Determination of trace copper by amalgam polarography with accumulation in certain materials used in phosphor production
Abstract Generated abstract
This paper develops amalgam polarographic procedures with accumulation for determining trace copper in calcium biphosphate and high-purity calcium carbonate used in phosphor production, where allowable copper levels are extremely low. For calcium biphosphate, copper is accumulated from hydrochloric acid solution and then measured after transfer in an ammonium acetate and tartaric acid supporting electrolyte, avoiding poorly resolved peaks and interference. For calcium carbonate, the authors select a calcium bromide and tartaric acid background, study possible copper loss during evaporation, and optimize electrolysis potential to reduce interference from bismuth and lead. The proposed methods give reported sensitivities of 4 x 10^-7 percent for calcium biphosphate and 6 x 10^-7 percent for calcium carbonate, with example analyses and reproducibility estimates presented.
Full Text
Determination of Traces of Copper by Amalgam Polarography with Accumulation in Some Materials Used in the Production of Phosphors. II. Determination of Copper in Calcium Biphosphate and Calcium Carbonate
L. G. Baletskaya, V. M. Pichugina, M. S. Zakharov
(Presented by the scientific-methodological seminar of the CTF)
The copper content in calcium biphosphate and calcium carbonate used in the production of phosphors must not exceed \(1 \cdot 10^{-5}\)—\(1 \cdot 10^{-6}\%\). A very promising method for determining trace amounts is amalgam polarography with accumulation (APA) [1, 2].
To convert calcium biphosphate into solution, we tested several mineral acids: \(\mathrm{HNO_3}\), \(\mathrm{HCl}\), \(\mathrm{HBr}\). Of the acids studied, hydrochloric acid with a concentration of not less than \(3N\) proved to be the best solvent for calcium biphosphate. It is not possible to carry out the amalgam-polarographic determination of copper directly in the resulting solution for the following reasons. The anodic peaks of copper in the resulting solution: 1) merge with those for bismuth, which may be present in calcium biphosphate, and 2) are diffuse in character. Well-shaped anodic copper peaks are obtained against a background of \(0.5N\) ammonium acetate \(+\) \(0.1N\) tartaric acid [3]. Therefore, the amalgam-polarographic determination of copper in calcium biphosphate was carried out by us using the APA method with transfer [4].
Copper accumulation was carried out from the solution obtained by dissolving calcium biphosphate in hydrochloric acid, and polarography after transfer was performed against a background of \(0.5M\) ammonium acetate \(+\) \(0.1M\) tartaric acid. We studied the reproducibility of determining copper by the APA method with transfer. In these studies, two series of experiments were carried out. In the first series the anodic copper peaks were obtained on the selected background without transfer, while in the second series the anodic peaks were obtained after transfer. The results of the study are presented in Table 1.
It is seen from the table that the error in the determination of copper does not exceed 10–15%, which is acceptable in determining trace amounts.
On the basis of the studies carried out, the following procedure is proposed for determining traces of copper in calcium biphosphate. A weighed portion
0.75 g of calcium biphosphate is placed in a quartz beaker and, with gentle heating by an infrared lamp in a box, is dissolved in 3 ml of 3 N hydrochloric acid. The beaker with the dissolved sample is placed in an electrolyzer, the design of which is described in [5]; the solution is purged with nitrogen for 10 minutes, and accumulation on the electrode is carried out. Electrolysis is conducted for 10 minutes at a potential of −1.0 V (sat. cal. el.). After electrolysis, the electrodes are washed in a second beaker with water (also freed of oxygen), and polarography is carried out in a third beaker on the selected supporting electrolyte. The impurity content is evaluated by the method of additions.
Table 1
Study of the reproducibility of copper determination by the APN method with transfer
Experimental conditions: copper concentration in solution 0.05 γ; supporting-electrolyte volume 3 ml; electrolysis potential −1.0 V (sat. cal. el.); $\tau_{\mathrm{el}} = 10$ min; polarograph sensitivity $3 \cdot 10^{-9}$ A/mm
| Depth of the anodic copper wave in polarography without transfer, mm | Depth of the anodic copper wave in polarography with transfer, mm | % transfer |
|---|---|---|
| 15 | 14 | 93.3 |
| 16 | 15 | 88 |
| 15 | 13 | 86 |
The maximum sensitivity of the procedure according to the three-sigma criterion under the conditions developed is $4 \cdot 10^{-7}\%$.
Using the proposed procedure, analyses of calcium biphosphate for copper content were performed. The results of the analyses and their statistical treatment are presented in Table 2.
Table 2
Results of the analysis of calcium biphosphate for copper content
| $X \cdot 10^{+6}\%$ copper content in CaHPO$_4$ | $X - \overline{X}$ | $(X - \overline{X})^2$ | Statistical treatment |
|---|---|---|---|
| 5.7 | −0.4 | 0.16 | Arithmetic mean: $\dfrac{53}{10} = 5.3.$ |
| 5.0 | +0.3 | 0.09 | |
| 5.0 | +0.3 | 0.09 | Mean square error of a single measurement: $\sigma = \dfrac{1.44}{10} = 0.144.$ |
| 5.0 | +0.3 | 0.09 | |
| 5.7 | −0.4 | 0.16 | Maximum possible error: $\Delta = 3\sigma = 0.144 \cdot 3 = 0.432.$ |
| 6.0 | −0.7 | 0.49 | |
| 5.0 | 0.3 | 0.09 | Mean square error of the arithmetic mean: $\sigma = \dfrac{\sigma}{\sqrt{10}} = 0.04.$ |
| 5.0 | 0.3 | 0.09 | |
| 5.6 | −0.3 | 0.09 | $X_{\mathrm{true}} = (5.3 \pm 0.04), 10^{-6}\%.$ |
| 5.0 | 0.3 | 0.09 | |
| 53.0 | 1.44 |
Determination of Copper in Calcium Carbonate
The literature [6] describes a procedure for determining copper in calcium carbonate with polarography against a background of $\mathrm{CaCl_2} + \mathrm{HCl}$. This procedure has a number of drawbacks: 1) on the indicated background, the anodic copper wave coincides with that for bismuth; 2) the sensitivity of the procedure reaches only $1 \cdot 10^{-5}\%$. We carried out studies to develop a procedure that does not suffer from the noted drawbacks.
Calcium carbonate can be converted into solution by many mineral acids. In order to select a supporting electrolyte on which the anodic copper wave would not merge with the anodic waves of other elements (Bi and Pb), whose anodic-wave potentials are close to that for copper, we obtained anodic waves in the following electrolytes: $mN \mathrm{Ca(NO_3)_2}$, $mN \mathrm{Ca_3(PO_4)_2}$, $mN \mathrm{CaBr_2} + 0.1\,N$ tartaric acid, $mN \mathrm{Ca(Br)_2}$, $mN \mathrm{CaBr} + 0.05\,N$ citric acid. The results of these studies are presented in Table 3.
Table 3
Peak potentials of the anodic waves of Cu, Bi, Pb in some electrolytes
| Background | Cu | Bi | Pb |
|---|---|---|---|
| $mN\,\mathrm{Ca(NO_3)_2}$ | +0.05 | +0.05 | −0.4 |
| $mN\,\mathrm{Ca_3(PO_4)_2}$ | +0.03 | +0.03 | |
| $mN\,\mathrm{CaCl_2}$ | −0.15 | −0.05 | |
| $mN\,\mathrm{CaBr_2} + 0.05\,N$ tartaric acid | −0.26 | no wave | −0.46 |
| $mN\,\mathrm{CaBr_2} + 0.05\,N$ citric acid | −0.26 | no wave | −0.46 |
Peak potentials of the anodic waves relative to the saturated calomel electrode.
It is evident from the table that the best background for the amalgam-polarographic determination of copper in the presence of bismuth and lead is a solution of $\mathrm{CaBr_2} + 0.05\,N$ tartaric acid.
In accordance with the selected background, calcium carbonate is dissolved in pure HBr. The excess acid is removed by evaporating the solution to dryness. The possibility of coprecipitation of traces of copper during evaporation of the solution was studied. The results of the studies are presented in Table 4.
Table 4
Results of studies to determine the coprecipitation of copper
| Evaporation temperature, °C | Increase in the height of the anodic wave from addition of 0.2γ Cu to the sample being analyzed before evaporation, mm | Increase in the height of the anodic wave from addition of 0.2γ Cu to the electrolyzer, mm | % determined |
|---|---|---|---|
| 220–230 | 28 | 27 | 96.4 |
| 220–230 | 26 | 25 | 96.1 |
| 220–230 | 23 | 24 | 95.8 |
| 250–270 | 26 | 25 | 96.3 |
| 250–270 | 27 | 26 | 96.3 |
| 250–270 | 28 | 25 | 89.3 |
It is evident from the table that, at the temperatures studied, copper coprecipitation is not observed.
We investigated the effect of the electrolysis potential on the depth of the anodic peaks of copper and lead (Fig. 1). It is evident from Fig. 1 that at an electrolysis potential of \(-0.5\) V (sat. cal. el.) the maximum depth of the anodic peak of copper is attained, whereas lead is reduced in insignificant amounts. Additional experiments showed that bismuth at this potential is practically not deposited on the electrode and, consequently, does not interfere with the amalgam-polarographic determination of copper.
Fig. 1. Dependence of the depth of the anodic peaks of copper and lead on the electrolysis potential against the background of CaBr\(_2\) + 0.5 N tartaric acid. Curve 1—for copper; curve 2—for lead. Experimental conditions:
1. \(C = 0.1\ \gamma/\text{ml}\) Cu; \(V = 2.5\ \text{ml}\); \(S = 4 \cdot 10^{-9}\ \text{a/mm}\); \(\tau = 10\ \text{min}\); \(r = 0.04\ \text{cm}\); \(W = 333\ \text{mV/min}\).
2. \(C = 0.1\ \gamma/\text{ml}\) Pb; \(V = 3\ \text{ml}\); \(S = 8 \cdot 10^{-9}\ \text{a/mm}\); \(\tau = 10\ \text{min}\); \(r = 0.04\ \text{cm}\).
On the basis of the investigations carried out, the following procedure is proposed for determining traces of copper in calcium carbonate. A 0.5 g sample of calcium carbonate is placed in a quartz beaker, and 1.5 ml of triply distilled HCl is added. The resulting solution is evaporated to dryness in an evaporation chamber [7]; to the dry residue of calcium bromide, 2 ml of 0.05 M tartaric acid is added, electrolysis is carried out for 15 minutes at \(-0.5\) V (sat. cal. el.), and polarography is performed at a polarograph sensitivity of \(3 \cdot 10^{-9}\ \text{a/mm}\).
Under the conditions developed, the sensitivity of the procedure by the three-sigma criterion is \(6 \cdot 10^{-7}\%\).
Analyses of high-purity CaCO\(_3\) for copper content were carried out by the proposed procedure (Table 5).
Table 5
Results of analysis of CaCO\(_3\) for copper content
| \(X \cdot 10^{-5}\), copper content in CaCO\(_3\) | \(X - \overline{X}\) | \((X - \overline{X})^2\) | Calculation |
|---|---|---|---|
| 1.5 | −0.07 | 0.0049 | Arithmetic mean: |
| 1.5 | −0.07 | 0.0049 | \(\dfrac{14.3}{10} = 1.43.\) |
| 1.3 | +0.13 | 0.0169 | |
| 1.2 | +0.23 | 0.0529 | Mean square error of an individual measurement: |
| 1.5 | −0.07 | 0.0289 | \(\sigma = \dfrac{0.201}{10} = 0.02.\) |
| 1.6 | −0.17 | 0.0289 | |
| 1.6 | −0.17 | 0.0289 | Maximum possible error: |
| 1.5 | −0.07 | 0.0049 | \(\Delta = 3\sigma = 0.02 \cdot 3 = 0.06.\) |
| 1.4 | +0.03 | 0.0009 | |
| 1.5 | −0.07 | 0.0049 | Mean square error of the arithmetic mean: |
| 14.3 | 0.201 | \(\sigma_0 = \dfrac{\sigma}{\sqrt{n}} = \dfrac{0.02}{\sqrt{10}} = 0.006.\) | |
| \(X_{\text{true}} = (1.43 \pm 0.006)\cdot 10^{-5}\%.\) |
Conclusions
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An amalgam-polarographic procedure has been developed for determining microconcentrations of copper in calcium biphosphate. The sensitivity of the procedure is \(4 \cdot 10^{-7}\%\).
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A procedure has been developed for determining traces of copper in high-purity calcium carbonate. The sensitivity of the procedure is \(6 \cdot 10^{-7}\%\).
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