants be determined (8) and the copper estimated in a separate sample. This is easily accomplished if the permanganate is first reduced in acid medium by dropwise addition of saturated ferrous ammonium sulfate solution and any excess of the latter is removed by boiling with a little bromine water. Copper is then determined by the method of Park (2),in which interference from iron is prevented by the
Vol. 16, No. 2
addition of fluoride. The accuracy of this procedure is indicated by the following results: Copper was determined in mixtures of 20 ml. of 0.1 N copper sulfate and 20 ml. of 0.1 N potassium permanganate; the titration values were 20.17, 20.19, and 20.16 ml. of thiosulfate, the theoretical being 20.19, indicating an average error of less than 0.1%. For a direct, accurate determination of small amounts of permanganate in the presence of copper, the method of Swift and Lee is probably the most convenient. ACKNOWLEDGMENT
The authors wish to express their appreciation to the Graduate School of the University of Minnesota for financial assistance which made this investigation possible. LITERATURE CITED
(1) Xapur, R. L., and Verma, M. R., I N D . ENQ.C H I M .ANAL. , ED., 13, 338 (1941). (2) Park, B., Ibid., 3, 77 (1931). (3) Swift, E. H., and Lee, T.L., Ibid., 14, 488 (1942).
Cupriethylene Diamine as a Solvent for Precise Determination
OF
Cellulose Viscosity
R. S. HATCH, Weyerhaeuser Timber Co., A modiRcation of the tentative standard A.C.S. method for the precise determination of cellulose viscosity has been developed, using a new solvent-cupriethylene
A
diamine.
TENTATIVE standard method for determining the viscosity of cellulose in cuprammonium hydroxide was presented by the Committee on the Viscosity of Cellulose, Division of Cellulose Chemistry, at the 74th Meeting of the AMERICANCHEMICAL SOCIETY, September, 1927. This was to a large extent a composite of methods which were in daily use by large manufacturers of cellulose derivatives and was based upon the wealth of experience gained in the research and control laboratories of these organizations. The present paper has to do with a method involving a new solvent and a different method of manipulation, which is offered as a rapid and precise method of measuring cellulose viscosity. That cuprammonium hydroxide is by no means the ideal dispersing agent for the determination of cellulose viscosity is evidenced by the many proposals in the literature for the use of other means. A solution of cupriethylene diamine has been suggested. This solvent is easily prepared and adjusted to constant composition, it may be stored under proper conditions a t room tempers, ture, and it shows little evidence of spontaneous decomposition over long periods of time. When cellulose is dispersed in cupriethylene diamine solvent, any considerable amount of atmospheric oxygen during the preparation of the dispersion must be avoided, but these dispersions are much less sensitive to the effects of atmospheric oxygen than are dispersions of cellulose in cuprammonium solvent. When cuprammonium is used as a solvent, it is necessary to use extreme caution to prevent the degrading effects of even minute amounts of oxygen while bringing about cellulose dispersion. The tentative A.C.S. method states that hydrogen or nitrogen must be especially purified to free it from traces of oxygen. I n dispersions of cellulose in cupriethylene diamine solvent, ordinary commercial nitrogen containing about 0,5y0 oxygen may be used with no appreciable effect, provided dispersion is brought about with reasonable rapidity. Furthermore, a cupri-
Pulp Division, Longview, Wash.
ethylene diamine solvent 0.5 molar in copper concentration is a much more efficient dispersing agent than any of the standard cuprarnmonium hydroxide solvents now in use. Straws and Levy (0were the first to describe the successful use of cupriethylene diamine solutions &s a solvent in the determination of cellulose viscosity. They pointed out the desirability of using 0.5 molar copper concentrations and gave clear directions for preparing the solvent. As a result;of experience extending over nearly two years, several modifications and refinements of their method have been worked out and standardized. The evolution of the method for rapid viscosity determination has been covered by the author (1). This paper describes more fully the method used for precise viscosity determinations where a high degree of precision is required and gives details of the present rapid method used as mill control. Most of the work done in the author's research laboratory has to do with wood cellulose, the viscosity of which, a t 1% concentration, is such that the time of fall of the l/ls-inch aluminum sphere in the standard viscometer is between 5 and 30 seconds. Under these conditions there will be no appreciable change in temperature during the time required for the ball to pass betwcen the two etched marks and it is only necessary to determine the temperature immediately after taking the time of fall of the sphere and apply the temperature correction factor. I n dealing with high-viscosity pulps where the time of fall of the standard sphere would exceed 30 seconds, it is desirable to bring the contents of the viscometer to 25' C. by means of a constant-temperature water jacket or to run viscosities a t lower cellulose concentration. The author hesitates to apply the Farrow and Neale equations for converting viscosities of solutions a t lower concentration to the standard 1% concentration because these equations apply only over very narrow limits of concentration. PRECISE M E T H O D
An unpressed sheet of pulp is air-dried to the point where it is in moisture equilibrium with the air of the balance room in
