ions in solution and other parameters entering into the electrolysis coefficients \(K_{e,1}\) and \(K_{e,2}\).

If \(z_1 = z_2\), then, taking approximately \(K_{e,1} \simeq K_{e,2}\), instead of (37) we obtain the simpler relation:

\[ C_u^{\max}=\frac{L}{\mu^{\,n+1} C_{u,2}^{\,n}} . \tag{39} \]

The equations obtained make it possible to estimate the conditions under which the formation of a solid phase of metal or an intermetallic compound in mercury may disturb the linearity of the calibration plot and cause errors in analysis using the AMP method.

References

1. A. G. Stromberg. Proceedings of the Siberian Branch of the Academy of Sciences of the USSR, 5, 76 (1962).
2. A. G. Stromberg, A. A. Kaplin. Journal of Analytical Chemistry, 20, 1043 (1965).

Purification of Reagent-Grade Carbazole

V. P. Lopatinskaya, E. E. Sirotkina, V. D. Pirogov

The reagent-grade carbazole of the “pure” grade supplied in our country by the Kharkov Coke-Chemical Plant (VTU No. RU-463-51 and MRTU 6-09-3815-67) is a product obtained by isolating carbazole from the anthracene fraction of coal tar. The methods of isolating carbazole at the Kharkov Coke-Chemical Plant are not constant, and therefore the purity of the reagent-grade carbazole produced varies substantially. This carbazole contains 5–8% impurities, mainly carbazole companions—anthracene and phenanthrene. A comparative estimate of the content of the principal components in reagent-grade carbazole of various batches and dates of manufacture is given in Table 1.

Table 1.

Results of analyses of reagent-grade carbazole of various batches

Continuation of Table 1

Data on the content in reagent-grade carbazole of any other components besides carbazole, anthracene, and phenanthrene are currently lacking. Our attempts to detect other components by thin-layer chromatography (TLC) on silica gel using various solvent systems (acetone, benzene, alcohols, dimethylformamide, and their mixtures) were unsuccessful, although some carbazole derivatives (see Table 2) can be separated and detected comparatively easily.

Table 2.

Chromatographic data for certain compounds on silica gel (solvent benzene–methanol in a ratio of 9:1)

Probably the amount of undetected impurities in the preparation is relatively small, or their $R_f$ coincides with the $R_f$ of the main components.

Our experience in synthesizing products based on reagent-grade carbazole supplied by the Kharkov Coke-Chemical Plant shows that, for most syntheses, the purity of the starting carbazole plays a re-

...a role and, naturally, the reagent-grade preparation cannot be used without additional purification. In this connection we undertook a search for convenient methods for the additional purification of reagent-grade carbazole, the results of which are given below. Carbazole 3 from the production batch of 23.06.64 was taken as the starting product; its principal characteristics are given in Table 1. The degree of purification of carbazole was monitored by the change in melting point ($T_{\mathrm{m}}$), the content of anthracene and phenanthrene, the change in the color of the carbazole, and by UV spectra recorded on an SF-4 type instrument at a substance concentration of 0.005 g per 10 ml of isopropyl alcohol and a layer thickness of 0.0061 cm. The standard was synthetic carbazole obtained by the Borsche method and purified to chromatographic purity; $T_{\mathrm{m}} = 246.2$–$247.0^\circ\mathrm{C}$, color pure white with bluish fluorescence.

In laboratory practice, for the purification of reagent-grade carbazole, repeated recrystallization of the preparation from acetone is used in most cases. In this case carbazole is obtained (see Table 3) with a high $T_{\mathrm{m}}$ ($245$–$246^\circ\mathrm{C}$) and has a pure white color. However, the yield based on the starting product does not exceed 20–30%, and solvent losses are very large.

Taking into account literature data /1/ on the quantitative interaction of anthracene with maleic anhydride, we somewhat modified and supplemented the acetone procedure, as a result of which it contains the following principal operations: 1) extraction of carbazole with acetone in Soxhlet apparatuses; 2) treatment with maleic anhydride of a solution of carbazole in xylene; washing of the excess maleic anhydride with a 0.1 N solution of caustic potash and then with water until the litmus reaction is neutral; 3) recrystallization from acetone. Carbazole purified by this procedure has $T_{\mathrm{m}}$ within the range $244$–$245^\circ\mathrm{C}$ and a pure white color (Table 3). Analysis shows the absence of anthracene in such carbazole; the phenanthrene content is up to 0.6% by weight.

But in this case as well the yield of pure product is low, and large solvent losses are not excluded. Moreover, the process is made still more labor-intensive and multistage.

Further development of the procedure for purifying reagent-grade carbazole by extracting anthracene with maleic anhydride and the subsequent removal of phenanthrene by recrystallization...

of the solvents showed that, in order to obtain carbazole with a purity of about 99%, it is sufficient to treat the reagent-grade product with maleic anhydride and then recrystallize it from aromatic hydrocarbons, best of all from xylene (the mixture of its isomers is most effective). In this case the purification procedure is as follows:

1. In the first stage, reagent-grade carbazole (250 g) and maleic anhydride (13 g) are charged into a round-bottom flask (capacity 5 l) equipped with a stirrer and a reflux condenser; xylene (4 l) is added, the mixture is heated to boiling, and boiled for 15 min. The solution is then cooled, and the crystallized carbazole is filtered off, washed with 0.1 N potassium hydroxide solution (to remove excess maleic anhydride) and then with water until the mother liquor is neutral, after which it is dried at a temperature of 80–100°C.

2. The carbazole (250 g) treated with maleic anhydride is charged into a round-bottom flask (capacity 5 l) and recrystallized from xylene (4.8 l).

The results of purification by this procedure are given in Table 3, item 3.

The UV spectrum of the purified carbazole in this case is very close to the spectrum of synthetic carbazole; the yield of pure product with a melting point in the range 244–246°C and a pure white color reaches 70–80%; the total impurity content does not exceed 1% by weight, and solvent losses are less than 20%.

Similar results are also obtained with double crystallization of reagent-grade carbazole (see Table 3, item 4). In this case the purification procedure is as follows: the reagent-grade product (250 g) is charged into a round-bottom flask (capacity 5 l) and recrystallized twice from xylene (4.8 l for each recrystallization), after which it is washed with benzene or alcohol (600 ml, to remove residual xylene and correspondingly reduce the drying time) and dried at a temperature of 60–70°C. By the procedure described, it is possible successfully to purify reagent-grade carbazole of any manufactured batches. The melting point of the purified carbazole becomes no lower than 243°C; the amount of impurities does not exceed 2.5% by weight. The UV spectrum is similar to that of synthetic carbazole. The yield of pure carbazole reaches 90% and depends on the impurity content in

Table 3

Results of purification of reactive carbazole by various methods

starting product. In the xylene filtrate after recrystallization there remains a certain amount of carbazole enriched with anthracene and phenanthrene (“tails”), from which, if necessary, an additional 2–5% of pure carbazole can be recovered. Solvent losses when using this purification procedure do not exceed 10%.

Increasing the number of recrystallizations gradually raises the purity of the carbazole. After the third recrystallization, the content of the principal substance in the carbazole, as a rule, exceeds 98%, and after the fifth or sixth recrystallization the carbazole is practically free of impurities and, when chromatographed by TLC on a fixed layer of silica gel using various solvents, gives a single spot. The melting point of such carbazole is 246.5–246.8°C.

Attempts to purify carbazole by crystallization from aromatic hydrocarbons, chiefly benzene and solvent naphtha, had been undertaken by a number of investigators earlier /3/, but the results were not very effective. In our case, the use of recrystallization from xylenes gave good results. The procedure is distinguished by reliability, simplicity, and economy, and can be applied for preparing carbazole for organic synthesis. Testing of the purification procedure at the Donetsk and Kharkov coke-chemical plants with batch charges of carbazole of more than 50 kg gave good results.

Conclusions

1. Some methods for purifying reagent-grade carbazole have been considered.

2. A procedure has been developed for purifying reagent-grade carbazole by recrystallizing it from xylene.

Literature

1. F. P. Lastovskii, Yu. I. Vainshtein. Technical Analysis in the Production of Intermediates and Dyes, Goskhimizdat, Moscow, 1958.

2. I. P. Zherebtsov. Candidate dissertation, State University, Tomsk, 1967.

3. Development of an integrated scheme for obtaining phenanthrene, anthracene, and carbazole for organic synthesis and plastics. Report on a scientific-research topic. Sverdlovsk, 1963.