A Procedure for Volumetric Determination of Gold By Means of Potassium Iodide and Arsenious Acid VICTOR E. HEKSCHLAG. 2013 Bryant Ave., New- York, 1v. Y.
S
other reducing agents. One might therefore expect interference from such elements in both Gooch and Morley's and Lenher's methods. Where potassium iodide is used, however, error from this cause may be reduced or eliminated in some cases by avoiding more than a slight excess of iodide and by keeping the acidity low. Rupp ( 7 ) precipitated metallic gold from an auric chloride solution with an excess of standard arsenious acid.
EVERAL volumetric niethods for gold have been described in the literature, none of which is as rapid and convenient as the one here presented. A11 involve a quantit,ative reduction from the auric state either to the aurous or to metallic gold, and all hut one require the absence of nit'ric aci 1. l i t r i c acid is removed in the manner recommended by Gooch and Morley ( 2 ) . The solution is evaporated several times with hydrochloric acid saturated with chlorine. The latter prevents partial decomposition to aurous chloride. The free chlorine is removed by neutralizing the acid with ammonium hydroxide, boiling gently, acidifying slightly with hydrochloric acid, heating to dissolve the fulminating gold precipitate formed with ammonium hydroxide, and again adding ammonium hydroxide and acidifying. The permanganate titration described by Juptner (3) and French (1) involves precipitation of metallic gold from an auric chloride solution free from nitric acid viith an excess of standard ferrous sulfate or othpr reducing agent. AuC13
+ 3FeSO4 +Au + Fe2(S04)3 + E'eC13
+ 4iiuc13 + 6H20+3Asz06+ 12HC1 + 4Au (3)
3As203
In this step the solution is heated until the colloidal gold disappears and the gold precipitate settles. The free acid is then neutralized with sodium bicarbonate, and the excess arsenious acid is titrated with standard iodine, using starch indicator. For microdeterminations, SzebellAdy and Vicziin (9) titrated auric chloride solutions, t o which sodium bromide was added, directly with arsenious acid. -4 gelatin solution prevented partial reduction to metallic gold. The titration was performed very slowly, with an interval of 15 t o 20 seconds following each drop, and the temperature was kept at 90" to 95' C. Pollard's method (6) applies to gold directly in the diluted aqua regia solution. In principle, it involves a titration with arsenious acid, whereby auric gold is reduced to the metallic state. Reduction from the auric to the aurous state is catalyzed by potassium iodide and mercuric chloride. Potassium bromide accelerates reduction from the aurous state to metallic gold, and in addition, it forms complex bromoauric acid, the color of which is removed by titration with arsenious acid. The procedure requires consideration of the following reactions in aqui regia: Concentrated hydrochloric and nitric acids form chlorine and nitrosyl chloride, slowly a t room temperature, mnre rapidly when heated.
(1)
Digestion on a steam bath accompanies this step, and the excess reducing agent is then titrated with potassium permanganate. The method is not applicable for gold in the presence of organic matter oxidized by permanganate. Peterson (5) suggested a method based on the reaction of auric chloride and potassium iodide whereby aurous iodide and free iodine are formed.
+
AuCI~ 3KI
+AuI
+ + 3KC1 12
(2)
In titrating the liberated iodine with sodium thiosulfate, however, he obtained results which were not satisfactory. Gooch and Morley ( E ) developed conditions whereby good results could be obtained on very small amounts of gold. According to their procedure, the weakly acid auric chloride solution is treated with a slight excess of potassium iodide, sufficient to precipitate aurous iodide and then redissolve it as the iodide complex. The liberated iodine is titrated to colorless with sodium thiosulfate, and a back-titration to a faint color is performed with iodine, adding starch indicator if desired. Dilute standard solutions, 0.01 W or 0.001 X , for example, are used. When using such dilute iodine solutions, a small correction is made, equivalent to that amount of the iodine which may be added to a blank containing starch before a color is produced. Lenher (4) investigated the use of sulfurous acid in the titration of auric chloride solutions. I t causes reduction of the yellow auric chloride t o colorless aurous chloride. Various salts prevent partial reduction to metallic gold during titration. Magnesium chloride or sodium chloride intensifies the yellow color of auric chloride. Potassium iodide or potassium bromide may also be added to produce a strong color. A wide range in the amount of free hydrochloric acid is permissible. Small or large amounts of gold may be determined in this way, but the sulfurous acid requires frequent restandardization. Rupp ( 7 ) held that the difficulty in applying Gooch and Morley's method for higher concentrations of gold mas caused by the instability of aurous iodide, which slo~vlydis ,ociated into metallic gold and iodine. Lenher's method, when potassium iodide is added to the auric chloride solution, is similar t o that of Gooch and Morley, but sulfurous acid is used instead of sodium thiosulfate. Lenher, however, obtained accurate results in determinations ranging from 0.0395 to 0.7301 gram of gold in a volume of 100 ml. Furthermore, he did not find a back-titration with iodine necessary. His results indicate that the main error is caused not by the instability of aurous iodide, but rather by a side reaction involving the aurous iodide complex and thiosulfate. Like auric gold, copper, iron, and platinum metals in their higher valence states also react with potassium iodide and with
3HC1
+ HK03 +C1z + NOCl + 2Hz0
(4)
When an aqua regia solution is diluted with water, nitrosyl chloride and chlorine recombine to form hydrochloric acid and nitric acid. YOC1
+ C12 + 2HzO
3HC1
+
"03
(5)
Before diluting there is always an excess of nitrosyl chloride, and after diluting the excess reacts with water to form nitrous acid and hydrochloric acid. NOC1
+ HzO +HXOz + HC1
(6)
When gold dissolves in aqua regia, it goes entirely into the auric state.
AU
+ 4HC1 + HNOa --+
HAuClp
+ 2H20 + NO
(7)
After solution of the sample, Pollard's method requires removal of the dissolved gases by bubbling air through the liquid as the latter is permitted to cool. This is necessary to avoid serious error from partial reduction to aurous chloride on diluting, because of the excess nitrosyl chloride originally present in the concentrated acid. A solution containing urea and potassium bromide is then added. A trace of nitrous acid, which catalytically starts the reaction between nitric acid and hydrochloric acid, is removed by the urea, and resolution of precipitated gold is thereby prevented. The mixture is heated to about 40" C. and titration is performed with a solution containing arsenious oxide, mercuric chloride, a small amount of potassium iodide, and some free hydrochloric acid. Auric gold oxidizes iodide as the latter enters and reappears in the solution. Iodine is thereby liberated, only to be reduced back to iodide by the arsenious acid. Ordinarily, in acid solution an equilibrium exists between arsenious acid and free iodine. AS203
561
+ 212 + 2H20 S
As205
+ 4HI
(8)
562
INDUSTRIAL AND ENGINEERING CHEMISTRY
In the presence of excess mercuric chloride, however, the iodide ion concentration is kept very low because of formation of the complex, HgC12.HgI2. This permits the reaction to go quantitatively to the right. At the same time, the concentration of iodide is too low to reduce such substances as copper, iron, and platinum from their higher valence states. Interference on account of reduction is removed in the case of all metals except thallium, palladium, and iridium. Reduction of gold to the metallic state causes formation of a precipitate which tends to make the solution opaque. This difficulty is overcome by passing air through the cold solution for a few minutes and then allowing the precipitatp to settle. When the broxn color of bromoauric acid disappears, a small amount of gold remains in solution. This is determined by adding indicator containing sodium fluoride and o-tolidine or o-dianisidine, and titrating with hydroquinone solution. By forming a colorless complex, the sodium fluoride prevents an interfering color with any iron that may be present. Ruthenium interferes in Pollard's method by forming a strongly colored solution which makes it difficult or impossiblp to find the end point. JThile this procedure is rapid and accurate, it would be somewhat more convenient if met'allic gold did not precipitate and tend to make the solution opaque during titration. It would be preferable, also, if the titration could be performed with a single standard solution instead of with two. The iodometric method given below has advantages in these respects. The reliability of the procedure has been established by a number of experiments in which good results were obtained under widely varying conditions.
Preparation of Standard Arsenite The arsenite may be prepared according to the directions given by Scott (8).
If 0.01 N solution is desired, dissolve 0.495 gram of pure arsenious oxide in a little 20 per cent sodium hydroxide. Neutralize the excess alkali m-ith dilute sulfuric acid, using phenolphthalein indicator, until the solution is just decolorized. Add 500 ml. of distilled water containing about 25 grams of sodium bicarbonate. If a pink color develops, remove it with a few drops of weak sulfuric acid. Dilute the solution to 1000 ml. and standardize it against a weighed amount of pure gold. The arsenite retains its strength practically indefinitely. Recommended Procedure If the gold content of a solution is to be determined, take with a pipet a sample containing about 40 mg. of gold. (Assume this to be 20 ml. or less.) In a fume hood add 15 ml. of concentrated hydrochloric acid and 5 ml. of concentrated nitric acid, and heat briefly. Remove flame and add 25 ml. of 5 per cent sodium hypochlorite. This will be accompanied by a mild effervescence of chlorine gas. Add 35 ml. of distilled water and boil gently for 10 minutes. Cool, and neutralize the acid with a saturated solution of sodium carbonate or with a solution of sodium hydroxide, testing by touching the stirring rod t o the edge of litmus paper. Adjust to slightly acid with dilute hydrochloric acid. Add a saturated solution of sodium bicarbonate until the gold solution reacts blue to litmus. Dissolve a fekv grams of potassium iodide in some distilled water and add the iodide rapidly, with stirring, to the gold solution. Add a few grams of sodium bicarbonate to ensure the presence of a sufficient excess, and titrate the liberated iodine with standard 0.01 A' arsenite. Add a few milliliters of starch indicator solution when the iodine color becomes faint. The end point is the complete disappearance of the starch-iodine color. The same procedure is applied to a solid, such as a gold alloy, the accurately weighed out sample of which is heated in the aqua regia until it dissolves. Experimental The standard solutions were prepared in the manner previously described by dissolving 5.0063 grams of arsenic trioxide and making the solution to volume, 1000 ml. The normality, 0.1012 N , was checked by standardization against a weighed amount of pure iodine. By proper dilution of the more concentrated solution, 0.01012 N arsenite was prepared. On the basis of Equations 2 and 8, the latter equation, of course, going completely t o the right, 1.000 ml. of the 0.01012 A; arsenite was theoretically equivalent to 0.998 mg. of gold.
Vol. 13, No. 8
The samples in experiments I, 11, V, VI, and VI11 were pure gold. The sample in experiment I11 was taken with a 1.00-ml. pipet from a solution of potassium aurocyanide, KAu(CN)*, known to contain 20.3 mg. of gold per ml. Since phosphate is often present in industrial gold-plating solutions, pure gold was taken in experiment IV and 0.3 ml. of phosphoric acid was added to see if the latter Rould interfere. To the pure gold sample of experiment VI1 were added 10 mg. each of copper, iron, silver nitrate. and nickel to test for interference.
TABLE I. RESCLTSWHICHILLUSTRATE ACCURACY OF PROCEDURE Expt.
I
I1
111 1V
v vI
VI1 VI11
Gold Taken
Arsenite
M g 18.9 19.4 20.3 22.9 34.7 44.2 233.7 410.2
Mi.
.
18.81 19.19 20.33 22.88 34.82 44.25 23.38 41.00
(0.01 N ) (0.01 3) ( 0 . 0 1 Iv) (0.01 N ) (0.01 A') (0.01 10.1 .W ( 0 . 1 ,V)
Gold Found
MQ. 18.77 19,lj 20.29 22.84 34.75 44.18 233.4 409.2
Deviation Mg.
-0.1 -0.2 0.0 -0.1 +o. 1 0.0 -0.3 -1.0
Discussion of Procedure Instead of bubbling air through the aqua regia solution as in Pollard's method, the reducing gas formed according to Equation 4 is more conveniently removed by introducing excess chlorine, then diluting and boiling out the excess. addition of sodium hypochlorite solution or potassium chlorate causes the desired liberation of chlorine. SaOCl KC103
+ 2HC1- KaC1 + H 2 0 + C12 + 6HCl +KC1 + 3HiO + 3C1,
(9) (10)
Effervescence attending the use of potassium chlorate is excessively rapid in warm, concentrated acid and mild in acid to which an equal volume of water has been added. If potassium chlorate is used instead of sodium hypochlorite, the recommended procedure is altered as follows: Add 1.0 gram of the material, heat the solution briefly, allow it t o stand about 5 minutes while effervescing, dilute with 60 ml. of water, boil gently 10 minutes, etc. The excess chlorine is removed readily from the diluted solution by gentle boiling. Seutrnlization of the acid does not cause precipitation of gold as a hydroxide or a carbonate. Upon addition of potassium iodide to the bicarbonate solution, the gold is reduced from the auric to the aurous state and iodine is liberated simultaneously according to Equation 2 . The aurous gold remains in solution as the iodide complex. The solution of auric gold treated with sodium bicarbonate is capable of oxidizing free iodine. This fact was discovered when a small amount of iodine solution or potassium iodide was added and the expected color with starch failed to appear. If potassium iodide solution is added rapidly, in large excess, and with stirring, no appreciable error occurs. The titration of liberated iodine with sodium arsenite is represented by Equation 8, which goes quantitatively to the right in bicarbonate solution. The reaction is rapid, the end point is sharp, and starch may be used or omitted, as preferred. There is no tendency for excess arsenite to precipitate metallic gold. Copper, iron, and other base metals when present do not interfere by reacting with potassium iodide. I n the bicarbonate solution they form basic carbonate precipitates. Those which are colored tend to interfere by making it less easy to determine the end point. Platinum and palladium do not precipitate and do not cause liberation of iodine, but they form strongly colored iodide complexes which make the end point difficult to determine. The color, similar to that of free iodine in potassium iodide solution, is removed by arsenite only when a large excess is added.
August 15, 1941
563
ANALYTICAL EDITION Summary
Literature Cited
Gold in a n aqua regia solution is determined very conveniently by adding sodium hypochlorite or potassium &lorate, diluting, and boiling out the free chlorine formed with these substances. The acid is then neutralized with a n excess of sodium bicarbonate, potassium iodide is added, and the liberated iodine is titrated with standard sodium arsenite, using starch if desired.
(1) French, H., M i n i w Enc. Wmld, 37, 853-5 (1912). (2) Gooch, F. A., and Morley. Ii. H., Am. J . Sci., (4)8, 261-6 (1899). (3) jkpher, H. p. oesterr. 2. B ~ ., ~ .yiittenW., ~ . 1880, 182-3. (4) Lenher, Y., J . Am. chem. SOC., 35, 733-6 (191, ( 5 ) Peterson. H., 2. ano~c.Chem.. 19.59-66 (1899 w. B., BuLz.Insf. Met. (La 331, pp. 23-5 (1932). (7) Rupp, E., Der.,35, 2011-15 (1902). (8) Scott, W. W., "Standard Methods of Chemioal Analysis", 5th ed., Vol. 1, pp. 453, 1203, New York, D. Van Nostrand Co.. 1939. (9) Su&&dy, L., and vioaiin,B., Oesferr. Ghem..zfo., 41, 431-5 (1938).
Acknowledgment Appreciation is expressed for C. B. F. Young's aid in providing materials necessary for the work.
An Improved Tangentimeter HOWARD P. SIMONS D c y a r t m e n t of Chemical Engineering, West Virginia University, Morgantown, W.
I
N PERFORMING calculations with data from drying
and filtration experiments, computing partial molal heat contents, partial heats of solution, and the like, it is frequently necessary to differentiate a function represented by a curve for which a n equation is not easily obhinable, in order to obtain rates or instantaneous values a t any desired point. Such differentiations are most conveniently performed by one of the well-known graphical methods, especially that involving the construction of a tangent to the curve and subsequently determining the slope of the tangent. It has heen recognized that the accurate location of tangent lines is attended with some little difficulty, particularly where the tangent is t o be constructed to a line of small curvature, and that the errors resulting from such inaccurate construction may often be of considerable magnitude.
Va.
possibility of confusion even when a large number of tangents are to be determined on a small portion of curve, and eliminating the necessity of defacing the curve that is being differentiated. Moreover, the instrument may be constructed very simply and inexpensively, is rugged, and may be made to any desired size, so t h a t tangent intercepts may be read accurately and quickly from curves of large dimensions. ,. .,
..,
Various meehaniral aids to the accurate construction of tangent lines have been used from time t o time. Latishaw ( 1 ) described n simple tsnaentimeter in which a piece of speculum metal at-
ing in of a h g e n&b& of tangent lines, as is freduently required, may often result in sohe little confusion, as well as defacing the curve. In 1930, Richards and Roope (8) described a tangent meter Jrhich made use of il prism mounted an a. vernier protrxtor scale, and this instrument was introduced commercially. Such an instrument, while accurate, made necessary the use of a factor to correct the observed tangent t o the curve scale. In addition, it m m somewhat expensive. The tangentimeter here described is a modification of that described by Latishaw, being capable of a similar degree of accuracy. It possesses the additional advantages of not,requiring the construction of any lines, thereby eliminating.tlic
FIGURE2. TANGENTIMETER IN USE
