Inorganic Spot Test for Copper FRIEDA GOLDSCHMIDT
AND
BINY4\.ITN R . DISHOK
Daniel Siefl Research Institute, Kehouoth, Palestine The appearance of a violet color when concentrated hydrobronlic acid reacts with salts of divalent copper is used as the basis of a spot test for copper. It requires only common inorganic reagents and has a sensitivit5 Comparable to that ,of copper tests with organic reagents.
N E of the numerous specific reactions for copper which have been proposed is the appearance of a violet color when concentrated hydrobromic acid reacts Kith salts of divalent copper.
The reaction can be used in presence of 10119 of gold, bismuth, (21 (III), iron, nickel, and cobalt, when the quantity of these metals does not excetd the maximum limits indicated in Table I
As early ab 1877 Cresti ( 3 ) applied this color reaction for the identification of copper metal, which he exposed to the vapors of hydrobromic acid and bromine. Sabatier (6) used concentrated hydrobromic acid or a mixture of potassium and phosphoric acid in order to detect the Cu (11)-ion in aqueous solution. He attributed the violet color which developed to the formation of a complex compound of cupric bromide and hydrogen bromide, as he succeeded in preparing an unstable dark-colored crystalline product of the approximate formula 3CuBrl.2HBr.6Hz0, probably identical with the metastable compound observed by Carter and Megson ( 2 ) in the system cupric bromidehydrobromic acid-water a t 25' C. The color. of the solution depends on the concentration of the hydrobromic acid; a t concentrations of 26% or more, the color is a pure violet, below 26y0 hrown to brown-violet. Scheringa ( 7 ) applied the reaction to the detection of copper in organic substances, and reported a sensitivity of 0.5 microgram. Augusti ( 1 ) used it as a microtest, and was able to detect 0.15 microgram of copper in one macrodrop. He stated that the presence of lead, cadmium, trivalent iron, silver, monovalent mercury, and monovalent copper interferes with the testc but that the influence of. the two latter cations can be eliminated by oxidation. Xirchhof (5) used the reaction for the detection of copper in rubber.
The color reaction may not appear or the test becomes uncertain for the following reasons: (1) the reducing properties of Hg*(I),Sn (11),or Fe (11) compounds; (2) the color of Au (111), Bi (111),or Fe (111) bromides which are present or are formed in the solution to be tested in concentrations higher than those indicated in Table I ; (3) the photochemical color change of silver compounds; and (4)the characteristic color of Cr (111),S i (11) and Co (11) compounds prrsent in concentrations higher than those indicated in Table I.
0
The authors have developed the reaction into a spot test for copper, and especially studied the interferences of other ions and their possible elimination. The method requires only common inorganic reagents and yet is of a sensitivity comparable to that of the known copper tests with organic reagents (4). The reagent used is an aqueous solution of ammonium bromide and phosphoric acid. This acid has proved superior to sulfuric acid, as it does not lead to the undesirable formation of free bromine, does not attack the filter paper, and largely suppresses the inter.ference by Fe (111)and Bi (111)ions. REAGENTS
1. Ammonium bromide-phoiphoric acid.
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Ammonium bromide (5 grams) is dissolved in a few cubic centimeters of water; sirupy phosphoric acid (d = 1.75) (4 cc.) is added, and the liquid is diluted with water to a total volume of 100 cc. 2. Saturated bromine water. 3. Silver 4. Sodium acetate, 1% solution. 5. Sodium fluoride, 4y0solution.
ELIMINATION OF DISTURBING I NFLUENCES
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Table 1.
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Permissible Quanitities of Metals
Foreign Cation Maximum Quantity Minimum Quantity Present in of Foreign of Copper Jon Copper Solution Cation per Detectable per t o RQ Tested Vacrodroo Macrodrop Grams Gram
Ratio of Copper Ion t o Foreign Cation
1. To eliminate the interference of the reducing substances, the solution to be tested is oxidized with saturated bromine water (reagent 2), and the test is carried out as follows: One drop of the solution to be tested is dried on the filter paper, one drop of saturated bromine water is dropped onto the spot and dried as before, one drop of reagent 1 is added and dried, and finally one drop of water is added and dried with a current of warm air. 2. In case the quantity of Au (111) compounds in the solution is higher than shown in Table I, a yellow-bro~-nring appears on the paper, and the reaction is not clear. This effect can be avoided if one boils, in a small test tube, one drop of the solution to be tested with one drop of reagent 1 and a few grains of reagent 3. When the brown color has disappeared, the ordinary test can be made with one drop of this diluted solution. It is now possible to ofdetect 500 microgram gold.0.2 microgram of copper _ _ in the presenre of
To eliminate the disturbing influence of the yellow color of Hi (111) bromide, one dror, of a 1% solution of sodium acetate (reagent 4) is added to onedrop of tLe test solution and one drop of reagent 1. Khite bismuth phosphate precipitates, and the test is now made as usual with one drop of this suspension. The main portion of bismuth phosphate collects in the center of the >pot. In this manner, 0.2 microgram of copper can be detected in a solution containing 5.0 micrograms of bismuth. The phosphoric acid present in reagent 1 converts the redbrown Fe (111) bromide into a complex ferriphosphoric acid, so that the spot test is possible, if the maximum quantity of iron indicated in Table I is not exceeded. In the presence of higher concentrations of iron, the test has to be modified as follows: One drop of the solution is boiled with one drop of saturated bromine water (reagent 2), so as to convert any Fe (11)salt present into Fe (111) compounds. To this solution, one drop of reagent 1 and one drop of a 4% sodium fluoride solution (reagent 5 are added); the latter is used to diminish the Fe (111) concentration. Upon heating the yellow-white ferric phosphate precipitates. The liquid is cooled and one drop of it is used for the test. Like bismuth (111) phosphate, ferric (111) phosphate collects in the center of the spot, and continued heating produces a
PROCEDURE
One drop of the solution to be tested is placed on filter paper (the authors used Green No. 803; Whatman filter paper gave a slight copper reaction with the reagent). After the spot is dried in a current of warm air, one drop of reagent 1 is brought into the spot and dried is the same way. In the presence of copper a violet color appears on the paper; on addition of a drop of water the copper compound is concentrated a t the outside border of the spot in form of a violet ring. The color fades rather quickly under the influence of atmospheric moisture and at very low copper concentration may even disappear. It returns, however, when the spot is dried again with warm air. (Cold air is not very effective.) In this way it is possible to detect 0.1 microgram of copper in one macrodrop (0.05 cc.). The limit concentration is therefore 1 to 500,000. With one microdrop of 0.05 cc. even 0.025 microgram of copper can be detected. 31'3
A N A L Y T I C A L CHEM1 STRY
374 Table 11. Maximum Quantities of Cations Foreign Cation Highest Quantity Minimum Quantity Present in of Forei n of Copper Ion Copper Solution Cation Use8 per Detectable per to Be Tested Macrodrop Macrodrop 0.2 Au (111) ' 500 0.2 Bi(II1) 500a 1 Fe (111) 200 0.2 Ag (1) 2500 a Maximum quantity permissible.
Table 111. Anion Present in Copper Solution
1:2500 1: 2500
1:200 1 : 12,500
Sensitivity of Test
Quantitya of Anion per Macrodrop
Minimum Quantity of Copper Ion Detectable per hlaorodrop
Y
0
Ratio of Copper Ion to Foreign Cation
Y
Nitrate I550 2090 Chlorate 3200 Bromate Iodate 4380 4800 Periodate These quantities correspond to solutions t o 0.5 mole.
0.1
0.5 0.5 0.5 0.5
brownish coloration of the center, surrounded by the violet ring. Thus, 1.0 microgram of copper can be detected in the presence of 200 micrograms of iron. 3. In the presence of silver compounds one drop of the solution to be tested is boiled for a short while with one drop of reagent 1. Silver bromide precipitates, and one drop of the liquid is taken for the ordinary test. This reaction allows detection of 0.2 microgram of copper in presence of 2500 micrograms of silver. Upon exposure to light, the silver bromide which collects in the center of the spot immediately turns gray. 4. If such colored substances as Cr (111), Ni (II), and Co (11) compounds are present, the test is applicable only if their concentration is not higker than that indicated in Table I. If large quantities are present, they must be removed from the solution by one of the ordinary analytical methods. Table I1 indicates the maximum quantities of foreign cations per macrodrop n-hich will not interfere Kith the modified specific procedures.
West (8) observed that nitrates, chlorates, bromates, iodates, and periodates give an intense yellow stain under the coqditions of this reaction. The color is obviously due to free bromine liberated by these highly oxidizing substances. In the case of nitrates present in the solution, the test is not affected adversely, as the violet ring is formed before the yellow color appears. In the presence of chlorates and bromates the violet ring appears first, but is immediately obscured by a deep yellow stain. Fortunately, the color fades quickly, and after renewed drying, only the violet ring, characteristic of copper, reappears. The situation is somewhat more. complicated in the case of iodates and periodates, which give a very persistent yellow stain. In this case, the reaction is performed in the following manner, utilizing the difference in capillarity of copper ions and these anions: One drop of the solution to be tested is dried on the paper, and one drop of reagent 1 is added and dried. Another two drops of reagent 1 are then brought onto the yellow spot. Upon drying, a yellow ring appears and, after some time, an outer violet ring. The authors have determined the sensitivity of the method in the presence of the oxidizing anions; their results are summarized in Table 111. LITERATURE CITED
Augusti, S.,Jfikrochemie, 22,139 (1937). Carter, S.R., and Megson, N. J. L., J . Chem. SOC.,1928,2954. Cresti, L., Ber., 10, 1099 (18977). Feigl, F., "Qualitative ..inalysis by Spot Tests," 3rd English ed., p. 74, New York, Elsevier Publishing Co., 1946. Kirchhof, Kautschuk, 14,163 (1938). Sabatier, P., Compt. rend., 118,980 (1894); quoted by GnielinKraut, 7th ed., Vol. V, Part I, pp. 708, 938. Szheringa, K., Chem. Zentr., 1925, I, 1771; Pharna. Weekblad, 62, 173 (1925). West, P. TV., private communication. RECEIVED September 30,
1946.
Melting-Point Bath liquids JONATHAN L. HARTWELL, .Vational Cancer Institute, National Institute of Health, C'. S . Public Health Service, Bethesda, M d .
HE paucity of published material relating to substitutes for Tconcentrated sulfuric acid as the bath liquid in the common melting-point apparatus is difficult to account for in view of the disadvantages of this almost universally used liquid, chief of which are its hygroscopicity and its hazards. I n particular, it is annoying and time-consuming when a melting-point determination must be interrupted because absorption of water vapor has lowered the boiling point of the bath below its expected value, and it is dangerous when one is working with unstable compounds or with compounds that melt near the boiling point of the acid. Yet the advantages of the type of apparatus employing a liquid heat-transfer medium, especially the Hershberg apparatus 1 2 ) for the accurate determination of corrected melting points by total thermometer immersion, seemed so great that a search was made for a substance that would be generally superior t o sulfuric acid. The classic treatises on organic chemical methods suggest only phosphoric acid, paraffin oil, paraffin, glycerol, castor oil, rapeseed oil, mixtures of sulfuric acid with inorganic sulfates, and mixtures of inorganic salts. Some of these have the same disadvantages as sulfuric acid; others are not liquid a t room tempera-
ture. More recently there have been recommended HB-40 (a), a product consisting mainly of a mixture of terphenyls, and silicone fluid type 550 (4, an aromatic silicone. After over 60 different organic substances distributed among several types Thich seemed to offer promise, had been investigated, two were ultimately selected as superior to the others and to those described in the literature--Aroclor 1248 and silicone oil 9981-LTNV-40. Both products are liquid a t room temperature, boil above 350" C. (corrected), are colorless, noncorrosive, highly stable to heat, nonhygroscopic, without objectionable odor a t elevated temperature, have a high flash point, and are commercially available in large quantities. The former gives tovic effects only upon prolonged inhalation of the vapor, n-liile the latter is nontoxic. EXPERIMEATAL
Preliminary Screening. A large number of substariccs w r c heated to boiling in an open beaker with a thermometer immersed in the liquid. Of these, 62 registered a temperature of 298O C. (corrected) or above. Some n-ere solids, as it was thought pos-
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