Sodiu m-p-(mercaptoacetamido)benzene Sulfonate as Colorimetric Reagent for Cobalt H. K. L. GUPTA
and N. C. SOGANI
Chemistry Department, Government College, Ajmer, lndia
- -
,Sodium p (mercaptoacetamido)benzene sulfonate is a highly selective and sensitive reagent for the direct determination of cobalt(l1). With cobalt(ll) it gives a brown-red watersoluble complex in the pH range of 6.5 to 7.5 with an absorbance peak a t 475 mF and a brown complex in the pH range 8.5 to 11.0 with an absorbance peak a t 390 mp. The system obeys Beer's law in the concentration range of 0.3 to 2.0 p.p.m. of cobalt and is stable toward time and temperature. The commonly associated elements do not interfere. Nickel gives a light brown unstable complex with the reagent and interferes. The reagent compares favorably with the others used for this purpose.
T
use of thioglycollanilides as gravimetric reagents for cobalt(I1) has been described (3, 4, 12, 17). Their high selectivity toward cobalt and fairly high sensitivity suggested that the sulfonic acid derivative of such a compound, either as such or in the form of the sodium salt, should prove a useful colorimetric reagent for cobalt. Sodium - p - (mercaptoacetamido) benzene sulfonate(I) HE
H O
is a very good colorimetric reagent for cobalt. It gives a water-soluble brownred complex in the pH range of 6.5 to 7.5 with an absorbance peak a t 476 mp. The empirical formula of the complex, determined by applying Job's continuous variation method (9), is CoR3, where R stands for the reagent. It can be best represented by the chelate structure (11) HzC-C=O B S - ~ - S O r ? ; a
\/'
Co/a
I1
In the pH range of 8.5 to 11.0, a brown complex is formed with an absorbance peak a t 390 mp. The empirical for918
H2C-C-ONa
In the pH range of 7.5 to 8.5, both forms of the cobalt complex seem to exist. The presence of diverse ions does not interfere in the direct determination of cobalt. However, some of the reagent is consumed by diverse cations and gives colorless complexes and additional reagent should be used. Under similar conditions, nickel(I1) also forms a light brown complex. Although it is unstable toward time, heat, and acid, complete removal of the interference is not possible. However, both cobalt and nickel can be determined simultaneously. Iron(I1) and (111) also form a light brown complex. A modified procedure is given to remove this interference. The reagent in solid state is very stable. In solution it is oxidized by atmospheric oxygen to disulfide (8), and should be properly stored or, preferably, freshly prepared before use. EXPERIMENTAL
I
&
mula of this complex is also CoR3. At higher pH, enolization of the keto group appears to take place (8) and the complex may be represented as:
ANALYTICAL CHEMISTRY
PREPARATIOK OF REAGENT. Several methods (1, 2, 12, 18, 19) for preparing thioglycollanilides have been described. In this investigation, a modification of the method of Misra and Guha Sircar (12) was used. It consists essentially in refluxing an aromatic amine with mercaptoacetic acid between 110' and 120' C. in an atmosphere of carbon dioxide. Sulfanilic acid is not acylated with mercaptoacetic acid under these conditions. However, its sodium salt can be easily acylated, because the basic characteristics of the amino group are completely masked by the presence of an acidic sulfonic group in the case of sulfanilic acid. To 9.7 grams of twice crystallized and dried sodium sulfanilate in a hard glass boiling tube were added 6 ml. of 80% mercaptoacetic acid. The reaction is exothermic. It was heated in an oil
bath kept between 120' and 130' C. An apparatus to give a continuous flow of carbon dioxide over the mass was made and fitted with a short air condenser. The mass started frothing, and the water vapor formed during the reaction was swept along with the carbon dioxide. I n about 3 hours the mass solidified. The air condenser was removed and the tube was further heated for about half an hour to expel the unreacted excess of mercaptoacetic acid. The solid mass was removed, rubbed to a fine powder in a pestle mortar with 99% ethyl alcohol, and filtered under suction. It was boiled with a large excess of 99% ethyl alcohol and filtered hot under suction. This operation was repeated to remove any adhering mercaptoacetic acid. Sodiump-(mercaptoacetamido)benzene sulfonate was dried a t 120" C. and the yield was about 8 grams. Analysis found, N, 5.12; S, 22.40%. Cs€&04S2SNa.H20requires K, 4.88; S, 22.29% REAGENT SOLUTION.A l.Oyow./v. solution of the reagent in distilled water was prepared fresh each time. STANDARD COBALTSOLUTION. Approximately 4.6 grams of reagent grade cobalt nitrate hexahydrate were dissolved in 260 ml. of water. Cobalt content was determined gravimetrically using 1-nitroso-2-naphthol. It was diluted to give 16 y of cobalt per ml. BUFFERSOLUTIONS.The first solution of pH 8.7 to 8.8, was prepared by dissolving 30.92 grams of boric acid, 37.27 grams of potassium chloride, and 6.56 grams of sodium hydroxide in 1 liter of water. The second, of pH 7.0 to 7.2, was prepared by dissolving 68 grams of potassium dihydrogen phosphate and 15.73 grams of sodium hydroxide in 1liter of water. SOLUTIONS OF DIVERSEIONS. Reagent grade soluble salts were dissolved in water to give 0.1% w./v. solution of the respective ion. IKSTRUMENTS. Absorbance measurements were made with a Spectronic-20 colorimeter using 1-cm. cuvettes (Bausch & Lomb Optical Co.). pH measurements were made with a Beckman pH meter Model H-2. Visual color comparisons were made in 50-ml. Nessler tubes of standard type. Sodium-p-(mercaptoacetamido) benzene sulfonate is a white crystalline compound, highly soluble in water and only slightly soluble in alcohol, acetone, and other organic solvents. It is
450
430
410
470
I 490
Wave Lengfh.m)r
Figure 1 .
Spectral characteristics of cobalt complex 1.6 p.p.rn. cobalt 1.
2.
pH 8.8 p H 7.0
quantitatively oxidized by iodine and this reaction has been used for its direct volunietric estimation. The purit'y of the compound and its stability in solution hare been determined by titrating it against standard iodine solut'ion. The compound in solution is stable for 6 hours and is oxidized to the extent of 7y0 in 30 hours, 13% in 53 hours, and 18y0 in 80 hours. SPECTRAL CHARACTERISTICS OF CoBALT COMPLEX. Five milliliters each of standard cobalt solution, containing 16 p.p.m. of cobalt, were taken in two 50-nil. flasks. To both were added 2 nil. of 10% sodium potassium tartrate and 5 nil. of 1% reagent solution. Varying amounts of 0.1% sodium hydroxide solution were added so that p H in one case was about '7.0 and in the other, about 8.8. The pH can also be adjusted by adding 5 ml. of the buffer solutions. The solutions were made up to volume and the absorbance, a t different wave lengths, was measured after about 1 hour. Because the reagent, solut'ioii has no absorbance in the visible range, water \vas used as the blank. Figure 1 gives the absorbance curves of cobalt complex a t pH i . 0 and 8.8. At pH 7.0 the color of' the complex is brown-red and gives an absorbance peak a t 4i5 mp. At pH 8.8 the color is b r o w and there is a gradual fall in the absorbance from 390 m& toward the higher wave lengths. EFFECTOF PH. ii. series of solutions was prepared as described and varying amounts of 1% sodiuni hydroxide solution were added so that the final pH of the solutions ranged between 5.0 and 11.0. Absorbance was measured after 1 hour a t both 475 and 390 nip. Figure 2 gi\-es the effect of change of pH on absorbance. At 4i5 nip the constant maximum absorbance is bet'ween 6.5 and 7.5, and a t 390 beheen 8.5 and 11.0. The absorbance at 475 nip in the pH range of 6.5 to i . 5 is slightly more than a t 390 in the pH range of 5.5 to 11.0. The stability of the complex at, a lower pH after its formation at, a higher
i Figure 2. 1. 2.
Effect of pH
At 475 mp At 3 9 0 m&
pH was studied. After all the soluobtained by this procedure are slightly tions had been added as described more intense than those obtained by above, it is diluted to about 40 ml. and keeping the solutions for definite inter10% v.,h. hydrochloric acid was added vals a t room temperature. so that the final pH after making up BEER'SLAW. The color development the volume was about 2.0. The color obeys Beer's law in both the pH ranges intensity remained unaltered for 15 of 6.5 to 7.5 and 8.5 to 11.0. The conminutes. centration range in the former case at REAGENT COKCESTRATIOX ASD ~ I O L E 475 mp is 0.29 to 2.0 and in the latter RaTIo. A series of solutions was precase a t 390 mp is 0.3 to 2.0 p.p.ni. of pared in which the mole ratio of cobalt cobalt. to reagent was from 1 to 2 to 1 to 80. SENSITIVITY OF REACTION.FiftyIn one set of experiments, the pH was milliliter solutions containing 2 nil. kept a t about 7.0 and in another, a t of reagent solution, 1 ml. of potassium about 8.8. Absorbance was measured dihydrogen phosphate buffer (pH 7.0), after 1 hour a t 475 mp in the former and and 0.1 to 1.0 p.p.m. of cobalt were prepared in Xessler tubes and kept for at 390 mp in the latter. There was a 30 minutes for full development of very gradual rise in absorbance indicatcolor. Cobalt (0.11 p.p.m.) was easily ing that the complex is in a highly distinguishable from a blank. Thus dissociated form in solution. At pH the sensitivity of the color reaction could 7.0 full development of color is ensured be taken to be 1 part of cobalt in 9,000,a t a 1 to 60 ratio of cobalt to reagent 000 parts of solution. Spot plate sensiand a t pH 8.5, a t a 1 to 45 ratio. tivity was determined to be 0.1 y of RATE OF REACTION -4xn STABILITY cobalt in 0.15 ml. of solution. OF COMPLEX.Experiments m-ere conRecommended Procedure. Cobalt ducted both a t pH 7.0 and 8.8 and, can be determined colorimetrically in a t interval?, the absorbance was measthe p H range of 6.5 to 7.5 a t 475 nip ured a t 475 and 390 mp, respectively. and pH 8.5 to 11.0 a t 390 mp. In the The complete development of color lower range, the specificity and sena t pH 7.0 took about 30 minutes and sitivity of the reagent are greater and a t p H 8.8, 50 minutes. The time for the possibility of precipitation of the complete development of color also dehydroxides of heavy metals is less than pends upon the excess of reagent added, in the higher. Hence, the lower pH Thus a t pH 7.0, by keeping the cobalt range was preferred. Here also two to reagent ratio a t 1 to 300, the time slightly different procedures w r e 1.5 minutes. The was reduced to adopted. colors a t both pH values were stable for 10 hours and showed an increase PROCEDURE I. TO a cobalt solution containing 15 to 100 y of the metal in a in the intensity by 5% after 24 hours. 50-ml. flask are added 2 ml. of 10% soEFFECTOF HEATISG. The solutions dium potassium tartrate solution, 5 were heated on a boiling water bath ml. of buffer solution (pH 7.0), and 5 for times varying from 1 to 1.5 minutes ml. of 1% reagent solution. The p H before making up to volume. They can also be adjusted by adding O.lyc were cooled immediately to room temsodium hydroxide solution. The soluperature. At higher temperatures the tion is made up to volume and absorbcolors faded out but reappeared on ance is measured, after 30 minutes, at 475 mp, using water as the blank. cooling. After making up the volume, PROCEDURE 11: The sample is prrthe absorbance was measured without pared as described above except that further waiting. The complete debefore making up to volume, it is kept velopment of color took about 5 minutes in a boiling water bath for 5 to 7 minof heating. Heating for more than 12 utes, cooled to room temperature in a minutes gave lower results. The colors water trough, and then made up to volVOL. 31, NO. 5, M A Y 1959
919
Table
Ion Zn(I1) Mn( 11) cum)
I.
Tolerance of Diverse Ions
Added as Acetate Chloride Sulfate Fe(I1) Sulfate Fe(II1) Sulfate Al(111) Sulfate Cr(II1) Sulfate Nitrate Chloride Tungstate Sodium Molybdate Sodium
#I,
Concn., P.P.M. 200 200 150 150 150 300 150 150 5 150 150
ume. Absorbance is measured, without further waiting, at 475 mp, This procedure gives slightly better results and takes less time. Tolerance of Diverse Ions. In slightly acidic or alkaline medium zinc(II), manganese(II), aluminum(II1) , chromium(II1), bismuth(III), mercury (11), lead(II), antimony(III), tin(II), titanium(IV), cerium(IV), Eirconium oxide(I1), uranium(IV), uranyl(11), thorium(1V) , vanadium(II1), platinum(IV), gold(III), and tungstate do not give any color reaction with the reagent and are not likely to interfere in the colorimetric determination of cobalt. Silver(1) gives a white precipitate with the reagent which is soluble in a n excess of the latter. Molybdate in acid medium gives a yellow color, which changes to green, and finally to blue, on standing. The reagent appears to reduce molybdate to molybdenum blue. Palladium(I1) in acidic medium gives a fairly intense and stable yellow. Copper(I1) gives a bluish black color which disappears on adding a n excess of the reagent. Yickel(I1). iron(II), and (111) give light brown unstable complexes. For studying the interference of diverse ions in the determination of cobalt. Procedure I1 was followed. Table I summarizes the tolerance of diverse ions as parts per million of ions. Except for palladium(II), where the limiting concentration has been found out, the tolerance beyond the limit mentioned in Table I has not been tested. Some reagent is consumed by the diverse ions and an extra amount should be used. For every milliliter of 0.1% solution of diverse ion, 2 ml. of 1% reagent solution should be added, except in the case of chromium(111) where 4 ml. of the reagent should be used.
920
ANALYTICAL CHEMISTRY
Direct determination of cobalt in the presence of nickel is not possible. The former is separated from the latter by precipitating it as potassium cobaltinitrite (IO). It is then dissolved in 2N sulfuric acid and is estimated colorimetrically by the procedure given above. Cobalt in Presence of Iron. Iron(I1) and (111) interfere in the usual procedure. This interference is eliminated by bringing the p H of the solutioii to between 2 and 4 by adding dilute hydrochloric acid. The sample is prepared as described. After it has been cooled to room temperature, it is diluted to about 40 ml., and 1 t o 1.5 ml. of 10% v./v. hydrochloric acid are added, so that the p H after making up the volume is between 2 and 4. Absorbance should be measured within 15 minutes because the color is not stable at a low p H for a longer period. DISCUSSION
Several colorimetric methods have been described for the determination of cobalt. 1-Nitroso-2-naphthol ( I @ , 2nitroso-1-naphthol ( H ) , nitroso-R salt (II), 2-nitroso-1-naphthol-4-sulfonic acid (19), ammonium thiocyanate ( I @ , terpyridyl (IS), o-nitrosophenol (?’), onitrosoresorcinol (14, 3-nitrososalicylic acid (16),and diethylthiocarbamate (6) are the reagents commonly used. Only a few are briefly discussed. The most widely used reagent is nitroso-R salt. However, the time of heating after the reagent and the nitric acid have been added and the final concentration of the acid are important factors which affect the intensity of the color. Recently, the use of 2-nitroso1-naphthol-4-sulfonic acid, which has a n identical chelating system has been reported. However, most of the common cations produce precipitates during the process, which must be removed by repeated centrifuging and washing. 3-Nitrososalicylic acid has been reported to be more specific than o-nitrosophenol and more convenient than o-nitrosoresorcinol and has been used for the separation and codetermination of cobalt and nickel. However, the cobalt complex must be extracted with petroleum ether before measuring the absorbance. Because of its similarity to the authors’ reagent, reference is made to o-(mercaptoacetamid0)-p-nitrophenol (6) which has been reported
as a colorimetric reagent for cobalt, The low solubility of the reagent in water and that of the complex even in alcohol are serious drawbacks with this compound. I n many respects sodium-p-(mercaptoacetamido)benzene sulfonate is superior to the reagents listed and can be used for the direct determination of cobalt in presence of almost all the commonly associated diverse ions. Its high sensitivity and selectivity may class it among the better reagents for cobalt. ACKNOWLEDGMENT
The authors express their gratitude to Shri Bhim Sen, Principal, Government College, Ajmer, for providing facilities in the college. REFERENCES
(1) Beckurts, H., Frerichs, H., J . prakt. Chem. 66, 174 (1902). (2) Benary, Erich, Ber., 46,2105 (1913). (3) Berg, Richard, Roebling, W., Zbid., 68, 403 (1935); 2. angezc. Chem. 48, 430, 597 (1935). 4) Bersin, Theodor, 2. anal. Chem. 85,
(6) Chilton,’J. M., ASAL. CHEJI.25, 1274 (1953). ( 7 ) Cronheim, George, IND.ENG.CHEW, ANAL.ED. 14, 445 (1942). (8) Feigl, Fritz, “Chemistry of Specific,
Selective, and Sensitive Reactions,” D. 234. Academic Press, Sew York.
1949. (9) Job, P., Ann. chim. 9, 113 (1928). (10) Kallmann, Silve, ASAL. CHEM.22, 1519 (1950). (11) McNaught, K. W., =Inalyst, 64, 23 (1939). (12) hlisra, R. M., Guha Sircar, S. S., J . Indian Chem. SOC.32, 127 (1955). (13) Moss, hf. L., hfellon, 11. G., IND. ENG.CHEW.,ANAL. ED. 15, 74 (1943). (14) Overholser, L. G., Yoe. J. H., Ibid., 15,310 (1943). (15) Perry, M. H., Serfass. E. J., ANAL., CHEX22, 565 (1950). (16) Snell, F. D., Snell, C. T., “Colorimetric Methods of hnalysie,” Vol. 11, p. 352, Van Nostrand, Sew York, 1955. (17) Swain, R. C., Rlisra, R. N., Guha Sircar, S. S., J. Indian Chem. SOC.33, 329 (1986). (18) Ubeda, F. D., Capitan, F., Anales real SOC. espaii. fis. y puim. (Madrid) 45B,413 (1949). (19) Van Bllan. J. A.. J . Am. Chem. SOC. ’-69,2914 (1946). ‘ (20) Wise, W. M., Brandt, W. W., ANAL. CHEX 26, 693 (1954). (21) Yoe, J. H., Barton, C. J., ISD.EVG. CHEX.,AXAL.ED. 12, -105 (1940).
RECEIVEDfor review May 28, 1958. -4ccepted December 8, 1958.
