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Precipitation of nickel sulphide from ammoniacal solutions by thioacetamide
Talanta. 1965, Vol. 12. pp. 357 to 362. Pergamon Press Ltd.
PRECIPITATION OF NICKEL SULPHIDE FROM AMMONIACAL SOLUTIONS BY THIOACETAMIDET DAVID
H. KLEIN,$’ DENNIS G. PETERS* and ERNEST H. SWIFT@ California Institute of Technology, Pasadena, California, U.S.A. (Received 23 July 1964. Accepted 13 December 1964)
Summary-The
rate of precipitation of nickel sulphide from a&noniacal solutions by thioacetamide is controlled mainly by two types of reaction : reactions betw=en the various nickel species and thioacetamide, and reactions between the nickel species and the sulphide produced by a thioacetamide-ammonia reaction. The sulphide produced by the hydroxide-catalysed hydrolysis of thioacetamide is relatively small. The rate expressions for these two predominant processes are, respectively, d[Ni(lI)] - = $0 k,lNi(NH,),P+l[CH~CSNH~IIH+]-a~L dt and -
v
= k[CH,CSNHJNH,]‘.
INTRODUCTION PREVIOUS
studies of the rates of precipitation of metal sulphides by thioacetamide In one (TAA) have indicated that two processes are involved in these precipitations. of these the TAA reacts with some constituent of the solution to liberate sulphide, which then forms the precipitate; in such cases the rate of sulphide formation limits the rate of the precipitation. Precipitation by the other process occurs without the intermediate ‘formation of sulphide from TAA. In the latter case the reaction appears to take place directly between the metal ion and TAA; this process is called the direct reaction. Within the accuracy of the measurements, the rate of precipitation of nickel sulpbide from acidic solutions has been found to be controlled by the direct reaction only.2 The rate equation for this precipitation at 90”, in units of moles, litres and minutes, is d [Ni2+] - = 2-2 x 10”’ [Ni2f][CH,CSNH,][Hf]-1’2. dt An experimental studyS of the precipitation of zinc sulphide by TA&from solutions buffered by ammonia and ammonium ion has indicated that the hydrated* zinc ion and the zinc ammines are precipitated via both routes. The direct reaction with each zinc species is characterised by a different value of the direct-reaction rate constant. A study of the precipitation of nickel sulphide by TAA from ammoniacal solutions is presented here. In such solutions, sulpbide is produced predominantly by a TMammonia reaction, the rate of which depends upon the square of the ammonia t Contribution No. 3145. $ Communications regarding this manuscript should be addressed to David H. Klein, Department
of Chemistry, Hope. College, Holland, Michigan, U.S.A. * Present address: Department of Chemistry, In.diana University, Bloomington, Indiana, U.S.A. 357
concentration. Washizuke4 has reported that precipitation occurs by a different mechanism in more concentrated ammonia solutions (0+1*2M); in these solutions the rate of precipitation of nickel sulphide appears to be dependent on the rate of a base-catalysed reaction between TAA, NH, and OH. fn the present systems, and in those studied by Washizuke, the rate of pr~ipitation by processes other than the direct reaction can be controlled by varying the ammonia concentration* EXPERIMENTAL Re~enls
Reagent-gradechemicals were used throughout. The TAA was Arapahoe Chemical Company material. Stock solutions were prepared and, when necessary, were standardised by conventional methods. The reaction solutions were prepared from distilled water and stock solutions of ammonia, ammonium nitrate and nickel@) nitrate. The ionic strength of the reaction solutions was adjusted to 0.30 with sodium nitrate.
The solution, contained in a 38 x WI-mm stoppered test tube, was preheated to siightly above W, the appropriate volume of TAA solution at room temperature added and the tube immersed in a water bath at 40” f 09. At timed intervals a sample of the suspension was removed and filtered solid disodium ethylenediaminetetra-acetate through a fritted glass filter into a tube containin (EDTA). The excess solid EDTA was removed by Agtration, and the concentration of nickel in the solution determined calorimetrically. Confirmatory analyses showed that this procedure gave results reproducible to within 3 %. The initial rates of precipitation reported below were obtained graphically from the experimental points. The re.ac&ns were fo8owed on& until a~p~ximat~ly 20% of tbe nicker was precipitated;
this reduced the &“t the nickel ammines.
of the i~rease
in ammonia concentration
RESULTS
AND
caused by release of ammonia from
DISCUSSION
In the subsequent discussion, the symbol [Ni(ll)] represents the total (volume formal) concentration of nickel in solution; therefore
The ammonia concentrations
shown in the rate expressions are for free ammonia,
Ammonia-thioacetamide reaction The rate of precipitation of nickel sulphide as a result of this reaction was assumed to be equal to the calculated rate of formation of sulphide. The rate expression is therefore
The value of the rate constant k at 40” has been found* to be 54 min-‘.
x
10-a lit&. mole-“.
Table I presents the results of experiments in which the initial nickel and TM concentrations were varied at constant PI-# and free ammonia ~~~~a~o~. The results indicate that the direct reaction rate is dependent on the Grst order of both Ni(II) and TAA concentration.
Precipitation of nickel sulphidc
359
Eflect of pH The rate of the direct reaction was studied over the pH range of 8.67 to 9.49. As shown in Table 1, at these hydrogen-ion concentrations, the rate appears to beinversely proportional to the two-thirds power of the hydrogen-ion concentration. TABLE
I.-EFFECT
OF NICK~L,THIOACETAMIDE AND HYDROGEN-ION CONCENTRATIONSON RATE OF PRECIPITATION OF NICKEL SULPHIDE
THEINFFIAL
Rate of direct TAA,
NW),
[H’]*‘B
VF
VF
pH
0.100 0.200 0.100 0.150 0.100 0.100
0~0100 0.0100 0.0200 0~0100 00100 00100
9.22 9.22 9.22 8.67 8.94 9.49
Observed rate, mole. We-‘. min-’
VM ’
7.1 7.1 7.1 1.7 I.1 4.7
x lo-’ x lo-’ x lo-’ r: lO-e I.~ 10 o 10 i
3.0 6.3 6.0 3.2 2.1 4.7
x x x x X .:
10-h 1O-5 1O-5 lO-5 10-b 10-i
(Free NH8 =y 0 08OVM;
: Calculated from observed rate by subtracting
reaction,’ mole. litre-I. min-’ 27 5.7 5.7 2.7 I.8 4.4
x x x x x x
10-e 10-6 10-J IO-” IO-5 10-s
temperature = 40”.) the rate of the NH,-TAA
K,b mole. litre-I. min-1 1.9 2.0 2.0 2.0 2.0 2.1
x x x x x x
10-a 10-a 10-e 10-n 10-e 10-8
reaction, 5.0
x
lo-*
[CH,CSNHJ [NH,]‘. b Calculated from the expression K = rate of direct reaction multiplied by [H+]p’S [CH,CSNH,][Ni(ll)] ’
E$ect of ammonia concentration Nickel in aqueous solution forms six ammine complexes, the general equilibrium expression for their formation being
[NW-4Jn2+l [Ni(NH,),_JNHs]
= Kn .
Bjerruml has determined the values of K,,. Therefore, for each concentration of free ammonia, the relative concentration of each nickel ammine and the average number of ammonia molecules co-ordinated to each nickel ion, ii, could be computed. This information is presented in Table II. Table III shows the dependence of the initial rates of the over-all precipitation and the direct reaction on the free ammonia concentration. At free ammonia concentrations greater than approximately 0.3 molal, the rate of the ammonia-TAA reaction becomes so fast that, the direct reaction process makes a relatively small contribution to the over-all precipitation rate. The observed relationship between rate and ammonia concentration for the direct reaction may be explained by assuming that each nickel species reacts at a different rate. In principle, therefore, a series of simultaneous equations of the form - y
= [CHsCSNH2](ke’[Ni2+] + k,‘[Ni(NHs)2+] +
ks’PWH2)2s+l + . . . + ka’lYiWH~s*l)
could be set up and solved for the value of kn’. It was observed in the analogous case of zinc,* however, that the ratios of successive rate constants were approximately constant, i.e., that k,,‘/k,’ = k,‘/ks’ = ks’/ks’, etc. In order to simplify the computations, the assumption was made that the ratios of successive rate constants are also
MO
TABLE
Iu.--EFFECT
INITIAL
Q-050
OG60 0.080 0.110 0.150 0.200 0.250 0*2tM~ 0.293”
33. G. I-Wats and E. H. Swrpr
D, H. Ikm,
0~0100 0~0100 O*OloO 0~0100 0.0300 0~02t?O 0~3Ocl 0~0200 0GBO
O~OSaO O*lOO 0~100 0,150 0.100 0*200 0.200 0*100 0*200
KATE
OF FRJX AMMONIA OF PRECIPITATION
2.9 4.5 3.1 3.6 4-4 6-9 7.4 5 13
x 1O-s x 10-s x 10-e x 1O-g x 10-S x 1O-s X 104 x 10-s X 10-S
CONCENTRATION OF NICKEL
2.8 4.3 28 2.7 3.3 2.9 I 3 4
x x x x x x x x x
1O-6 10-s 1O-6 IO-6 10-5 10-s IO-” 10-6 10-e
ON
THE
SULPHIDE
5.6 4.3 2.8 1.8 1.1 0.72 0.18
x 10-e x IO-2 x
x x x x -
m-a
10-a 10-s IO-8 lo-*
2.8 4.3 2.8 2.5 3.3 2.9 3 2 4
x x x x x x X X x
10-g 10-s 10-S 10-J 10-J 10-S 10-s 10-s 10-S
fpH = 9-22; temperrrature= 40”) * Computed from observed rate by subtracting the rate of the NE&-TM reaction; 5-O x lo-$ [CH&SNH,J [NH,]‘. Rate of direct reaction 0 Computed from the expression K’ = [CHJ!SNH,] [Ni(II)J ’ c Computed from the second-order rate constants for each nickel ammine species. BpK = 10-04.
for the nickel ammines. Based on this assumption the best values of the individual second order rate constants, ka’, expressed in litre. mole-l. min-l, are
constant
k,’ = 2.9 k,’ = 540 x 10-l k,’ = 8.6 x 1O-2 kS’ = 1.5 x lO-2 k; = 2.5.x 1O-a lC; = 44 X lo”’ b’ = 7.5 x 10-e. The rates of the direct reaction calculated on the basis of these rate constants are also presented in Table III.
The true rate constants, knr were obtained from the second order constants, 4’, measured at pH 9.22 and 40”. The best values for these constants, in units of moles, l&es and minutes, are incorporated into the following equation, which describes the
Precipitation of nickel sulphide
over-all rate of precipitation a&amide at 40”.
361
of nickel sulphide from ammoniacal solution by thio-
$_ 3.5 X lO-‘~i~H~~~] x 1O??WHa),z+j
+ 6-O x l~~i~H~a~]
+ 1.0
+ 1.8 x 10-g[Ni(NHa)4@] + 34
x 10-10[Ni(NH3)~+] + 5.3 x lW11[Ni(NH3),rB+]). Efect of temperature To determine the dependence of the pr~ipi~tion on temperature, experiments were performed at 25”, 40°, 50”, 60’ and 70” with solutions in which the initial concentrations of the reagents were as follows: [Ni(II)] = @02OOVF, CH3CSNH, = 0*05OOVP, NH&l = 0.06OVF; total initial NH, = 0*38VF, and calculated free ligand NH, = O-29VM. The initial rates for each temperature are given in Table IV. Because of uncertainties concerning the values of the formation constants of the nickel ammines at the higher temperatures, and because of the likelihood of volatilisation of some of the ammonia, no attempts were made to calculate the temperature dependence of the ammonia-TM or the direct-reaction processes. TAB&EW.-EFFECT OF TEMPERATURE ON RATEOF PRECXPITA=ON
Temperature. “C z 50 60 70
Initial over-all rate of precipitation, x 106 0.9 4.5 1: 50
Initial Conditions: NW) = 0.02OVF; CH&SNH,= 095OVF; NH&l = 0.06OVF; free ligand NHI = @29VM; total initial NH* = 0*38VF. ANALYTICAL
CONSIDERATIONS
These studies have shown that the rate of precipitation of nickel sulphide by thioacetamide can be varied over a wide range by the proper selection of the reaction temperature and the ~n~ntrations of the reagents. For example, calculations indicate that in a solution initially 0.01 VF in nickel(II), O*lOVF in tbioacetamide, 0*3OVF in free ammonia and having a pH of 10, the quantitative precipitation of nickel sulphide should require about 700 min at 40” ; however, at 90” the precipitation should be quantitative within about 5 min. The use of thioacetamide for the precipitation of nickel sulphide appears to have several advantages over the conventional hydrogen sulphide or ammonium sulphide p~ipi~tions. When a solution confining nickel nitrate, aqueous ammonia, arnmomum chloride and thioacetamide is heated at W, a dense, granular precipitate of nickel sulphide is obtained and the clear, colourless supernatant can be easily separated from the precipitate by decantation. Thus, the precipitation of nickel sulphide by
D. H. KLBIN, D. G. Pmzna and E. H. SWIFT
362
thioacetamide avoids the troublesome aspects of the ammonium sulphide precipitation, namely (I) : the voluminous precipitate which can be filtered only with great difficulty, (2) the yellowish to black colour of the filtrate caused by the presence of colloidal nickel sulphide, and (3) the lengthy procedure required to coagulate this precipitate. However, when thioacetamide is employed for the precipitation of sulphides and where subsequent operations are to be made with the same solution, the effects of the excess thioacetamide together with its hydrolysis and reaction products must be considered. It seems possible that thioacetamide could be used to advantage as a substitute for hydrogen or ammonium sulphide in the separation of zinc, nickel and cobalt as sulphides from ammoniacal solutions in which aluminium, chromium and in some cases manganese, are present as tartrate or oxalates complex ions. Before thioacetamide can be employed for such separations, however, it will be necessary to determine experimentally the behaviour of each of the elements involved under the conditions to be used for the separation. Acknowle@nenrs-The authors are grateful for financial support from the National Science Foundation during the course of this investigation. Discussions with Fred C. Anson and Dwight M. Smith have been helpful in the preparation of the manuscript. B-Die Geschwindigkeit der F&hung von Nickels&id aus ammoniakalischen LiJsungen mit Thioacetamid wird hauptSHchlich dumb zwei Reaktionstypen kontrolliert : Reaktionen zwischen den verschiedenen Nickelspezres und Thioacetarnid und Reaktionen zwischen Nickel und dem aus der Reaktion zwischen Ammoniak und Thioacetamid stammenden Sulfid; die aus der durch Hydroxylionen katalysierten Hydrolyse von Thioacetamid stammende Sulfidmenge. ist relativ klein. Die Geschwindigkeitsausdriicke fur die beiden vorherrschenden Prozesse sind d[Ni(lI)] - dt
= nio k,[Ni(NH&,*+][CHICSNH,][H+]-*/*
dN(lOl
- -
dt
= k[CH,CSNH,][NH,]*.
R&us&--On contr8le la vitesse de presipitation du sulfure de nickel a partir de solutions ammoniac&s au moyen de thioac&amide essentiellement par deux types de reactions: reactions en& les differentes esp&ces du nickel et la thioacttamide, et &actions entre le nickel et le sulfure produit par une reaction thioac&amide-ammoniaque. (Le sulfure produit par l’hydrolyse du thioa&amide catalys&e par I’hydroxyde est relativernent faible.) Les expressions de la vitesse pour ces deux processus p&dominants sont, respectivement : d]Ni(II)] - =nie dt _
k,[Ni(NH8),*+][CHsCSNHs][H+]-*‘s
et -
d[Ni(Il)] = k[CH,CSNH,][NH,]* dt
.
REFERENCES i J. Bjerrum, Metal Ammine Formation in Aqueous Solution. P. Haase and Son, Copenhagen, 1941. * D. F. Bowersox, D. M. Smith and E. H. Swift, Talanta, 1959,2, 142. * D. H. Klein and E. H. Swift, ibid., 1%5,12, 349. 4 S. Washizuke, Jqan Analyst, 1963, 1520. 6 G. F. F. Lundell, H. A. Bright and J. I. Hoffman, Applied Inorgat& Analysis. Wiley and Sons, New York, 1953. * E. H. Swift, System of Chemical Analysis. Freeman and Co., San Francisco, 1939.
PRECIPITATION OF NICKEL SULPHIDE FROM AMMONIACAL SOLUTIONS BY THIOACETAMIDET DAVID
H. KLEIN,$’ DENNIS G. PETERS* and ERNEST H. SWIFT@ California Institute of Technology, Pasadena, California, U.S.A. (Received 23 July 1964. Accepted 13 December 1964)
Summary-The
rate of precipitation of nickel sulphide from a&noniacal solutions by thioacetamide is controlled mainly by two types of reaction : reactions betw=en the various nickel species and thioacetamide, and reactions between the nickel species and the sulphide produced by a thioacetamide-ammonia reaction. The sulphide produced by the hydroxide-catalysed hydrolysis of thioacetamide is relatively small. The rate expressions for these two predominant processes are, respectively, d[Ni(lI)] - = $0 k,lNi(NH,),P+l[CH~CSNH~IIH+]-a~L dt and -
v
= k[CH,CSNHJNH,]‘.
INTRODUCTION PREVIOUS
studies of the rates of precipitation of metal sulphides by thioacetamide In one (TAA) have indicated that two processes are involved in these precipitations. of these the TAA reacts with some constituent of the solution to liberate sulphide, which then forms the precipitate; in such cases the rate of sulphide formation limits the rate of the precipitation. Precipitation by the other process occurs without the intermediate ‘formation of sulphide from TAA. In the latter case the reaction appears to take place directly between the metal ion and TAA; this process is called the direct reaction. Within the accuracy of the measurements, the rate of precipitation of nickel sulpbide from acidic solutions has been found to be controlled by the direct reaction only.2 The rate equation for this precipitation at 90”, in units of moles, litres and minutes, is d [Ni2+] - = 2-2 x 10”’ [Ni2f][CH,CSNH,][Hf]-1’2. dt An experimental studyS of the precipitation of zinc sulphide by TA&from solutions buffered by ammonia and ammonium ion has indicated that the hydrated* zinc ion and the zinc ammines are precipitated via both routes. The direct reaction with each zinc species is characterised by a different value of the direct-reaction rate constant. A study of the precipitation of nickel sulphide by TAA from ammoniacal solutions is presented here. In such solutions, sulpbide is produced predominantly by a TMammonia reaction, the rate of which depends upon the square of the ammonia t Contribution No. 3145. $ Communications regarding this manuscript should be addressed to David H. Klein, Department
of Chemistry, Hope. College, Holland, Michigan, U.S.A. * Present address: Department of Chemistry, In.diana University, Bloomington, Indiana, U.S.A. 357
concentration. Washizuke4 has reported that precipitation occurs by a different mechanism in more concentrated ammonia solutions (0+1*2M); in these solutions the rate of precipitation of nickel sulphide appears to be dependent on the rate of a base-catalysed reaction between TAA, NH, and OH. fn the present systems, and in those studied by Washizuke, the rate of pr~ipitation by processes other than the direct reaction can be controlled by varying the ammonia concentration* EXPERIMENTAL Re~enls
Reagent-gradechemicals were used throughout. The TAA was Arapahoe Chemical Company material. Stock solutions were prepared and, when necessary, were standardised by conventional methods. The reaction solutions were prepared from distilled water and stock solutions of ammonia, ammonium nitrate and nickel@) nitrate. The ionic strength of the reaction solutions was adjusted to 0.30 with sodium nitrate.
The solution, contained in a 38 x WI-mm stoppered test tube, was preheated to siightly above W, the appropriate volume of TAA solution at room temperature added and the tube immersed in a water bath at 40” f 09. At timed intervals a sample of the suspension was removed and filtered solid disodium ethylenediaminetetra-acetate through a fritted glass filter into a tube containin (EDTA). The excess solid EDTA was removed by Agtration, and the concentration of nickel in the solution determined calorimetrically. Confirmatory analyses showed that this procedure gave results reproducible to within 3 %. The initial rates of precipitation reported below were obtained graphically from the experimental points. The re.ac&ns were fo8owed on& until a~p~ximat~ly 20% of tbe nicker was precipitated;
this reduced the &“t the nickel ammines.
of the i~rease
in ammonia concentration
RESULTS
AND
caused by release of ammonia from
DISCUSSION
In the subsequent discussion, the symbol [Ni(ll)] represents the total (volume formal) concentration of nickel in solution; therefore
The ammonia concentrations
shown in the rate expressions are for free ammonia,
Ammonia-thioacetamide reaction The rate of precipitation of nickel sulphide as a result of this reaction was assumed to be equal to the calculated rate of formation of sulphide. The rate expression is therefore
The value of the rate constant k at 40” has been found* to be 54 min-‘.
x
10-a lit&. mole-“.
Table I presents the results of experiments in which the initial nickel and TM concentrations were varied at constant PI-# and free ammonia ~~~~a~o~. The results indicate that the direct reaction rate is dependent on the Grst order of both Ni(II) and TAA concentration.
Precipitation of nickel sulphidc
359
Eflect of pH The rate of the direct reaction was studied over the pH range of 8.67 to 9.49. As shown in Table 1, at these hydrogen-ion concentrations, the rate appears to beinversely proportional to the two-thirds power of the hydrogen-ion concentration. TABLE
I.-EFFECT
OF NICK~L,THIOACETAMIDE AND HYDROGEN-ION CONCENTRATIONSON RATE OF PRECIPITATION OF NICKEL SULPHIDE
THEINFFIAL
Rate of direct TAA,
NW),
[H’]*‘B
VF
VF
pH
0.100 0.200 0.100 0.150 0.100 0.100
0~0100 0.0100 0.0200 0~0100 00100 00100
9.22 9.22 9.22 8.67 8.94 9.49
Observed rate, mole. We-‘. min-’
VM ’
7.1 7.1 7.1 1.7 I.1 4.7
x lo-’ x lo-’ x lo-’ r: lO-e I.~ 10 o 10 i
3.0 6.3 6.0 3.2 2.1 4.7
x x x x X .:
10-h 1O-5 1O-5 lO-5 10-b 10-i
(Free NH8 =y 0 08OVM;
: Calculated from observed rate by subtracting
reaction,’ mole. litre-I. min-’ 27 5.7 5.7 2.7 I.8 4.4
x x x x x x
10-e 10-6 10-J IO-” IO-5 10-s
temperature = 40”.) the rate of the NH,-TAA
K,b mole. litre-I. min-1 1.9 2.0 2.0 2.0 2.0 2.1
x x x x x x
10-a 10-a 10-e 10-n 10-e 10-8
reaction, 5.0
x
lo-*
[CH,CSNHJ [NH,]‘. b Calculated from the expression K = rate of direct reaction multiplied by [H+]p’S [CH,CSNH,][Ni(ll)] ’
E$ect of ammonia concentration Nickel in aqueous solution forms six ammine complexes, the general equilibrium expression for their formation being
[NW-4Jn2+l [Ni(NH,),_JNHs]
= Kn .
Bjerruml has determined the values of K,,. Therefore, for each concentration of free ammonia, the relative concentration of each nickel ammine and the average number of ammonia molecules co-ordinated to each nickel ion, ii, could be computed. This information is presented in Table II. Table III shows the dependence of the initial rates of the over-all precipitation and the direct reaction on the free ammonia concentration. At free ammonia concentrations greater than approximately 0.3 molal, the rate of the ammonia-TAA reaction becomes so fast that, the direct reaction process makes a relatively small contribution to the over-all precipitation rate. The observed relationship between rate and ammonia concentration for the direct reaction may be explained by assuming that each nickel species reacts at a different rate. In principle, therefore, a series of simultaneous equations of the form - y
= [CHsCSNH2](ke’[Ni2+] + k,‘[Ni(NHs)2+] +
ks’PWH2)2s+l + . . . + ka’lYiWH~s*l)
could be set up and solved for the value of kn’. It was observed in the analogous case of zinc,* however, that the ratios of successive rate constants were approximately constant, i.e., that k,,‘/k,’ = k,‘/ks’ = ks’/ks’, etc. In order to simplify the computations, the assumption was made that the ratios of successive rate constants are also
MO
TABLE
Iu.--EFFECT
INITIAL
Q-050
OG60 0.080 0.110 0.150 0.200 0.250 0*2tM~ 0.293”
33. G. I-Wats and E. H. Swrpr
D, H. Ikm,
0~0100 0~0100 O*OloO 0~0100 0.0300 0~02t?O 0~3Ocl 0~0200 0GBO
O~OSaO O*lOO 0~100 0,150 0.100 0*200 0.200 0*100 0*200
KATE
OF FRJX AMMONIA OF PRECIPITATION
2.9 4.5 3.1 3.6 4-4 6-9 7.4 5 13
x 1O-s x 10-s x 10-e x 1O-g x 10-S x 1O-s X 104 x 10-s X 10-S
CONCENTRATION OF NICKEL
2.8 4.3 28 2.7 3.3 2.9 I 3 4
x x x x x x x x x
1O-6 10-s 1O-6 IO-6 10-5 10-s IO-” 10-6 10-e
ON
THE
SULPHIDE
5.6 4.3 2.8 1.8 1.1 0.72 0.18
x 10-e x IO-2 x
x x x x -
m-a
10-a 10-s IO-8 lo-*
2.8 4.3 2.8 2.5 3.3 2.9 3 2 4
x x x x x x X X x
10-g 10-s 10-S 10-J 10-J 10-S 10-s 10-s 10-S
fpH = 9-22; temperrrature= 40”) * Computed from observed rate by subtracting the rate of the NE&-TM reaction; 5-O x lo-$ [CH&SNH,J [NH,]‘. Rate of direct reaction 0 Computed from the expression K’ = [CHJ!SNH,] [Ni(II)J ’ c Computed from the second-order rate constants for each nickel ammine species. BpK = 10-04.
for the nickel ammines. Based on this assumption the best values of the individual second order rate constants, ka’, expressed in litre. mole-l. min-l, are
constant
k,’ = 2.9 k,’ = 540 x 10-l k,’ = 8.6 x 1O-2 kS’ = 1.5 x lO-2 k; = 2.5.x 1O-a lC; = 44 X lo”’ b’ = 7.5 x 10-e. The rates of the direct reaction calculated on the basis of these rate constants are also presented in Table III.
The true rate constants, knr were obtained from the second order constants, 4’, measured at pH 9.22 and 40”. The best values for these constants, in units of moles, l&es and minutes, are incorporated into the following equation, which describes the
Precipitation of nickel sulphide
over-all rate of precipitation a&amide at 40”.
361
of nickel sulphide from ammoniacal solution by thio-
$_ 3.5 X lO-‘~i~H~~~] x 1O??WHa),z+j
+ 6-O x l~~i~H~a~]
+ 1.0
+ 1.8 x 10-g[Ni(NHa)4@] + 34
x 10-10[Ni(NH3)~+] + 5.3 x lW11[Ni(NH3),rB+]). Efect of temperature To determine the dependence of the pr~ipi~tion on temperature, experiments were performed at 25”, 40°, 50”, 60’ and 70” with solutions in which the initial concentrations of the reagents were as follows: [Ni(II)] = @02OOVF, CH3CSNH, = 0*05OOVP, NH&l = 0.06OVF; total initial NH, = 0*38VF, and calculated free ligand NH, = O-29VM. The initial rates for each temperature are given in Table IV. Because of uncertainties concerning the values of the formation constants of the nickel ammines at the higher temperatures, and because of the likelihood of volatilisation of some of the ammonia, no attempts were made to calculate the temperature dependence of the ammonia-TM or the direct-reaction processes. TAB&EW.-EFFECT OF TEMPERATURE ON RATEOF PRECXPITA=ON
Temperature. “C z 50 60 70
Initial over-all rate of precipitation, x 106 0.9 4.5 1: 50
Initial Conditions: NW) = 0.02OVF; CH&SNH,= 095OVF; NH&l = 0.06OVF; free ligand NHI = @29VM; total initial NH* = 0*38VF. ANALYTICAL
CONSIDERATIONS
These studies have shown that the rate of precipitation of nickel sulphide by thioacetamide can be varied over a wide range by the proper selection of the reaction temperature and the ~n~ntrations of the reagents. For example, calculations indicate that in a solution initially 0.01 VF in nickel(II), O*lOVF in tbioacetamide, 0*3OVF in free ammonia and having a pH of 10, the quantitative precipitation of nickel sulphide should require about 700 min at 40” ; however, at 90” the precipitation should be quantitative within about 5 min. The use of thioacetamide for the precipitation of nickel sulphide appears to have several advantages over the conventional hydrogen sulphide or ammonium sulphide p~ipi~tions. When a solution confining nickel nitrate, aqueous ammonia, arnmomum chloride and thioacetamide is heated at W, a dense, granular precipitate of nickel sulphide is obtained and the clear, colourless supernatant can be easily separated from the precipitate by decantation. Thus, the precipitation of nickel sulphide by
D. H. KLBIN, D. G. Pmzna and E. H. SWIFT
362
thioacetamide avoids the troublesome aspects of the ammonium sulphide precipitation, namely (I) : the voluminous precipitate which can be filtered only with great difficulty, (2) the yellowish to black colour of the filtrate caused by the presence of colloidal nickel sulphide, and (3) the lengthy procedure required to coagulate this precipitate. However, when thioacetamide is employed for the precipitation of sulphides and where subsequent operations are to be made with the same solution, the effects of the excess thioacetamide together with its hydrolysis and reaction products must be considered. It seems possible that thioacetamide could be used to advantage as a substitute for hydrogen or ammonium sulphide in the separation of zinc, nickel and cobalt as sulphides from ammoniacal solutions in which aluminium, chromium and in some cases manganese, are present as tartrate or oxalates complex ions. Before thioacetamide can be employed for such separations, however, it will be necessary to determine experimentally the behaviour of each of the elements involved under the conditions to be used for the separation. Acknowle@nenrs-The authors are grateful for financial support from the National Science Foundation during the course of this investigation. Discussions with Fred C. Anson and Dwight M. Smith have been helpful in the preparation of the manuscript. B-Die Geschwindigkeit der F&hung von Nickels&id aus ammoniakalischen LiJsungen mit Thioacetamid wird hauptSHchlich dumb zwei Reaktionstypen kontrolliert : Reaktionen zwischen den verschiedenen Nickelspezres und Thioacetarnid und Reaktionen zwischen Nickel und dem aus der Reaktion zwischen Ammoniak und Thioacetamid stammenden Sulfid; die aus der durch Hydroxylionen katalysierten Hydrolyse von Thioacetamid stammende Sulfidmenge. ist relativ klein. Die Geschwindigkeitsausdriicke fur die beiden vorherrschenden Prozesse sind d[Ni(lI)] - dt
= nio k,[Ni(NH&,*+][CHICSNH,][H+]-*/*
dN(lOl
- -
dt
= k[CH,CSNH,][NH,]*.
R&us&--On contr8le la vitesse de presipitation du sulfure de nickel a partir de solutions ammoniac&s au moyen de thioac&amide essentiellement par deux types de reactions: reactions en& les differentes esp&ces du nickel et la thioacttamide, et &actions entre le nickel et le sulfure produit par une reaction thioac&amide-ammoniaque. (Le sulfure produit par l’hydrolyse du thioa&amide catalys&e par I’hydroxyde est relativernent faible.) Les expressions de la vitesse pour ces deux processus p&dominants sont, respectivement : d]Ni(II)] - =nie dt _
k,[Ni(NH8),*+][CHsCSNHs][H+]-*‘s
et -
d[Ni(Il)] = k[CH,CSNH,][NH,]* dt
.
REFERENCES i J. Bjerrum, Metal Ammine Formation in Aqueous Solution. P. Haase and Son, Copenhagen, 1941. * D. F. Bowersox, D. M. Smith and E. H. Swift, Talanta, 1959,2, 142. * D. H. Klein and E. H. Swift, ibid., 1%5,12, 349. 4 S. Washizuke, Jqan Analyst, 1963, 1520. 6 G. F. F. Lundell, H. A. Bright and J. I. Hoffman, Applied Inorgat& Analysis. Wiley and Sons, New York, 1953. * E. H. Swift, System of Chemical Analysis. Freeman and Co., San Francisco, 1939.