Sorption-Photometric Method for the Determination of Trace Copper and Iron from a Single Sample in Certain Materials Used in Phosphor Production
Abstract Generated abstract
A sorption-photometric procedure is described for determining trace copper and iron from a single sample of materials used in phosphor production, including luminophor L-34, hydrochloric acid, calcium carbonate, and ammonium and calcium hydrogen phosphates. The method decomposes samples in concentrated hydrochloric acid, concentrates copper and iron chloride complexes on AN-2F or ceralite FF anion exchangers, elutes them with dilute hydrochloric acid, and determines copper with diethyldithiocarbamate in chloroform and iron with 2,2′-dipyridyl. Interference from iron in the copper determination is suppressed with Trilon B and ammonium citrate. Tests on artificial mixtures and production materials indicate an analysis time of 1.5 to 2 hours and relative errors generally not exceeding about 10 to 15 percent.
Full Text
SORPTION–PHOTOMETRIC METHOD FOR THE DETERMINATION OF TRACES OF COPPER AND IRON FROM A SINGLE SAMPLE IN CERTAIN MATERIALS USED IN PHOSPHOR PRODUCTION
Yu. T. Tilchuk, R. D. Glukhovskaya, E. D. Lukker, G. N. Dembovskaya
(Presented by the scientific seminar of the Department of Analytical Chemistry)
As a result of studying the processes of sorption of iron by anion exchangers from hydrochloric acid solutions and its desorption from chromatographic columns, we previously developed [1] a highly sensitive sorption–photometric method for determining traces of iron in phosphor L-34, hydrochloric acid, calcium carbonate, and ammonium and calcium hydrophosphates, with a sensitivity of \(10^{-6}\)—\(10^{-7}\%\). The method is based on the photometric determination of iron with 2,2′-dipyridyl after its separation and concentration on the anion exchanger AN-2F or on ceralite FF.
The data published in this collection, obtained in studying the processes of copper sorption on anion exchangers and its desorption from chromatographic columns, enabled us to develop optimal conditions for the sorption–photometric determination of traces of copper and iron from a single sample in certain materials used in phosphor production. The relevant enterprises are interested in such analyses.
The method developed is based on decomposition of a weighed sample of the substance being analyzed in 8–9 N HCl, separation and concentration of the resulting chloride complexes of copper and iron by the anion exchanger AN-2F or ceralite FF in the chloride form, elution of copper and iron from the anion-exchange columns with a 0.1 N HCl solution, and photometric determination in the eluates of copper as a complex with diethyldithiocarbamate in chloroform and of iron as a complex with 2,2′-dipyridyl.
In the work, a 0.1% aqueous solution of sodium diethyldithiocarbamate, a 2% solution of 2,2′-dipyridyl in 0.001 N HCl, triply distilled water, and especially pure hydrochloric acid were used; the latter was practically completely purified of traces of copper and iron by isothermal distillation and subsequent chromatographic purification [1].
Sodium diethyldithiocarbamate as a reagent for determining traces of copper has a number of advantages over other reagents [3]. Many extractants are used for extraction of the copper complex. The best is chloroform. The literature [3—7] gives contradictory data on the optimal acidity of the solution during extraction, and there are no sufficiently precise data on the influence of traces of iron. Our experiments showed that the copper complex is extracted most completely by two 5 ml portions of chloroform at pH 4–5. This pH is maintained with an ammonia–acetate buffer mixture.
Study of the effect of various additions of iron to \(5\ \mu\mathrm{g}\) of copper in the determination of copper with diethyldithiocarbamate showed that an equal amount of iron increases the optical density of the solution by 6–7%,
twofold amount—by 57%, and a tenfold amount—by a factor of 9. To eliminate the interfering effect of iron, our procedure uses solutions of Trilon B and ammonium citrate. Experiments showed that for each determination it is sufficient to take 2 ml of an approximately molar solution of ammonium citrate and 5 ml of a 10% solution of Trilon B.
Copper does not interfere with the determination of iron with 2,2′-dipyridyl even when present at 200 times the iron content [8].
The method developed was used for the determination of traces of copper and iron from a single sample in phosphor L-34, in hydrochloric acid, calcium carbonate, and ammonium and calcium hydrophosphates.
Course of analysis
Preparation of the chromatographic column is carried out in the usual manner [2]. When concentrated HCl is analyzed, it is converted to 8–9 normal acid. For this, one volume of water is added to every three volumes of acid and the mixture is stirred. When analyzing the phosphor, calcium carbonate, and ammonium and calcium hydrophosphates, an accurately weighed portion of the substance being analyzed is treated with 5–10 ml of especially pure 8–9 N HCl.
Ten ml of the same acid is passed through the anion exchanger at a flow rate of 2–3 ml/min; then, at the same rate, the solution being analyzed is passed through, and the column is washed with several portions of HCl of the same concentration. The anion exchanger thereby becomes dark brown. To avoid breakthrough of copper, the total volume of 8–9 N HCl must not exceed 50–60 ml. The sorbed chloride complexes of copper and iron are eluted from the column with 0.1 N HCl solution. As this acid moves through the column, the anion exchanger becomes lighter. When a dark-brown layer 2–3 cm thick remains at the bottom, the stopcock is closed, it is rinsed with water, and the eluate is collected in a 25-ml volumetric flask to the mark and thoroughly mixed. Copper is determined in one aliquot of this solution, and iron in another.
Determination of copper. An aliquot of the eluate is transferred to a separatory funnel. Five ml of Trilon B solution, 2 ml of ammonium citrate, 5 drops of phenolphthalein, and concentrated NH₄OH are added until a faint pink color appears. Then 2 ml of an ammonia–acetate buffer with pH 4.2 and 2 ml of sodium diethyldithiocarbamate are added, and after thorough mixing the colored copper complex is extracted with two 5-ml portions of chloroform, shaking each time for two minutes. The chloroform extracts of the copper complex are poured into a dry 25-ml volumetric flask. The volume of the solution is brought to the mark with chloroform, mixed thoroughly, and the optical density is measured on a photoelectric colorimeter-nephelometer in cells with a 50-mm layer thickness, using blue light filter No. 3 with a transmission maximum at 453 mμ. Chloroform serves as the reference solution. A blank experiment is run simultaneously. In this case, water is taken instead of the eluate.
The optical density of the blank is subtracted from the optical density of the solution being analyzed, and from this difference the copper content in the aliquot, in μg, is found from the calibration curve. To construct the calibration curve, the experiments are carried out under the same conditions, except that, instead of an aliquot of the eluate, appropriate amounts of a standard copper solution are taken. The copper content in the substance being analyzed, in percent, is calculated by the formula:
\[ \% \mathrm{Cu}=\frac{B_{\mathrm{Cu}}\cdot V_k}{10000\cdot H\cdot V_{\mathrm{al}}}, \]
where
\(B_{\mathrm{Cu}}\) is the copper content in the aliquot, μg;
\(V_k\) is the volume of the volumetric flask, 25 ml;
\(H\) — sample weight of the substance analyzed, g;
\(V_{\text{al}}\) — aliquot volume, ml.
Determination of iron. An aliquot portion of the eluate is transferred to a 25 ml volumetric flask, the appropriate reagents are added, and iron is determined photocolorimetrically with \(\alpha,\alpha'\)-dipyridyl [2].
The percentage content of iron in the substance analyzed is calculated by the formula:
\[ \% \mathrm{Fe}=\frac{B_{\mathrm{Fe}}\cdot V_k}{10000\cdot H\cdot V_{\text{al}}}, \]
where
\(B_{\mathrm{Fe}}\) — weight of iron in the eluate aliquot, μg;
\(V_k\) — volume of the volumetric flask, 25 ml;
\(H\) — sample weight of the substance analyzed, g;
\(V_{\text{al}}\) — aliquot volume, ml.
The accuracy and reproducibility of the method were checked on artificially prepared mixtures and by parallel determinations of traces of copper and iron in various materials. Some of the experimental data are given in Tables 1 and 2.
Table 1
Sorption-photometric determination of copper and iron in artificially prepared salt mixtures
| Anion exchanger | Taken, μg | Taken, μg | Number of determinations | Found, μg | Found, μg | Relative error, % | Relative error, % |
|---|---|---|---|---|---|---|---|
| copper | iron | copper | iron | for copper | for iron | ||
| AN-2F | 5 | 5 | 4 | 5.18 | 5.13 | +3.6 | +2.6 |
| AN-2F | 5 | 5 | 6 | 4.71 | 4.77 | −5.8 | −4.6 |
| Ceralite FF | 5 | — | 4 | 4.98 | — | −0.4 | — |
| Ceralite FF | 5 | 5 | 3 | 4.85 | 5.21 | −3.0 | +4.2 |
| Ceralite FF | 10 | 5 | 4 | 9.48 | 5.26 | −5.2 | +5.2 |
| Ceralite FF | 15 | 5 | 2 | 13.83 | 4.90 | −7.8 | −2.0 |
Table 2
Sorption-photometric determination of traces of copper and iron from one sample in certain materials
| Substance, grade | Number of determinations | Found, % | Found, % | Max. deviations from the mean value, % | Max. deviations from the mean value, % |
|---|---|---|---|---|---|
| copper | iron | for copper | for iron | ||
| Luminophor L-34 | 7 | \(1.4\cdot 10^{-4}\) | \(1.6\cdot 10^{-3}\) | ±18 | ±14 |
| CaCO\(_3\), analytical grade | 10 | \(1.3\cdot 10^{-5}\) | \(3.8\cdot 10^{-4}\) | ±25 | ±8 |
| CaHPO\(_4\), analytical grade | 8 | \(6.0\cdot 10^{-4}\) | \(1.9\cdot 10^{-4}\) | ±22 | ±5 |
| CaHPO\(_4\), technical | 6 | \(4.9\cdot 10^{-5}\) | \(4.0\cdot 10^{-4}\) | ±17 | ±15 |
| \((\mathrm{H}_4)_2\mathrm{HPO}_4\), Kalibraug | 4 | none | \(1.91\cdot 10^{-4}\) | — | ±10 |
| \((\mathrm{H}_4)_2\mathrm{HPO}_4\), technical | 10 | \(5.1\cdot 10^{-4}\) | \(1.2\cdot 10^{-2}\) | ±20 | ±12 |
Conclusions
A sorption-photometric method has been developed for determining, from a single sample, traces of copper and iron in luminophor L-34, hydrochloric acid, calcium carbonate, and ammonium and calcium hydrogen phosphates. Copper and iron are concentrated by anion exchangers in the form of chloride complexes. The determination is completed by photometry of copper in the form of its complex with diethyldithiocarbamate, and of iron with 2,2′-dipyridyl.
The duration of the analysis is 1.5–2 hours. The relative error does not exceed ±10–15%.
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