New lodometric Determination of Cobalt Based on Formation of lodopentamminecobalt( Ill) Nitrate RICHARD G. YALMAN Antioch College, Yellow Springs, O h i o
The reaction between cobalt and iodine in ammoniacal solutions of ammonium nitrate to form iodopentamminecobalt(II1) nitrate can be used for the determination of cobalt(I1). The reaction is rapid and oxygen does not need to be excluded. The end point can be determined either potentiometrically or by using the starch indicator. Copper, nickel, etc., do not interfere, nor do aluminum, iron, and chromium in the presence of tartrate.
of an iodine solution of sufficient strength, so as to provide a 10 to 50% excess. iifter all of the ammonium nitrate has been dissolved with the aid of magnetic stirring, add 5 ml. of concentrated ammonia. Solutions which are acid may be conveniently neutralized by the addition of a 6-V ammonium carbonate solution until the formation of carbon dioxide stops. The resulting ammoniacal solutions, which will have a pH of about 9, should not be heated nor, unless a very large amount of acid has been neutralized, do they need to be cooled. The precipitation of apple green iodopentamminecobalt(II1) nitrate will begin as soon as the ammonia has been added and !vi11 be complete within 5 minutes. The excess of iodine is then determined potentiometrically by the addition of n standard arsenic(111) solution. The end-point potential in these titrations was the same as that observed during the standardization of the iodine solutions (+0.27 volt us. SCE). The results obtained are listed in Table I. Khen the same procedure was used with a starch end point, less satisfactory results were obtained (Table 11). I n these experiments 6 ml. of starch solution were added just before the end point was reached and the reaction vessel was a 250-ml. Erlenmeyer flask. When large amounts of cobalt were present, the bulky green precipitate of iodopentamminecobalt( 111) nitrate made the end point difficult t o observe. By filtering the precipitate onto a mediumpore, sintered-glass Gooch crucible, washing with two small portions of 2 M ammonium nitrate, and combining the washings with the original filtrate, a sharper end point was obtained.
W
H I 3 iodine is added to ammoniacal solutions of cobalt containing ammonium nitrate, chloride, bromide, or iodide, the formation of the corresponding salt of iodopentamminecobalt(II1) occurs (9). The preparation of t,hese compounds is direct' and rapid, and does not require the exclusion of air. Because the direct ferricyanide titration of cobalt either as the cobalt( 11)-ammonia ( 1 , 6) or the cobalt(I1)-ethylenediamine ( 2 ) complex does require the removal of air from the initial solutions, it seemed worth while to investigate t'he analytical j)ossibilities of the reaction involved in the formation of the iodo~ientamminecohalt(II1j salts. Further studies are needed. EXPERIMENTAL
INTERFERENCES WITH METHOD
I t was found that iodopentamminecohalt(II1) is stable in the presence of arsenic(II1). ;iccordingly, an excess of iodine was used in each experiment and, after the reaction had occurred, the solutions were titrated with standard arsenic(II1). Because the solutions of the iodo complex ion are yellowish green and the iodo complex hydrolyzes to form the red-violet hydroxopentamminecobalt(III), the end point. of the iodine-arsenic reaction was determined potentiometrically. It was subsequently found that the starch end point could also be used. Attempts t o determine cobalt gravimetrically by the formation of iodopentamniinecobalt(111) salts were unsuccessful.
Although an exhaustive study of the metals and of the various comple\ing agents was not made, it is evident that any substance which can oxidize iodide or cobalt (permanganate, persulfate, etc.) or reduce iodine (cyanide) must be absent. Many metals which interfere in their lower oxidation states do not do so in their highest oxidation state. These include arsenate, antimonate, chromate, molybdate, vanadate, and tungstate. Cad-
Table I.
Apparatus. A Beckman RIodel G p H meter was used as a potentiometer. Bright platinum and calomel electrodes having 24-inch shielded leads were used. The titrations Tere done in a three-necked 200-ml. round-bottomed flask and the solutions were magnetically stirred. Reagents. C ~ B A L T ( ISULFATE. I) A desired amount of reagent grade cobalt(I1) sulfate was dissolved in 2 liters of distilled water. The solution was standardized by determining the cobalt mntent in 100-nil. aliqtiots by the method of Smith ( 5 ) . A precision iTithin 0.02570 was observed. Aliquots of the stock solution \yere diluted as required.
Potentiometric Determination of Cobalt
Cobalt Taken, JIg. 64 68
16,17
3.234
IODISE.The standard technique for preparing iodine solutions from resublimed iodine and reagent grade potassium iodide was followed. Hon-ever, it was found that, in the ammoniacal solutions better result,s were obtained when t,wice the usual amount of potassium iodide was present. Thus for the preparation of 0.05S iodin? solutions, 6.3 grams of iodine and 25 grams of potassium iodide per liter of solution Tvere used. T h e iodine solutions were standardized potentiometrically against standard arsenic(II1) using sodium bicarbonate. The same titer was observed with an ammonia-ammonium ion buffer a t a pII of 9. These solutions xvere restandardized each t,ime they were used. STASDARD ARSESIC(III). A solution of 0.10005 arsenic(II1) was prepared from Sational Bureau of Standards arsenic trioxide according to their directions. Aliquots viere diluted to prepare solutions of 0.05000 and O.OIOOOaV arsenic(II1). Potentiometric Method Adopted. To 25 ml. of a solution containing from 1 t o 260 mg. of cobalt in a round-bottomed, tlirec-necked flask add 25 grams of ammonium nitrate and 26 ml.
Table 11.
PH 8.8 9.0 8.8 9.0 8.8 8.9 8.9 8.9 9.0 9.0 9.0 9.0
Cobalt Found, hIg. 64,65 64,68 64.71 64.71 16.16 16.1.5 16.18 16.18 3.243 3.219 3.215 3,227
LIg.
59.00
-n. in -0.20 0.10 0.10 0.28 -0.46 -0.60 -0.22
Cobalt Found, hI@. 58.55 58.72
Relative Error,
58.88
-0.25" 0.225 -0.3Za -0.77 0.20 -0.86 -0.60 -1.16 -0.60 -0.24 -0.78
69.13 59,80
11.71 11.82 11.76 11.73 2,950 2 91.5 2.932 2 943 2 927 Precipitate filtered before titrating. 1 1 80
91
% -0.05 0.00 0.11 0.11
Determination of Cobalt Using Starch Indicator
Cobalt Taken,
a
Relative Error,
70
-0.73 -0.,46
92
ANALYTICAL CHEMISTRY
mium, copper, nickel, and zinc which form ammino complexes or mercury(I1) which forms an iodo complex do not interfere in the presence of excess ammonia or iodide, respectively. However, elements which form insoluble hydroxides or iodides in ammoniacal solutions will carry down cobalt and interfere n-ith the proposed method. Metals in this group include aluminum, bismuth, chromium, iron, tin, silver, lead, mercury(I), and manganese. The results of a nuniher of experirnenh on the prevention of interference by various metallic ions on the proposed iodometric determination of cobalt are summarized in Table I11 and are discussed in more detail below. Effect of Presence of Nickel, Copper, Zinc, and Cadmium. These elements form ammino complexes and, as a result, lower the pH of the solutions. It was found that when t,he p H of the s7lution \vas less than 8.5, the rate of formation of iodopentamminecobalt(II1) was slow and the analysis was low. This can be remedied by the addition of sufficient ammonia to raise the p H to 9 (Table IV). Because of the deep blue color of the copper-ammine complex ion, the starch end point cannot be used in the determination of cobalt in the presence of copper. Effect of Presence of Silver, Lead, and Mercury. Silver, lead, and mercury( I) will precipitate as the iodides in acid solutions. The precipitates do not occlude cobalt. However, they cause a loss of iodide ion and increase the hydrolysis of iodine in the :immoniacal solutions and lon- results in the cobalt determination will be obtained. This may be remedied by precipitating these elements by t'he addition of pot,assium iodide before the addition of the iodine solutions. Khen mercury(I1) is present, t,he stable mercury-iodide comples is formed and more iodidc must also be added. Effect of Presence of Aluminum, Bismuth, Chromium, Iron, and Tin. I n the presence of these elements which forti1 insoluhlc hydroxides in ammoniacal solutions it was necessary to add 5 eomplexing agent in order to prevent the occlusion of cobalt (Table V). Fluoride, oxalate, and pyrophosphate, n-hirh do not interfere with the formation of iodopentamminecobalt(III), !rere unsatisfactory. Ethylenediaminetetraacetic acid forms a very stable complex n-ith cobalt(I1) and interferes with the formation of the iodo complex. The oxidation of the cobaltethylenediaminet,etraacetjc acid complex by iodine occurs very slowly. .Uthough citrate does not interfere with the formation of iodopentamminecohalt(III), its reaction with iodine to form tetraiodoacetone and iodoform is cat,alyzed b y the presence of cobalt as well as other nictals ( 4 ) . Thus in the presence of chromium, iodoform is quickly formed. Satisfactory result's can be obtained by using tartrate, which is added as t'artaric acid to the initial solutions. The result,s of a number of experiments on the determination of cot)alt in the presence of chromium are listed in Table V. Similar oliservations were made n-ith aluminum, bismuth, irori, and tin. Manganese. I n the ammoniacal solutions manganese( 11) T?-as oxidized by iodine to a mixture of manganese(II1) and manganese(1V) hydroxides. I n the presence of tartrate oxidation of manganese(I1)-tartrate to manganese(II1)-tartrate complex occurred. I n the absence of oxygen only partial oxidation occurred. Attempts to prevent completely the oxidation of manganese(I1) in the presence of tartrat,e x-ere unsuccessful. The reaction appeared from the few experiments carried out here to be oxygen catalyzed. I n order to determine cobalt directly by the format,ion of iodopentamminecobalt(III), manganese must be removed as the dioxide in st,rongly acid solutions. DISCL-SSIOS
The formation of iodopentaniminecobalt(II1) may be written as follows:
2Co++
+ 10 SHa
A
I;
=
2Co(XH3)jIf+
+ I-
(1)
This reaction is reversible (8) and n-hen the reduced solutions
Table 111. Effect of Metallic Ions on Iodometric Determination of Cobalt Amount,
Ion
ME.
Interference
CII. Cd, Ni, Zn
100 500
H p ( I ) ,€Ig(II),P b , Ag
100 io0 100
+ ++
AI, Bi, C r U I I ) , Fe, Sn AIn
Reagent Required
-
Addn. airiiiionia
-
Addn. iodide Tartrate
-
25
Table IV. 4nal)sis of Cobalt Solutions Containing Cadmium, Copper, Nickel, Zinc, and \lercur) Cohalt Present,
Element Present,
M g
.\Ig
PH
Cobalt Found, hlg
Pio of Cxpts.
0 00 6 20 6 42 6.38 6.43 6.37
2 2 2
0 0 0 37
2
6 40 R 40 i, 40 6.40
a
Copper 1320 R Copper 1320 8 4 Coppei 1320 8 8 Cadmium 200 8 8 ii , 4 0 Nirkel 224 9.0 0.40 Zinc 224 8.8 r, 40 bIercury(I1) 100 8 8 1; 40 Mercury(I1) 100 9 0' 0.300 gram of potassium iodide added
2
'Tahle V. Determination of Cobalt in Presence of Chromium C o h u l t Present, Llg. b4 . 0 ti4 0 ii4.0 ii4 . 0 til.
0
Ij4,O 'l
1,
Chromiiilii Present, lIg.
100 100 100 100
Coniplexing Agent
Cobalt Found, llg.
3-0. Expts. of
5 ,2
2 2
9.15
EDT.l Citrate Tartrate
100
100
62b
2.4 74 63 8
i:2 7
.imount of cobalt after a-uinic of ciirolniuin hydroxide for 1 hour. Cobalt added after preriliitation of chromium.
itre allowed to stand, the starch-iodine color slowly forms. This may be the cause of the low results observed when the amount of cobalt present i? of the order of 10 nig. or less. I n order to prevent the reduction of iodopentamniinecobalt(II1) bj- iodide ion a series of experiments !vas carried out using iodine dissolved both in alcohol and in concentrated ammonium bromide solutions. Under these conditions iodine will hydrolj-ze to a greater extent than it will in solutions containing iodide ion and the hypoiodite ion formed may be air oxidized to iodate ion ( 3 ) . Because the latter ion does not react, x i t h cobalt(II), these experiments were performed in a hydrogen atmosphere. Although a number of factors such as the order of addition of the reagents and the length of time, the temperature and the p H of the reaction xere also varied, only 30 to 95% of the cobalt present could be found. The oxidation of cobalt(I1) by hypoiodous acid ion was also investigated. Iodine monochloride, iodine dissolved in silver nitrate solutions, and iodine shaken with mercuric oxide were used as sources of hypoiodous acid. The reactions were carried out in oxygen-free solutions buffered with ammonia and ammonium salts at pH's of 9 to 9.5. K i t h small amounts of cobalt the same yellow-brown color that occurs during the air oxidation of cobalt-ammonia solutions was observed; in more concentrated solutions both iodopentamminecobalt(II1) nitrate and decammine p-peroxo dicobalt(II1) nitrat,e were formed. Separate experiments with the latter binuclear complex prepared according to the direction of Werner ( 7 ) showed that it reacts rapidly and irreversibly with solutions of iodine according to the equation:
(SHS)jCoOOCo(XH:~);--+
+ 12
02
+ 2Co(IKH3)51-'+
(2)
The occurrence of these reactions, therefore, would give high rather than low cobalt, resulte.
93
V O L U M E 2 8 , N O . 1, J A N U A R Y 1 9 5 6 Table VI.
Effect of Oxygen on Determination of Cobalt
(Rate of huhhling, 2 nil. per minute) Cohalt Found," Time, Cobalt Present, Rlg. 31g. Minutes 11.80
11.78 11.75 11.55
1 20 120
No. of
Expts. 2 2 2
standard iodine solution were added and the excess iodine was determined potentiometrically in the usual way. The results of these experiments, recorded in Table VI, indicate that decammine M-peroxo dicobalt(II1) ion is very stsablein the dilute cobalt and ammonia solutions present in the proposed analytical procedure.
a Iodine solutions added a f t e r oxygen was bubbled through ammoniacal solutions of cobalt.
If, on the ot,her hand, decanimine p-peroxo dicobalt(II1) v-ere formed by the air oxidat,ion of cobalt(I1) ammines and subsequently x a s converted, perhaps catalytically, int,o hydroxopent,amminecobalt( 111) or hexamminecobalt(II1) by some react.ion other than that of Equation 2, then low cobalt results would be served. A series of experiments was carried out in order to study the stability of decammine M-peroxo dicobalt(II1) using the following procedure: Oxygen, which had previously been passed through a solution of ammonium hydroxide, was bubbled through 25 ml. of a solution of cobalt sulfate containing 25 grams of ammonium nitrate and 5 ml. of concentrated ammonia a t the rate of 2 ml. per minute. After a predetermined length of time, 25 ml. of a
4CKYOW LEDGlRI ENT
The author \Tishes to thank Allan Fischlowitz and Delmrah Edelman for making marly of the cobalt analyses. LITERATURE CITED
P., and Maassen, G . . A r c h . Eisenhiittenzo. 9, 487 (1935). bsa~. C H E X 27, 777 (1955). (2) Diehl, H., and Butler. J . P., (3) LIc.ilpine, R. IC.,J . Chem. Educ. 26, 362 (1919). (4) Qureshi, lI.,and Veeraiah, K., Current Sei. (India) 15, 132 (1) Dickens.
(1946).
(5) Smith, J. D. hI., Chern. In&. 44,5391 (1925). (6) Tomicek. 0..and Freiberner. F.. J . A m . Chem. Soc. 57.801 11936). ( 7 ) Terner, d.,Ann. 375, 1 (7910) ( 8 ) Yalman, R. G., J . Am. Chem. Soe. 75, 1842 (1953). (9) Yalman, R. G., Ibid.,77, 3219 (1955). RECEIVED for review July
21, 1955.
Accepted Octoher 6, 1955.
Interaction of Platinum Group Elements with 1,2,3-Benzotriazole RAY
F. WILSON
and LOUBERTA E. WILSON
D e p a r t m e n t o f Chemistry, Texas Southern University, Houston, T e x .
This paper is part of a general study of the interactions of the platinum metals with 1,2,3-benzotriazole. Methods for the prai irrietric and for the amperometric determination of palladium(I1) chloride, in acetic acid-sodium acetate buffer, have been developed using 1,2,3-benzotriaaole as a precipitant. From 3 to 60 mg. of palladium u ere determined gravimetrically with an average error of less than 0.1 mg. The amperometric method has been applied to 0.2 to 6 mg. of palladium. When the concentration of palladium is greater than 0.2 mM, the a\erage relative analytical error is within &0.3%. The methods are recommended primarily for the accurate determination of chloride solutions of palladium containing only traces of other platinum elements.
W
studying the interaction of palladium (11) and 1 2 3-benzotriazole (15)i t was observed that palladium r a s HI? precipitated quantihtively in this reaction. 1,2,3-Benzotriazole has been studied as a precipitant for silver by Remington and \Toyer (11), Tar:isevich ( I Z ) , and Cheng ( 2 ) . Curtis (4)has ex;%minedthis reagent :is :L precipitant for copper. These studies indicate t,hat iron(I1 j , nirkel(IT), cobalt(II), zinc, and cadmium :ire also previpitated l)j, this reagent. When an excess of 1,2,3-benzotriazole is added to an acetic acid-sodium acetittc huffer solution containing palladium(I1) c.hloride in the ahsencc or presence of sodium dihydrogen (ethylenedinitri1o)tetraacetic acid. a white colored coordination compound is formed, which, n-hcn dried, corresponds to the formula Pd(C6H4NHNl)pCI? (151. In excess palladium, 1,2,3-benzotriazole forms a reddish-hron-n colored coordination compound; its formula is Pd(C,H,SHS,)Cl? ( 1 5 ) . GRAVIIUETRIC DETERJIINATION OF PALL.IDIUM The gravimet'ric determination of palladium has been t h e subject of several investigations during the past few decades. Pal-
ladium has been determiiirtl gravimetrically xith salicylaldosime ( 7 ) ) with p-furaldoxime ( ~ 7 ) n-ith ~ 1,2-cyclohexanedione ( l e ) , with dimet,hylglyosime (Io),and with phenylthiourea, thiophenol, and thiobarbituric acid ( 3 ) . Of these gravimetric methods, the dimethylglyosinie procedure has been used very widely. I t was the purpose of this investigation to ascertain the possibility of obtaining reproducible and sufficiently accurate stoichiometric results in the direct gravimetric determination of palladium(I1) xith l ,2,3-benzotriazole in acetic acid-sodium acetate buffer, and to determine the estent of interferences from certain diverse ions on the determiriation of palladium. EXPERIIIENTAL
Reagents and Solutions. A 5-gram sample of palladiuni(I1) chloride, obtained from Coleman and Bell Co., was dissolved in 10 ml. of concentrated hydrochloric acid and diluted t o 1 liter with distilled water. This solution was standardized gravimetrically, using modifications of the Gilchrist-\Tichers procedure (6). Reagent grade rhodium( III), iridium( IV), platinum( IV), ruthenium( III), iron( I11 chromium( 111), aluminum( 111), zinc, magnesium, nickel(II), and cobalt(I1) chlorides \\-ere used to prepare solutions of these elements. Osmium(VII1) solution was prepared from osmium tetroxide, using the procedure of A y e s and Wells ( 1 ) . Platinum(I1) solut'ion was prepared from potassium tetrachloroplatinate(I1). Gold(II1) solution was prepared from chloroauric acid trihydrate. 1,2,3-Benzotriazole, East,man Kodak Chemical KO.2759, was recrystallized tn-ice from chloroform and dried a t room temperahre. A n-eighed amount n-as dissolved in 125 ml. of glacial acetic acid and diluted to 250 ml. with distilled water. The concentration of this solution was checked by modifying the method of Cheng ( 2 ) ; the results obtained hy this method agree closely with the calculated concentration of 1,2,3-benzotriazole. (Ethylenedinitrilo jtetrnacetic acid (Versenate) solution n-as prepared by dissolving 10 grams of the disodium salt of (ethylenedinitri1o)tetraacetic acid, analytical reagent grade from T'ersenes, Inc., in 1 liter of distilled water. -4buffer solution was prepared, which was 251 in acetic acid and in sodium acetate.
