V O L U M E 23, N O . 12, D E C E M B E R 1 9 5 1
1863
Table 11. Reproducibility of Copper and Zinc Determinations (In five portions of corn tissue from plots with and without minor element fertilizer, recovery of added amounts of copper and zinc, and noninterference of nickel, cobalt, and antimony) Copper Determination Zinc Determination Flask Tissue Element Added to Flask I n dry Deviation I n dry Deviation KO. Weight Cu Zn Ni Co Sb Total Net6 tissue from mean Total h’etb tissue from mean Grams y Y Y Y Y Y ^I P.p.m. % Y Y P.p.m. % 1
2 3 4 5 6
7
8 9 10 11 12
1.399 1.429 Blank 1.248 1.333 1.227
0 0 0 9.7 0 0
Mean
,
1.294 1.491 1.247 1.267 1.235 Blank Mean
,
9.7 0 0 9.7 0 0 ,,
0 12.5 0 0 0 12.5
0 9.0 9.0 0 0 0
..
..
0 12.5 0 0 0 0
0 0 9.0 0 0 0
..
..
4.9 0 0 0 4.9 0
..
0 0
0 8.2 0 0
Corn Tissue 25.3 25.7 1.7 35.2 26.5 24.5
..
Corn Tissue from 0 0 4.9 0 0 8.2 0 0 0 8.2 0 0
..
..
..
from Check Plot 23.6 16.9 24.0 16.8 0 0 23.8 19.1 24.8 18.6 22.8 18.6 18.0
-6.1 -6;7 +6.1 4-3.3 +3.3
1.2
44.9 43.3 53.3
42.5 115.9 0 43.7 42.1 39.6
30.4 81.1 0 35.0 31.6 32.3
..
..
..
32.3
45.5 69.3 46.8 53.8 49.5 1.2
44.3 55.6 45.6 52.6 48.3
34.2 37.3 36.6 41.5 39.1 0 37.7
..
Plots Treated with iilinor Element Fertilizer 38.8 27.4 21.2 4-2.7 33.2 31.5 21.1 +2;2 23.4 21.7 17.3 37.8 26.4 20.8 4-0.7 25.8 24.1 19.5 -5.8 c 0 0 0 .. .. 20.6 ..
43.7 129.6
0
,.
..
-5.9 E
c
+8.4 -2.2 0.0
..
.
-9.4 -1.2 -3.0 i.lO.0 +3;8
Blank and added copper subtracted from total b Blank and added zinc subtracted from total. C Not included in mean.
a
of the diffusion current due to 40 micrograms of zinc. As the cobalt content of plant tissue rarely exceeds 1 p.p.m. ( 1 ) compared to 20 to 40 p.p.m. of zinc, its influence on the zinc determination would rarely exceed 2% and can be neglected, except with plants with low zinc content. A wave for nickel was found a t -0.90 volt, but it was well separated from the copper and zinc waves. Many alfalfa samples analyzed have shown a measurable wave corresponding with that for nickel. However, an accuracy of better than = t l O o / , for nickel determination would be difficult to attain consistently, as nickel often occurs in plants in lesser concentrations than copper A more accurate determination of nickel would require modification of the procedure to separate it from copper. LITERATURE CITED
(1) Beeson, K. C., E. S. Dept. Agr , M i s c . Pub. 369 (1941).
Cholak, J., and Hubbard, D. M., IND. ENG.CHEM.,ANAL.ED., 16,333 (1944).
Cranaton, H. A., and Thompson, J. E., Ibid., 18,323 (1946). Cummings, R. W.,and Reed, J. F., Soil Sci. SOC.Am. Proc., 5 , 167 (1941).
Heller, X., Kuhla, G., and Machek, F., Mikrochemie, 23, 78 (1937).
Jackson, M.L., Chatterjee, B., Whittig, L. D., and Kittrick, J. A., unpublished manuscript. Kolthoff, I. hl., and Lingane, J. J., “Polarography,” New York, Interscience Publishers, 1941. Riches, J. P. R., h’ature, 158, 96 (1946). Stout, P. R., Levy, J., aiid Williams. L. C., Collection Czechoslov. Chem Communs., 10, 129 (1938).
Walkley, A , Australian J . Exptl. Bzol. Med. Sci., 20, 139 (1942). RECEIVED January 23, 1951. A portion of a thesis submitted b y R. G. Blenzel in partial fulfillment of the requirements for the degree of doctor of philosophy, Department of Soils, University of Wisconsin. J u n e 19jO. T h e work was supported in part by a fellowship grant to the Wisconsin Agricub tural Experiment Station b y the Tennessee Corp., .4tlanta, Ga.
Photometric Determination of Traces of Silver E. B. SANDELL AND J. J. NEUMAYER School of C h e m i s t r y , University of Minnesota, Minneapolis, M i n n . Trace quantities of silver can be isolated by precipitation with stannous chloride from hydrochloric acid medium when tellurium is used as collector, and determined photometrically with p-diethylaminobenzqlidenerhodanineas repgent. A s little as 0.5 p.p.m. of silver can be determined when a 1-gram sample is taken. The method is intended for the determination of minute amounts of silver in sulfides, meteoritic iron, plant ashes, etc.
S
MALL amounts of silver in complex inorganic materials are usually determined by cupellation with final weighing. Optical spectrography has also been applied in the final determination after isolation of silver in a lead button. In one such procedure ( I ) , the limit of detectability is reported as 0.1 p.p.m. of silver. The photometric procedure described is a rather general one, which can be applied in the presence of much‘iron, lead, copper, nickel, cobalt, etc., as well as amounts of the noble metals likely to be encountered in natural materials. Silver is isolated by precipitation with stannous chloride from hydrochloric acid solution with tellurium as collector; it is doubtless present in the pre-
cipitate as telluride. The colorimetric or photometric determination is carried out with p-diethylaminobenzylidenerhodanine as reagent. This compound (actually the methyl analog) was first used as a qualitative reagent for silver by Feigl, and has since been applied to some extent in the determination of silver ( 3 ) . The rhodanine reagent has the advantage of great sensitivity and good selectivity. The red-colored product formed n-ith silver is insoluble, so that the determination is based on the photometry of a colloidal suspension. In an indirect method described by Ringbom ( 2 ) silver is precipitated by p-dimethylaminobenzylidenerhodanine,the washed precipitate is dissolved in potassium cyanide solution, and its
1864
ANALYTICAL CHEMISTRY
amount is found from the color intensity of the yellow solution (rhodanate ion). Trace quantities of silver in copper metal were successfully isolated and determined in this way. PHOTOMETRIC DETERMINATION OF SILVER
The effect of acidity, concentration of reagent, and time of standing on the determination of silver was studied. The most favorable conditions were found to be an acidity of 0.05 N in nitric acid and a final reagent concentration of 0.002%. The acid concentration must be carefully controlled because the rhodanate ion ( R - ) concentration (and the solubility of the silver rhodanate) is a function of hydrogen ion concentration:
+
HR.H+*HR H+ HR+H+ RAg+ R-@AgR
+
+
K~
= 2
x
10-4
K~ = 7 x 10-7 Ka.*.= 8 X
The above values of the constants are for 20% ethyl alcohol solutions a t an ionic strength of 0.05 a t room temperature. Reproducible results are obtained. The optical density of the suspension shows but little change with time under the conditions of the procedure. Thus, a solution containing 1 p.p.m. of silver gave the following extinctions (1-em. cell) after standing: 5 minutes, 0.215; 10, 0.215; 20, 0.215; 30, 0.214; 45, 0.213. The maximum color intensity is reached almost a t once on mixing in the range 0.05 to 2 p.p.m. of silver. The relation between the color intensity of the colloidal suspension and the silver concentration is linear up to 1 p.p.m. of silver. The straight line passes through or extremely close to the origin (extinction of blank deducted). The use of a protective colloid such as starch or gelatin has no advantage. Gold, palladium, and mercury also react sensitively with the rhodanine reagent in acid medium. Gold does not accompany silver into the final solution in the separation, but palladium xi11 do so. The interference of up to 5 micrograms of palladium in a final volume of 10 ml. or 15 micrograms in a final volume of 25 ml. can be prevented by addition of dimethylglyoxime; the dimethylglyoximate does not precipitate. Larger amounts of palladium give a perceptible precipitate, but palladium will rarely be encountered in amounts that cannot be made harmless by addition of dimethylglyoxime without filtration. This precipitate can probably be filtered off and silver determined in the filtrate, but this procedure has not been tested. Small amounts of mercury (100 micrograms) carried through the procedure caused no error; in appreciable amounts, mercury can be eliminated a t the beginning of the analysis by igniting the solid sample. The small amount (1 mg.) of tellurium used as collector does not interfere in the silver determination. The acidity of the final solution is readily adjusted to the specified value by evaporating the nitric acid solution of the tellurium precipitate to dryness and taking up the residue in a measured volume of dilute nitric acid. The sensitivity of the method, as defined by the amount of silver in a column of so'lution 1 sq. em. in cross-sectional area giving an extinction (log lo/l)of 0.001, is 0.005 microgram of silver a t 490 mp, As 0.5- or even 1-gram samples of most materials can be taken, the relative sensitivity of the procedure is 0.05 to 0.1 p.p.m. of sample when the final volume is 10 ml. and a I-cm. absorption cell is used. The visual sensitivity is about 0.05 microgram of silver per square cm. Inasmuch as tubes 1 sq. em. in cross section can be used in the colorimetric comparison, this value may be taken as the limit of the colorimetric method. SEPARATION OF SILVER
The isolation of silver by coprecipitation in tellurium is carried out in 2 N hydrochloric acid. At this acidity silver is precipitated virtually completely by stannous chloride in the presence of 1 mg. of tellurium; higher chloride concentrations appear to have an unfavorable effect on the completeness of precipitation. The
solubility of silver chloride in 2 iV hydrochloric acid a t room temperature ip greater than 50 mg. per liter, so that this acid concentration keeps in solution amounts of silver for which the method is intended. No difficulty is experienced in separating microgram quantities of silver from 0.5 gram of iron, cobalt, nickel, and zinc. These metals, as well as copper, gave no color when carried alone through the procedure. Lead in large amounts precipitates as lead chloride, but this can be dissolved by washing with dilute hydrochloric acid. Arsenic does not interfere. The only common metal hindering precipitation of silver is copper. The precipitation of more than 5 micrograms of silver from 50 ml. of solution containing more than 0.2 gram of copper is incomplete. In the presence of 0.5 gram of copper, up to 2 micrograms of silver can be recovered satisfactorily (Table I). When enough copper is present to interfere, the filtrate from the first precipitation can be treated with more tellurite to recover most of the remainder of the silver. In this way, 10 micrograms of silver can be successfully separated (95% recovery) from 0.2 gram of copper. Relatively much copper is carried down with elemental tellurium and compound formation may occur. Apparently silver must compete with copper to be incorporated in the tellurium precipitate.
Table I. Photometric Determination of Silver after Coprecipitation in Tellurium Addition
Silver Taken,
Silver Found,
Y
Y
4.0 20.0 3.0 5.0 4.0 5.0 1.0 2.0 5.0 10.0 1.0 2.0 5.0 15.0 1 .o 10.0 3.0 10.0 5.0 10.0 5.0 5.0 5.0 5.0 5.0 5.0
4,2 19.5 3.1 5.1 4.0 4.7 1.05 2.1 5.0 9.6b, 9.5C 1.1 1.9 4.7 14.4 1.1 10.2
None
0.5 gram X i (as nickel chloride) 0 . 5 gram Co (as cobalt chloride) 0.2 gram Zn (as zinc chloride)
0 , 2 gram . I s (as arsenious oxlde)
0.2 gram P b (as lead nitrate)" 0 . 2 gram Cu (as copper sulfate)
0.5 gram Cu (as copper sulfate) 5 0 ~
:OyYAURI
lOOy Aur'I
15y Pd" 15y Pd"
50y Pt'v lOOy PtIv 1 O y each Ru"' a n d OsVI" 1Oy each Rh"' and Ir'V 1 mg. MovI 1 mg. WV' 10 m each Sb*II, Bi, V", and Ti"' 50y
&I
3.2d 9.9d
4.8 9.5
4.Y 5.1 4.8 4.8 5.0 4,9
Lead chloride precipitated: dissolved by washing tellurium precipitate with five 5-ml. portions of 1 N hydrochloric acid. b Two tellurium precipitations (sum of 6.5 a n d 3.17 -4g). 5 Two tellurium precipitations (sum of 7.5 a n d 2 . 0 Ag). ~ d Dimethylglyoxime added. 0
Gold and the platinum metals are precipitated by stannous chloride, but only palladium is dissolved when the tellurium precipitate is treated with nitric acid. N o retention of silver by small amounts of these metals was found. Accurate results for silver can still be obtained when the ratio of gold to silver is 10. Small amounts 'of other elements (Table I ) cause no interference. In larger amounts, some of these-e.g., tungsten-can cause difficulty and require modification of the procedure. The effect of the alkaline earths, magnesium, manganese, aluminum, chromium, the rare earths, etc., was not investigated
1865
V O L U M E 2 3 , NO. 1 2 , D E C E M B E R 1 9 5 1 because it is difficult to see how they can affect the reduction of silver ion by stannous chloride. SPECIAL SOLUTIONS
p-Diethylaminobenzylidenerhodanine, 0.05 gram in 100 ml. of absolute ethyl alcohol. Stannous chloride dihydrate, 15 rams in 100 ml. of 2 N hydrochloric acid. Remove any insofuble material by filtration. Prepare fresh a t reasonable intervals. Tellurium tetrachloride, 1 mg. of tellurium per ml. Treat 100 mg. of precipitated tellurium with 1 or 2 ml. of nitric acid and evaporate to dryness. Add 1 ml. of hydrochloric acid and again evaporate t o dryness. Dissolve the residue in 10 ml. of hydrochloric acid and dilute to 100 ml. with water. Nitric acid, ca. 1.25 N . Dilute 40 ml. of concentrated (16 N ) nitric acid to 500 ml. with water. The solution must be chloridefree. Standard silver solution, 0.001% silver as the nitrate in 0.050 N nitric arid, obtained by diluting a 0.01 or 0.1% solution prepared from metallic silver or silver nitrate (AgNOa/Ag = 1.57). PROCEDURE
The sample solution (0.5 to 25 micrograms of silver) may conveniently have a volume of 50 ml. and should be approximately 2 1' in hydrochloric acid. It should not contain strong oxidizing agents such as nitric acid. Add 1 ml. of tellurium solution, mix, and add with stirring 10 ml. of stannous chloride solution (or more as re uired to reduce ferric iron and copper and give a brown colloidd precipitate of tellurium, followed by an excess of 10 ml.). Heat to boiling and keep near the boiling point for 0.5 hour or until the precipitate is well-coagulated. Collect the precipitate in a small filter crucible. Wash the precipitation beaker and crucible carefully with 50 ml. of 1 N sulfuric acid, and then with 50 ml. of water. It is very important to remove all traces of chloride and other foreign substances. Dissolve the tellurium precipitate by adding three 1-ml. portions of hot concentrated nitric acid and wash the filter crucible with several small portions of water. A bell jar-type filtration apparatus is best used in this operation, eo that the nitric acid solution and the washings can be collected directly in a 25-ml. beaker or Erlenmeyer flask. Evaporate the solution to dryness. Take up the residue in 0.40 ml. of 1.25 N nitric acid and 1 ml. of water if the sample contains less than 3 micrograms of silver (determination in a final volume of 10 ml.) or 1.00 ml. of the acid and 1 ml. of water if more than 3 micrograms of silver are present (final volume 25 ml.). After transferring the solution to the appropriate volumetric flask, dilute with water just to the neck
of the flask. Add 0.40 ml. of rhodanine reagent to the 10-ml. flask or 1.00 ml. to the 25-ml. flask. Dilute to volume with water and mix by rapidly inverting several times. Obtain the transmittancy of the suspension in a 1-cm. (or deeper) cell a t about 495 mp within 30 minutes. Prepare the standard curve for the determination of silver in a final volume of 10 ml. by taking 0 to 3.0 micrograms of silver and adding sufficient nitric acid to make its concentration 0.050 N after dilution to volume, taking into account the amount of acid added with the standard silver solution. Dilute with water just to the neck of the flask and add 0.40 ml. of rhodanine reagent. Dilute to volume with water, mix, and determine the transmittancy. Prepare the standard curve for the determination of silver in a final volume of 25 ml. by taking 0 to 25.0 micrograms of silver and proceeding as described above, but increase the amount of rhodanine reagent to 1.00 ml. Determine the transmittancy of the sample and standard solutions after about the same period of standing. For less than 0.5 microgram of silver, visual comparison by the standard series method can be applied with advantage. The final volume should be kept a t or below 5 ml., so that the comparison can be made in 1.2 X 8 cm. flat-bottomed glass-stoppered tubes. The volume of rhodanine reagent is proportionately reduced from the volume given above. The final acidity is maintained a t 0.050 N . The absorption cell or tubes should be cleaned with dilute (6 N ) nitric acid after each determination to dissolve any precipitate that may have been deposited. Notes. If lead chloride is precipitated in the isolation of silver, first wash with the minimum volume of 1 N hydrochloric acid to dissolve it. When copper is present in fairly large amounts (0.2 gram) and the quantity of silver is likely to be greater than 5 micrograms, precipitation of silver is incomplete (see above). In suc$ a case, add 1 ml. of tellurium solution to the filtrate from the first tellurium precipitation, coagulate the precipitate, and proceed as before. When palladium is present (
