a solution of nickel oxalate
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THE PRECIPITATION OF NICKEL OXALATE
BY J. A. ALLEN
Department of Chemistry, University of Tasmania, Hobart, Tasmania, Australia
Received February 16, 1953
The rate of formation of precipitates of nickel oxalate in solutions containing nickel sulfate and oxalic acid has been studied by turbidimetric and dilatometrie techniques which cover successive stages of the reaction. Equations have been derived relating the rate of formation to the initial concentrations of nickel sulfate and a oxalic acid in the solution. A mecha nism based on the formation and subsequent rearrangement ofs complex ion is suggested.
The work described in this paper is part of a general study of the properties of the oxalates of divalent heavy metals in the solid state. Nickel oxalate is an example of this class of compounds and may be prepared by the reaction in solution between a soluble nickel salt and oxalic acid. This reaction does not, however, proceed by a simple ionic double decomposition mechanism: an induction period is observed and the rate of precipitation depends markedly on the concentra tions of the reactants in solution. The present work was undertaken to establish these relation - ships.
Experimental
Stock solutions of nickel sulfate and oxalic acid of normal
strength were prepared from high grade chemicals and di- luted as required. When a solution of nickel sulfate of say, 0.3 N is added to one of oxalic acid of the same strength the order of addition is immaterial-no precipitate at first appears. After some minutes the solution becomes cloudy, the solid particles increase in size and subsequently settle out. Two techniques have been employed to follow
these processes.
(1) Turbidimetric Method. This method is useful for which is carried out in the cell of a photoelectric colorimeter. The
instrument scale, graduated logarithmically from 0 to 100, gave directly the percentage of light cut off by the solution. A green filter was used throughout and the instrument was adjusted to read zero with distilled water in the cell. The solutions were mixed in the required amounts to make up a total volume of 10 ml., the cell inserted in the colorimeter
and the scale reading followed with time as the optical den sity of the solution increased. As it is not possible to relate the scale reading, R, directly
to the amount of precipitate present, the assumption was made that dR/dt was proportional to the rate of formation of the precipitate. The method was used only for the very early part of the reaction when no visible settling took place. (i) Dilatometric Method.-The required solutions were mixed and drawn rapidly into a dilatometer. The rate of change in volume of the reaction system as the precipitation proceeded, which is proportional to dH/dt where is the reading of the liquid level in the capillary, was taken as a measure of the rate of precipitation. The volume of the bulb of the dilatometer was 3 ml. and the capillary diame ter was 0.8 mm. The solutions and dilatometer were main- tainted at 20 0.05°.
These two methods are complementary in that the part of the reaction measured in the colorimeter precedes that which causes a significant increase in volume in the dilatome- ter.
Results
plotted against the excess concentration of oxalic acid, C, are shown in Fig. 1. For concentrations of oxalic acid equal to or greater than those of nickel
sulfate
S. = h,C + ha
(1)
where h, and ha are both functions of the concentra tion of nickel salt, E, given by
A = kE
Ag = kE/
The over-all equation is, therefore
S EO + EN
(2)
(3)
(4)
where k = 2820 and k = 43.6 when C and E are expressed as normalities and S as the number of scale divisions per minute.
40 -
30
715
8 20
с
10
D
-0.3
-0.1 0.1 0.3 Concentration of excess oxalic acid, C.
0.5
Fig. 1.--A, 0.4 N; B, 0.3 N; C, 0.25 N; D, 0.2 N; E,
0.15 N.
The dilatometric experiments were carried out on the same plan. The graph of the capillary reading against time again gave an s-shaped curve with a long central straight portion, the slope of which, Sa, was taken as the rate of formation of the precipi tate. The results are shown in Fig. 2 in a manner acid equal to or greater than those of nickel sulfate
In the turbidimetric experiments, the graph of the scale reading against time gave an s-shaped curve with a long central straight portion, the slope of which, S, was taken as the rate of formation of the precipitate. Measurements were made at five differ- ent oxalic acid concentrations. The slopes, S.. analogous to Fig. 1. For concentrations of oxalie
(1) N. V. Sidgwick, "The Chemical Elements and their com pounds " Vol. 2. Oxford University Press, London,