New Organic Reagents for the Photometric Determination of Trace Aluminum in High-Purity Materials
Abstract Generated abstract
The paper evaluates cyanoformazan-2, chlorocyanoformazan, and arsonocyanoformazan as organic photometric reagents for determining trace aluminum in high-purity materials. Using spectrophotometric measurements, the authors characterize pH dependence, complex composition, absorption maxima, molar absorption coefficients, contrast, photometric sensitivity, and interference from selected cations and anions. All three reagents form 1:1 aluminum complexes with an optimal complexation pH of about 4, but arsonocyanoformazan shows weaker analytical characteristics. Chlorocyanoformazan is identified as the most promising reagent, with high sensitivity and contrast and comparatively favorable resistance to anionic interferences, while cyanoformazan-2 is also considered suitable for sensitive aluminum determination.
Full Text
NEW ORGANIC REAGENTS FOR THE PHOTOMETRIC DETERMINATION OF TRACES OF ALUMINUM IN HIGH-PURITY MATERIALS
V. A. MALEVANNY, M. P. ERMAKOVA, Yu. L. LELCHUK¹
(Presented by the scientific seminar of the Department of Analytical Chemistry)
In the search for more sensitive and specific reagents for the photometric determination of traces of aluminum in high-purity materials, we have studied the analytical properties of cyanoformazan-2, chlorocyanoformazan, and arsonocyanoformazan, synthesized earlier [1, 2] by one of us.
Cyanoformazan (1,5-di(2-hydroxy-5-sulfophenyl)-3-cyanoformazan), chlorocyanoformazan (1,5-di(2-hydroxy-3-sulfo-5-chlorophenyl) and arsonocyanoformazan (1-(2-hydroxy-5-sulfophenyl)-5-(2-arsonophenyl-3-cyanoformazan), in the structure of their molecules, resemble the structure of triphenylmethane dyes, which belong to the most valuable of the currently used photometric reagents for aluminum.
Experimental Part
Solutions of the reagents and of aluminum sulfate with a concentration of \(10^{-3}\ M\) were used in the work. The aluminum sulfate solution was freshly prepared. The required pH was established with acetate buffer solutions and monitored on a pH meter. The absorption spectra of the reagents and their complexes with aluminum were recorded on an SF-10 spectrophotometer in the range 400–750 mµ. A dye solution at the same pH was used as the compensating liquid.
The dependence of the optical density of the aluminum complexes on the pH of the solution was studied in the pH range 1–11 with a tenfold excess of reagent. The pH values of the indicator transition of the reagent \((\mathrm{pH}_{1/2}^{c})\), the pH of half-complex formation \((\mathrm{pH}_{1/2}^{k})\), and the quantities \(\Delta \mathrm{pH}\) and \(\dfrac{\Delta D}{\Delta \mathrm{pH}}\), characterizing, respectively, the reproducibility of the reaction and the sensitivity of the complex to acidity fluctuations by 0.5 pH unit, were determined. These characteristics were determined in accordance with the instructions of Academician A. K. Babko [3]. The data obtained are given in Table 1.
For photometric reagents Babko considers the best values to be pH 3.0 and \(\dfrac{D}{\mathrm{pH}} = 1\). The cyanoformazans studied by us fully meet these requirements. Cyanoformazan-2 is the least sensitive to pH fluctuations.
¹ K. I. Banokina participated in the work.
Table 1
Effect of pH on the complex formation of reagents with aluminum
| Name of reagent | pH optimum | pH 1/2 K | pH 1/2 C | ΔpH | $\dfrac{\Delta D}{\Delta \mathrm{pH}}$ |
|---|---|---|---|---|---|
| Cyanoformazan | 4.0 | 2.6 | 7.0 | 4.4 | 0.02 |
| Chlorocyanoformazan | 4.0 | 2.1 | 7.0 | 4.9 | 0.15 |
| Arsonocyanoformazan | 4.0 | 2.3 | 7.0 | 4.7 | 0.14 |
The identical value of the optimal pH for complex formation for all three reagents, as well as the pH of the indicator transition, is explained by the analogous structure of the skeletons of their molecules.
The most important analytical characteristics of organic reagents are the values of contrast ($\Delta \lambda$) and of the molar absorption coefficients of the complex ($E_k$) and the reagent ($E_r$). The absolute values of the molar absorption coefficients of the complexes were determined by the Komar method [4], the photometric sensitivity (p.s.) and the absolute difference between the molar absorption coefficients of each complex and the corresponding reagent—by the Babko method [5], and the compositions of the complexes—by the Ostromyslensky–Job method [6]. The data obtained are given in Table 2.
Table 2
Spectrophotometric characteristics of aluminum complexes with cyanoformazans
| Name of reagent | Composition | $\lambda_{\text{work}}$, mµ | $\lambda$, mµ | $E$, µmol$^{-1}$ cm$^2$ complex Al | $E$, µmol$^{-1}$ cm$^2$ reagent | p.s. | ΔE |
|---|---|---|---|---|---|---|---|
| Cyanoformazan 2 | 1 : 1 | 640 | 170 | 27800 | 160 | 174 | 27590 |
| Chlorocyanoformazan | 1 : 1 | 670 | 210 | 30300 | 160 | 190 | 30200 |
| Arsonocyanoformazan | 1 : 1 | 594 | 134 | 15200 | 830 | 18 | 14370 |
From these data it is evident that the most sensitive reagent is chlorocyanoformazan. It also has the maximum values of contrast and photometric sensitivity. This permits recommending it as a sensitive reagent for aluminum. Arsonocyanoformazan, which has the poorest characteristics, was not studied further by us.
An extremely important characteristic of any reagent is the degree of its specificity. There is no generally accepted method for characterizing this parameter. We consider it appropriate, for cations, to express it as the ratio of the apparent value of the molar absorption coefficient of the complex of the given reagent with aluminum to the apparent value of the molar absorption coefficient of the complex of the reagent with the interfering cation. We call this quantity the specific sensitivity and denote it by the letters SS. Determination of SS is carried out at the optimal pH and $\lambda_{\text{work}}$ characteristic of the given aluminum complex with the reagent.
The greater the SS, the larger, in comparison with aluminum, an excess of the interfering cation may be present in the solution being analyzed without special masking.
The data obtained are given in Table 3.
From these data it is evident that, in their SS toward interfering cations, the cyanoformazans studied differ little from the best reagents of the triphenylmethane-dye group. Cyanoformazan 2 is less sensitive to impurities of Fe$^{3+}$, Cr$^{3+}$, Ti$^{4+}$, Zn$^{2+}$, and Pb$^{2+}$ than chlorocyanoformazan.
Table 3
Specific sensitivity of cyanoformazans with respect to reference cations
| Reagent name | Fe$^{3+}$ | Fe$^{2+}$ | Cr$^{3+}$ | Cr$^{6+}$ | Ti$^{4+}$ | Zn$^{2+}$ | Pb$^{2+}$ | Cu$^{2+}$ | Cb$^{3+}$ | V$^{5+}$ |
|---|---|---|---|---|---|---|---|---|---|---|
| Cyanoformazan 2 | 11.2 | 14.2 | 3.7 | 3.1 | 7.1 | 122 | Does not affect | 8.0 | 435 | 1.0 |
| Chlorocyanoformazan | 5.7 | 14.0 | 1.8 | 2.0 | 5.3 | 13.8 | 29.4 | 4.9 | 520 | 1.4 |
Table 4
Specific sensitivity of cyanoformazans with respect to reference anions
| Reagent name | SO$_4^{2-}$ | Cl$^-$ | TGK | AK | TGK:AK = 1:1 | PO$_4^{3-}$ | citrate | tartrate | NO$_3^-$ | EDTA |
|---|---|---|---|---|---|---|---|---|---|---|
| Cyanoformazan 2 | 8.0 | 8.0 | 7.0 | 7.0 | 6.0 | 4.0 | 0.02 | 4.0 | 8.0 | P |
| Chlorocyanoformazan | 9.1 | 8.0 | 9.1 | 8.0 | 9.0 | 8.4 | 1.2 | 9.0 | 8.4 | P |
The SS index of the reagent with respect to anions in Table 4 is expressed by us as the number of moles of the interfering anion that does not affect the value of $E_k$. In this table we have used the following designations: TGK—thioglycolic acid, AK—ascorbic acid, EDTA—Trilon B, P—the aluminum complex with the reagent is destroyed. Trilon B destroys aluminum complexes with cyanoformazan-2 and chlorocyanoformazan. The citrate ion also has a considerable interfering effect. However, both with respect to the citrate ion and to all the other anions studied, the aluminum complex with chlorocyanoformazan is less sensitive than that with cyanoformazan-2. This circumstance permits the conclusion that the most valuable reagent is chlorocyanoformazan. As a means for masking Fe$^{3+}$, it is advisable to use a 1:1 mixture of ascorbic and thioglycolic acids.
Conclusions
The principal analytical characteristics of aluminum complexes with cyanoformazan-2, chlorocyanoformazan, and arsonocyanoformazan have been studied. It has been shown that chlorocyanoformazan and cyanoformazan-2 can be used as highly sensitive reagents for the photometric determination of traces of aluminum in high-purity materials. By a number of indicators, chlorocyanoformazan is the most valuable reagent.
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