Determination of Copper in Metallurgical Analysis Use of 2,B’-Biquinoline R. J. GUEST Radioactivity Division, Department of Mines and Technical Surveys, Ottawa, O n t . , Canada Difficulty is often found in determining small amounts of copper in many types of material. A spectrophotometric method utilizing 2,2’-biquinoliue (cuproine), which is specific for copper, has been found suitable for copper analyses on a wide range of sample material. In the procedure, the purple complex formed by 2,2’-biquiuolinewith cuprous ion is extracted with n-amyl alcohol. Shaking time and pH were found to have an important effect on the extraction of the copper-cuproine complex. Results were obtained both on synthetic and actual samples. The procedure, being very rapid and simple in execution, is especially suited for routine work.
G
RAVIMETRIC ahd volumetric methods are not suitable for the determination of trace amounts of copper. A lnumber of spectrophotometric methods have been proposed for ‘the analysis of small amounts of this element. The often-used reagents such as diethyldithiocarbamate and dithizone are not apecific for copper, and procedures based on their use are subject to interference from a number of commonly occurring ions (1,4,8). Recently, reagents specific for copper have received much attention from investigators. Two of these reagents 2,2’-biquinoline and 2,9-dhethyl-l,lO-phenanthroline have been used in spectrophotometric procedures for copper. Breckenridge, Lewie, and Quick (2) showed that copper(1) forms a purple complex with 2,2’-biquinoline (cuproine) in alcoholic or acetic acid media. Hoste (5,7) investigated the specificity of this reaction and stated that the reagent is completely specific for copper(1). The effect of colored ions can be eliminated by extracting the copper-2,2‘biquinoline complex with an organic solvent immisrible with water. The reagent has been used for the determination of copper in tap water, alloy steel, and vegetable matter ( 3 , 6). More recently, Smith and McCurdy (9) have used 2,9-dimethyl-l,10phenanthroline (neocuproine) for the determination of copper(1) in the presence of iron.
APPARATUS
Beckman spectrophotometer, Model B Beckman p H meter Fisher safety centrifuge PROCEDURE FOR SOLID SAMPLES
Weigh out accurately 1 gram of the ore into a 250-ml. beaker. Add 20 ml. of 6 N hydrochloric acid and boil the sample for 10 minutes; cool. Add 5 ml. of concentrated nitric acid and 10 ml. of 18 N sulfuric acid, and evaporate the sample t o dryness on a hot plate. Dissolve the soluble salts in 5 ml. of concentrated hydrochloric acid and 50 ml. of water. Bring the sample t o a boil and digest below boiling for 15 minutes. Filter off the insoluble residue on Whatman #40 paper and wash the residue with 1% hydrochloricacid (v./v.) solution, If the residue is suspected of containing copper, ignite and then fuse or sinter with sodium peroxide. Acidify the melt and add t o the main solution. Dilute the filtrate t o an appropriate volume in a volumetric flask. Place a suitable aliquot into a 100-ml. beaker and add 5 ml. of 10% hydroxylamine hydrochloride (w./v.) and 5 ml. of 10% tartaric acid (w./v.). Use the same treatment for a blank de-
1.9
1.8
1.7
EXPERIMENTAL
An attempt was made t o apply the 2,2’-biquinoline method for use in this laboratory. The general recommendations found in the literature (5, 7 ) were followed, but early efforts to obtain reproducible values on standard copper solutions were unsuccess-
ful. hccording to Hoste ( 5 ) ,extraction of the complex is complete at a pH of 3.0. Tests in this laboratory indicated that a pH of between 4.4 and 7.5 is required for complete extraction of the copper-2,2’-biquinoline complex in 30 seconds (Figure 1). It is shown in Figure 2 that extraction of the copper varies with shaking time if the p H falls below optimum levels. Whereas extraction of 107 of copper was not complete after 8 minutes shaking a t pH 3.5, extraction was complete in 30 seconds when the pH was raised t o 5.0. A shaking time of 30 seconds was satisfactory for amounts of copper up to a t least 1757 as long as the p H w’as kept within the required limits. Tests indicated that hydroxylamine hydrochloride solution could be successfully used for up to a week. Boiling the sample after addition of the reducing agent did not alter the copper values. A calibration curve was drawn up covering a range up t o 1407 of copper in 10 ml. of 0.02% 2,2’-biquinoline in n-amyl alcohol and using a wave length of 545 mp. The color of the complex was found to be stable for periods of a t least one week.
1.6
1.5 1.4
1.3
1Y
1.2
a9 OB 0.1
0.6
0.5
0.4
0.3
0.2 0.1
0.0
Figure 1. Effect of pH on Extraction of Copper2,t’Biquinoline Complex with n-Amyl Alcohol
0. 1757 Cu with 30-
or 120-second shaking
X. lOOr Cu with 30- or 120-second shaking 0 . 607 Cu with 30- or 120-second shaking
1484
1485
V O L U M E 25, NO. 10, O C T O B E R 1 9 5 3 termination along with every group of samples. By means of a p H meter standardized a t p H 4, adjust the p H of the solution to 5.0 to 6.0 with 50% ammonium hydroxide. Transfer the sample t o a 60-ml. separatory funnel, keeping the volume of sample plus washings a t 40 ml. Apply silicone grease t o the stopper and stopcock of each separatory funnel. Add by pipet 10 ml. of 0.02% 2,2’-biquinoline (Brickman and Co., Montreal, Canada) in n-amyl alcohol (w./v.). Shake the mixture for 1 t o 2 minutes. Discard the aqueou8 layer. Draw off the organic layer into a 1.5-cm. centrifuge tube and centrifuge the sample for 1 minute t o clear up any cloudiness which may be present in the organic extract. Read the sample against a reagent blank in the spectrophotometer using a wave length of 545 mp and 1-cm. cells.
Table 11.
Comparative Results on Typical Samples
Sample
Type of Sample
NBS l l e
Steel 5 % us08 added Phosphor-bronse Bauxite
NBS 63b KBS 69
XBS 98
Aluminum alloy Zinc-base alloy 10% us08 added Plastic clay
S B S 108
Suelter
NBS 85a NBS 94a
Copper Present, 0.105 0.106 78.0 0.03 (as CuO) 2.48 1.08 1.08 0.009 (as CuO) 0.0004
0.107 0.106 77.0 0.02 (as CuO) 2.47 1,OQ 1.08 0.009 (as CuO) None
PROCEDURE FOR SOLUTION SAMPLES
For solution samples, take an appropriate aliquot, add 3 to 5 drops of concentrated hydrochloric acid and dilute t o the required volume in a volumetric flask with distilled xater. Place an appropriate aliquot into a 100-ml. beaker, add hydroxylamine hydrochloride and tartaric acid, and continue as for solid samples.
a
Grams per liter of Cu obtained by dithizone method (1).
APPLfCATlON OF METHOD
Synthetic Samples. The reported specificity of 2,2’-biquinoline for copper(1) was investigated by adding a number of ions commonly found in minerals and ores t o synthetic copper solutions. Results of these tests are found in Table I. I’one of the ions tested interfered in the copper determination. The extraction procedure eliminated the effect of any ions which are themselves colored. If ions which form insoluble chlorides are present, the precipitated chlorides can be filtered o f f before the extraction is carried out. This modification %‘ascarried out successfully in cases where large amounts of such ions as mercury or silver were present. Small amounts of these ions caused no trouble. The presence of moderate amounts of oxidizing agents, such as nitric acid, does not affect results. National Bureau of Standards Samples and Ore Samples. -4series of National Bureau of Standards samples and ore samples was analyzed for copper. The samples were dissolved by standard acid treatment, with any insoluble material being fused with sodium peroxide and added t o the main solution. The results of these tests are shown in Table 11. Uranium oxide (CSOs)was added t o three of the samples, but in no case was the copper value affected.
.700
,600
,500 W
2
,400
a
m
K
0 w m ,300 a
.200
.I00
0
1
2
3
4
5
6
7 6 9 1 0 SHAKING TIME IN MINUTES Figure 2. Effect of Shaking Time on Extraction of Copper-2,2’-Biquinoline Complex with n-.4myl Alcohol 1. 1Oy C u , p H 2. 20y Cu, p H 3. 20y Cu, p H 4. 60y Cu,p H
Table I. Test 1
2 3 4 5 6 7 8 9
10
11
12 13 14 15
3.5 3.5 5.0 4.5
Effect of Other Ions on the Determination of Copper by 2,2’-Biquinoline Contaminant, Added, y 5,000 Ti02 4,000 Bi 5,000 Co 20,000NiCh 2,000 13,000 Na 15,000 Fe 50,000 Fe 12,000UaOs 5,000 UoOa 3,000 A s + 3,000PZOl 3,000.4s+ 3,000 P206 170 SnClt 1 x lO‘S0, 1 x 10sxoi
v
Copper Present, y 30.1 30.1 30.1
30.1
Copper Found, y 29.9 29.9 31.0 29.8 29.7 30.2 12.1 0.7 12.1 Kone detected 30.7
..
None detected
30.1 30.1 30.1
29.8 30.2 30.2
30.1 30.1 30.1 12.0 12: 0
..
RESULTS AND DISCUSSION
Copper values obtained by the 2,2’-biquinoline method compare very favorably with Bureau of Standards values and with results obtained on ore samples by the dithizone method ( I ) . Reproducibility of results was satisfactory on all samples analyzed. Hoste ( 5 ) states that 2,2’-biquinoline is a specific reagent for copper(1). Work carried out in this laboratory substantiates this claim. The extraction procedure was found to be essential for many of the samples analyzed. Under the conditions of the procedure, such ions as titanium and iron when present in large quantities give definite colors. As only the copper-2,2’-biquinoline complex is extracted by the n-amyl alcohol, the coloration of these contaminants is without effect. The substitution of isoamyl alcohol for n-amyl alcohol gave a small increase in the color intensity. Brief tests indicated that the two amyl alcohols gave about the same results in determinations of extraction rate and optimum p H conditions. Because a plentiful supply of n-amyl alcohol was available, this compound was used for all work described. It was found essential that the p H of the sample solution be kept between 4.4 and 7.5 for complete extraction of copper. This finding does not agree with Hoste ( 5 )who indicates that complete extraction of copper is obtained with 30-second shaking a t a p H greater than 3.0. For analysis of all samples, it was found satisfactory t o adjust the pH of the sample to 5.0 t o 6.0. Varying the shaking time from 30 t o 120 seconds had no effect on results, if optimum p H values were maintained.
1486
ANALYTICAL CHEMISTRY
No trouble was caused by precipitation of such ions as iron and aluminum a t p H values as high as 7.5, providing tartaric acid was present. If large quantities of ions which form insoluble chlorides are present, it is necessary to remove the insoluble compounds by filtration. SUMMARY AND CONCLUSIONS
A rapid, accurate method for determining copper spectrophotometrically is based upon the colored complex formed by 2,2'-biquinoline and copper( I). This complex can be extracted by n-amyl alcohol from a weakly acid solution. The procedure is suitable for samples containing between 0.001 and 10.0% copper, but material containing larger amounts of copper can also be successfully analyzed. The technique required is simple and results are reproducible under routine conditions. After sample dissolution, a trained analyst can complete a single determinntion in less than 15 minutes.
LITERATURE CITED
Bendix, G. H., and Grabenstetter, Doris, IND. ENG. CHEM., ANAL.ED., 15, 649-52 (1943). Breckenridge, J. G., Lewis, R. W. +J., and Quick, L. A,, Can. J . Research, B17, 258 (1939). Gillis, J., Hoste, J., and Fernandez-Caldas, E., Anales edufo?. u. fisol vegetal (Madrid), 9, 585-91 (1950). Greenleaf, C . A., J. Assoc. Ofic.Agr. Chemists, 25, 385 (1942). Hoste, J., A n a l . Chim. Acta., 4, 23-37 (1950). Hoste, J., Heiremans, A., and Gillis, J., Mikrochemie cer. Mikrochim. Acta., 36/37,349-61 (1951). Hoste, J., Research (London),1, 713-5 (1948). Sandell, E. B., "Colorimetric Determination of Traces of Metals," Vol. 111, p. 305, N e w York, Interscience Publishers, 1950. Smith, G. F., and McCiirdy, W.H., Jr., AXAL.CHEM.,24, 371-3 (1952). RECEIVEDfor review February 12, 1953. Accepted July 2, 1953
Determination of Ascorbic Acid by a New Colorimetric Reaction MORTON SCHMALL, CHARLES W. PIFER, AND ERNEST G. WOLLISH Products Control Laboratory, Hoffmann-La Roche, Inc., Nutley, N. J . Most methods for the colorimetric determination of ascorbic acid are based on the reduction of 2,6-dichlorobenzenone indophenol or the wupling of dehydroaswrbic acid with 2,4-dinitrophenylhydrazine, both of which are subject to interferences. A new assay method for ascorbic acid involves the reaction with diazotized 4-methoxy-2-nitroaniline in acid medium, followed by development of a blue color in alkaline solution. This color, with a maximum absorbancy at 570 mM, is compared with standards in a suitable photoelectric colorimeter. Because of the sensitivity of the reaction it is possible to determine very small quantities of ascorbic acid. The simplicity of the procedure permits rapid analysis, suitable for routine control. The method is highly specific for the determination of ascorbic acid in the presence of dehydroascorbic acid and all other vitamins normally found in pharmaceutical preparations. It has also been applied to various fruit j~iicesand processed foods.
A
GREAT number of methods have been proposed for the de-
termination of ascorbic acid. The majority of these are based on the oxidatian of ascorbic acid with 2,6-dichlorobenzenone indophenol ( I , 2 ) or iodine, both of which suffer from lack of specificity. A different approach was used by Roe and Kuether 17,8),who oxidized ascorbic acid t o its dehydro form and coupled the latter with 2,4dinitrophenylhydrazine to produce a red color, which is measured photometrically. Another method (18)is based upon the oxidation of ascorbic acid and interfering compounds with cucumber juice and the selective reduction of dehydroascorbic acid with Staphylococcus albus or Escherichia coli. Scudi and Ratish (10, 11) reacted ascorbic acid with a known quantity of diazotized sulfanilamide and determined the excess of the reagent. These authors assumed a reduction of the diazonium salt by ascorbic acid. Weidenhagen and Wegner (IS) treated ascorbic acid with toluene-diazonium sulfate and advanced a mechanism, inr-olving a reduction of the diazonium salt, folloQ-eclby cleavage, rearrangement, and coupling of the reaction product. The 6nnl compound is described as almost colorless. I n order to apply the reaction of ascorbic acid with diazonium salts to the development of a photometric method, it was considered necessary t o find an intensely colored reaction product. Various diazonium salts tested were found t o result in only yellowish-colored compounds, in both acid and alkaline medium,
which color appeared t o be insufficiently distinct from the sample blank such as can be expected from a multitude of preparations. However, when diazotized 2-nitroaniline reacted n-ith ascorbic acid, a yellow color resulted in acid solution, which turned purplish in alkaline medium. When diazotized 4methoxy-2-nitroaniline was used, a vivid blue color was obtained under the same conditions. I t was found that the blue color of the latter compound was more desirable for analytical purposes, as its E:?!. value was considerably greater and the blank due to the reagents was much loll-er than with the 2-nitroaniline reagent. Therefore, diazotized 4-methoxy-2-nitroaniline was adopted as reagent for a photometric method. Owing to the enediol character of ascorbic acid, the reaction was found t o be highly specific for this vitamin in presence of all other knon-n vitamins. The actual reaction mechanism has not yet been fully elucidated. The blue color formrd in alkaline solution is reversible upon acidification. The absorption spectrum of the blue color with a maximum a t 570 mp is shown in Figure 1. Plotting concentration of ascorbic acid against galvanometer readings obtained with a Klett-Summerson photoelectric colorimeter] a graph resulted, as shown in Figure 2. For concentrations between 0.5 and 2.0 mg. of ascorbic acid per 200 ml. of final solution, a straight line was obtained, which, however, did not pass through the point of origin. Therefore] the
