Gravimetric Determination of HexamminecobaIt(III) as the Perchlorate Mary A. Taylor, James A. Baur,' and Clark E. Bricker Department of Chemistry, Uniuersity of Kansas, Lawrence, Kan
THEDETERMINATION of hexamminecobalt(lI1) in solution is usually done by decomposing the complex ion in alkaline solution and then analyzing for either or both of the constituents. This technique is not specific for this complex ion, and any free cobalt(l1) or ammonia in the solution would interfere with the analysis. However, Federov, et al. ( 1 ) have suggested that hexamminecobalt(II1) can be precipitated as Co("&T1Cl~ and determined gravimetrically. This salt is not particularly easy to prepare and requires a reagent that is not always available. It is known ( 2 ) that when a solution of C O ( " ~ ) ~ C O ( C ~ ~ ) ~ is acidified with perchloric acid, a yellow precipitate forms which has been presumed to be the perchlorate salt of hexamminecobalt(II1). Several workers have prepared Co(NH3)@(CIO,), and studied some of its properties. These studies include thermal stability (3, 4), solubility in chloride-perchlorate mixtures (1, 5), and method of preparation. This perchlorate salt is known to be soluble only to the extent of 0.013 mole per is easily preliter at 18" C (6, 7). Because Co("&(C104)3 pared, is sparingly soluble in water, and has been found t o be thermally stable at temperatures below 260"C, it seemed likely that this compound could be used for the gravimetric determination of hexamminecobalt(II1). EXPERIMENTAL
Preparation of Co(NH&Cla and CO(NH~)~(NO~)~. These two salts were prepared by the methods described in "Inorganic Synthesis" (8) except that 30 % hydrogen peroxide was used instead of air to oxidize cobalt(I1) to cobalt(II1). Solutions of the hexamminecobalt(II1) nitrate and chloride were prepared by dissolving the pure salts in water. The concentration of hexamminecobalt(II1) in the chloride solution was determined by analyzing aliquots for ammonia by a Kjeldahl procedure. The amount of hexamminecobalt (111) in the nitrate solution was found by evaporating aliquots of the solution to dryness, drying at 120"C and weighing the residue. A sample of chloropentamminecobaIt(II1) chloride was prepared by the method of Willard and Hall (9). A solution of this salt was standardized by the Kjeldahl procedure. Precipitation and Properties of Co(NH3)6(CIO&. A known volume of a standardized solution of hexammine1 Present Address, California State College at Fullerton, Fullerton, Calif.
(1) V. A. Federov, V. E. Mironov, and F. Ya. Kul'ba, Zh. Neorgan Khim., 7, 2528 (1962). (2) J. A. Baur, Doctoral Thesis, University of Kansas, 1967. (3) A. A. Zinov'ev and V. I. Naumova, Zh. Neorgan. Khim., 7, 52 (1 962). (4) M. Viltange, Mikrochim. Ichnoanal. Acta, 2-4, 461 (1964). (5) V. E. Mironov and V . A. Federov, Zh. Neorgan. Khim., 7, 2524 (1962). (6) F. Ephrairn and P. Mosirnann. Ber., 55B, 1613 (1922). (7) F. Ephraim and P. Mosirnann, 56B, 1531 (1923). (8) "Inorganic Syntheses," Vol. 11, McGraw-Hill, New York, 1946, pp 216-19. (9) H. H. Willard and D. Hall, J . Am. Chem. Soc., 44,2220 (1922).
436
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ANALYTICAL CHEMISTRY
cobalt(II1) chloride was placed in a beaker and diluted with water t o 25 ml. Then, 5 ml of 6 0 % perchloric acid was added with stirring and the resulting mixture was allowed to stand at room temperature for approximately 1 hour. The precipitate was collected on a weighed sintered glass crucible (medium porosity ) and washed five times with 5-ml portions of 10% perchloric acid. The crucible and its contents were then dried at 120" C to constant weight, Weighed amounts of precipitate obtained by this procedure were analyzed for ammonia by a Kjeldahl procedure and for perchlorate by precipitation with nitron. Average experimental results were identical to the theoretical percentages of ammonia and perchlorate which are 22.2 and 64.9 %, respectively. The weights of precipitates obtained from a number of different volumes of hexamminecobalt(II1) chloride solution are shown in Table I. These results show conclusively that the precipitate must be CO("~)~(CIO& and that the recovery of this salt from a solution containing excess perchlorate ions is virtually quantitative and reproducible. Equally good results were obtained for the recovery of the perchlorate salt when solutions of hexamminecobalt(II1) nitrate instead of the chloride were used. The quantity of perchloric acid needed for quantitative precipitation of the hexamminecobalt(II1) was determined by taking eight equal aliquots, each containing 0.670 millimole of hexamminecobalt(II1) in 25 ml of water and adding varying amounts of 60% perchloric acid. These results are summarized in Table I1 and suggest that the precipitation is neither complete nor reproducible unless a t least 5 ml of 6 0 z acid is used. The results in Table I indicate that this volume of acid is quite satisfactory to precipitate as much as 2.5 millimoles of hexamminecobalt(II1). A 10M sodium perchlorate solution was used as the precipitant in place of perchloric acid in a series of experiments, The wash solution, however, was always 10% perchloric acid. The precipitation of C O ( N H ~ ) ~ ( C I Owas ~ ) ~not so rapid as with the acid and the recovery was somewhat erratic. With 5 ml of 10M sodium perchlorate as the precipitant, the recoveries were from 2 to 10 parts per thousand low; with 15 ml of the sodium perchlorate, the weights of the precipitates were 2 to 11 parts per thousand high. Thus, the use of sodium perchlorate as a precipitant cannot be recommended in this procedure. Very few cations other than potassium and hexamminecobalt(II1) are known to precipitate with perchlorate. Since the solubility of ammonium perchlorate is over 10 grams per 100 grams of water at 0' C and increases rapidly with increasing temperature, the possible interference of ammonium ions was not specifically investigated. Indirectly, however, ammonium ions were shown not to interfere because added hexammine-nickel(I1) which decomposes in acid solution to nickel and ammonium ions did not give high results (see Table 111). Since the proposed method involves the precipitation of a polycharged cation, it is likely that coprecipitation will be significant. For this reason, the effect of some foreign ions on the recovery of C O ( ~ H & O ~was ~ ~determined. ) ~ The results are listed in Table 111 and indicate that there is a slight interference from a variety of ions that d o not precipitate as the perchlorate when present alone. I t should be mentioned
Table I. Recovery of Co(NH3)&10& from Co(NH3)eC13 Solutions Co(NH&C13 solution taken, ml. of 0.0670M 5.00 5.00 10.00 10.00 15.00 15.00 20.00 20.00 25.00 25.00 25.000 0
Wt of ppt grams 0.1543 0.1540 0.3083 0.3074 0.4622 0.4608 0.6170 0.6153 0,7700 0.7670 1,1376
Co(NHs)s8+ taken, millimoles 0.335 0.335 0.670 0,670 1.005 1.005 1,341 1.341 1.676 1.676 2.475
CO(~ddClO4)a found, millimoles
Deviation, parts per thousand
0.336 0.335 0.671 0.669 1.006 1.003 1.342 1.339 1.675 1.669 2.475
+3.0 0.0 $1.5 -1.5 +1.0 -2.0 +0.7 -1.5 -0.6 -4.2 0.0
0.0990M Co(NHs)& used in this case.
that hexamminechromium(II1) does precipitate as the perchlorate if the concentration of the ions is sufficiently large. The most serious interferences shown in Table 111 are with chloropentamminecobaIt(II1) and hexamminechromium(II1). I n the study of the interference of chloropentamminecobalt (111), the rate of addition of the precipitant was varied. With 0.10 millimole of chloropentamminecobalt(II1) and 0.670 millimole of hexamminecobalt(II1) present, the 5 ml of 60% perchloric acid was added either dropwise with stirring or rapidly with stirring. The difference in the interference found was negligible-varying from 3.0 t o 3.6% in all cases. Thus, the interference of the pentammine cannot be attributed solely t o occlusion of the foreign ions into the crystalline precipitate because the degree of contamination should decrease as the rate of precipitation decreases. Over the range studied, the interference of chloropentamminecobalt(II1) and hexamminechromium(II1) shown in
Table 11. Effect of Quality of Precipitant Volume
CO(~&(ClOa)3 found Deviation from millimoles 0.670 millimoles
mz
Wt of ppt grams
0.5 0.5
0.2381 0.1972
0.518 0.429
-23Z - 36
1 .o 1 .o
0.2814 0.2852
0.612 0.621
-8.7 -7.3
3.0 3.0
0.3056 0.3061
0.665 0,666
-0.7 -0.6
5.0 5.0
0,3083 0.3074
0.671 0.669
+o. 1
HC104ml
-0.1
Table 111. Interferences with Recovery of Co(NH&O10& Formula
NaHCO, BaC12 CaO LiCl ZnS04
Interference
Millimoles present
Co(NH+a+ taken milhmoles
1 .o 0.010 0.051 0.101 0.060 0.302 0.603
0.827 0.670 0.670 0.670 0.670 0.670 0.670
0.828 0.675 0.686 0.694 0.679 0.693 0.704
$0.7 $2.4 $3.6 $1.3 $3.4 +5.1
0.670
0.672
$0.3
0.670
0.680
+1.5
0.670
0.670
5
Deviation, $0. 1
2.4
FeS04
CoClr CuClr HgClz MnClt NiC& CrCli WHm4)4
CO(~HMC~O& found, millimoles
0.7) 0.4 0.0
0.03 0.03
All of these substances were present simultaneously.
VOL 40, NO. 2, FEBRUARY 1968
437
Table I11 does follow a Langmuir adsorption isotherm of the form, w = k Cn,where w = the weight of material adsorbed k and n are constants C = the concentration of the potentially interfering substance.
For chloropentamminecobalt(III), the values of k and n were found to be 0.56 and 0.69, respectively, whereas for hexamminechromium(III), the values were 0.13 and 0.55, respectively. Thus, the interference of these ions is probably due t o preferential adsorption on the growing crystals of Co(NH3)6(C10r)3 followed by the inclusion of the foreign ions into the crystal lattice. The other ions studied as possible interferences d o not show this effect because any adsorption that they may undergo does not lead to incorporation into the crystal lattice. Heating perchlorate salts, especially if they are moistened with perchloric acid, may be considered a dangerous operation. This would certainly be true if any organic matter were present. I n all the precipitations carried out in this study, no indication of violent or nonviolent degradation of the precipitate was observed. However, in no case was any organic matter present because the precipitates were always washed thoroughly with 10% perchloric acid before heating. When the moist precipitates were heated at 120" C , a white smoke was evolved until constant weight was reached. This white smoke, when dissolved in water, gave a n indication of being perchloric acid. Some precipitates were heated for as long as 48 hours at 120" C and these precipitates weighed the same as they did after 3 or 4 hours of heating. One gram of Co("&(CIO& contains 2.176 m i k n o l e s of hexamminecobalt(II1). The gravimetric factor for converting the grams of precipitate to grams of hexamminecobalt(II1) is 0.3505.
Even though the solubility of Co(NH&olO& has been reported at 18" C (6, 7), n o value for its solubility at 25" C was found. A saturated solution of the perchlorate salt in water was prepared and aliquots of this solution were analyzed for ammonia by a Kjeldahl procedure. The solubility was found t o be 0.022 mole per liter. Using this value for the was calculated to be solubility, the K,, for Co("&(C101)~ 6.3 x at 25" C and a t the ionic strength of the saturated solution. This method for the gravimetric determination of hexamminecobalt(II1) is quite specific even though some ions d o coprecipitate and thereby cause high results. The method gives results reliable and reproducible to within 3 parts per thousand if at least 0.3 millimole or 48 milligrams of hexarnminecobalt(111) is present. Apparently, small losses due to sohbility cause the results to become less accurate when smaller amounts of hexarnminecobalt(II1) are to be determined. Although no studies were made to find the upper limit of this procedure, no difficulty was encountered in precipitating 2.5 millimoles of hexamminecobalt(II1). Furthermore, it is possible that this method could be extended to the gravimetric determination of cobalt(I1) if these ions were oxidized with hydrogen peroxide in the presence of ammonia t o hexamminecobalt(II1) prior to the precipitation. The reverse of this analytical method, namely, the use of hexarnminecobalt(II1) to precipitate perchlorate, is not quantitative. This is not surprising when it is realized that excess perchlorate has a far greater effect in reducing the solubility of the salt than does hexamminecobalt(II1). RECEIVED Sept. 5,1967. Accepted Nov. 29,1967. This work was supported by a National Science Foundation grant for undergraduate research participation.
Spectrophotometric Determination of Ferric Acetylacetonate in Uncured Terpolymer with Nitrosoresorcinol Robert B. Lewl Chemical and Materials Laboratory, Aerojet-General Corporation, Sacramento, Calg.
THEUSE OF FERRIC ACETYLACETONATE Fe(AA)3 as a catalytic agent for polymerization of high molecular weight polymers is well known ( I , 2). A small amount of this metal chelate causes a severalfold increase in the rate of polymerization of epoxy crosslinked butadiene, acrylic acid, and acrylonitrile terpolymer. Thus, it became desirable to develop a rapid method for its determination. Ferric acetylacetonate and terpolymer have strong absorption bands in the violet and blue regions of the visible spectrum. The region from 400 to 500 mp is thus not suitable for the determination of the iron chelate. If a chromogenic agent, which forms a color complex with ferrous ions of greater stability than ferrous acetylacetonate and absorbs in a different region of the visible spectrum were added, the deterPresent address, Sacramento State College, Department of Chemistry, 6000 "J" St., Sacramento, Calif. (1) James R. Fischer (to Aerojet-General Corp.), U. S. Patent 2,933,462 (April 19, 1960). (2) Erwin Windemuth (to Farbenfabriken Bayer Akt.-Ges.), U. S. Patent 2,897,181 (July 28, 1959).
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ANALYTICAL CHEMISTRY
mination of Fe(AA)3 could be rapidly accomplished. Ferric acetylacetonate can be easily reduced to ferrous acetylacetonate with hydroxylamine hydrochloride. Overholser and Yoe (3) in their investigation of the determination of cobalt with nitrosoresorcinol reported that this chromogenic agent reacted with ferric ion forming a green water soluble complex. In this work, the author observed that the reaction of nitrosoresorcinol and ferric ion resulted in the formation of a brown complex, whereas a green complex was produced exclusively with ferrous ions. The formation of the ferrous-nitrosoresorcinol complex was rapid and the resulting color could be used for quantitative determination of Fe(AA)3in uncured terpolymer. Maximum absorption of the complex occurs in the region of 690-700 mp where the terpolymer offers practically no interference. Effects of several variables related to the formation of ferrous-nitrosoresorcinol complex in a nonaqueous solvent were studied. (3) L. G. Overholser and J. H. Yoe, IND. & ENG. CHEM.,ANAL. ED., 15, 310 (1943).
