Qualitative Detection and Photocolorimetric Determination of Nitrites with Phenothiazine

P. B. KRISTALEV

(Presented by the scientific seminar of the Departments of Analytical and General Chemistry, KhTF)

Because most salts of nitrous acid are soluble in water, the qualitative detection and quantitative determination of nitrites are carried out colorimetrically. For this purpose, reactions are used in which nitrite ion oxidizes such substances as diphenylamine, potassium iodide, indigo, and others [1].

However, these substances are oxidized by a number of other oxidizing agents, and in particular by nitrate ion, which more often accompanies nitrites in natural samples.

The widely used Griess method, based on the formation of azo dyes, according to the investigations of V. N. Zimarev, B. V. Mikhanchuk, and R. E. Oshcherovich, is also not free from substantial shortcomings [2, 3].

In addition to antipyrine and pyramidon, proposed earlier and experimentally tested by us [4], which showed their almost complete unsuitability for determining nitrites among nitrosation reactions, chromotropic acid has recently been studied by S. Ya. Shnaiderman [5]. The need for increased acidity of nitrite solutions (pH from 5 to 1) and the attainment of stability of the solution color after 3–4 hours also complicate its practical application.

In addition to the 1-phenyl-3-methylpyrazolone proposed earlier by us [4], in the present work phenothiazine (thiodiphenylamine) was tested for these purposes; it gives colored nitroso-phenothiazine, which at low nitrite concentration colors the solution red, while at increased nitrite concentration a sparingly water-soluble precipitate of red-violet color is formed.

Phenothiazine was synthesized by us according to Ackermann [6] by fusing diphenylamine with powdered sulfur at 140–160°. After three recrystallizations, the preparation melted at 180°.

Qualitative Detection of Nitrites

Mixing a 1% alcoholic solution of phenothiazine and soluble salts of 24 different anions showed the absence of formation of colored compounds with the reagent.

In an acidic medium the reagent is oxidized by chromate ion and permanganate ion, with formation of a green-colored solution.

The sulfite anion in acidic solution decolorizes nitrosophenothiazine. Its interfering effect is eliminated by precipitation as lead sulfide upon addition of a solution of lead acetate.

Cations of alkali and alkaline-earth metals, as well as chromium (III), manganese (II), cobalt (II), tin (IV), cerium (III), mercury (II), and iron (II), do not interact with the reagent solution.

Iron(III) ions interfere with the determination of nitrites; in a weakly acidic medium with the reagent they give green-colored solutions.

The interfering effect of iron(III) is eliminated by adding citric acid. Testing a series of organic solvents (benzene, toluene, xylene, chloroform, carbon tetrachloride, carbon disulfide, dichloroethane, dibromoethane, ether, and isoamyl alcohol) showed that isoamyl alcohol dissolves and extracts the colored nitroso-phenothiazine. The alcohol is only slightly soluble in water (2.5 g per 100 ml) and is nonvolatile (b.p. 130°).

The ability of nitroso-phenothiazine to be extracted into a nonaqueous layer makes the qualitative reaction specific and makes it possible to carry out quantitative colorimetric determinations of nitrites.

Quantitative determination of nitrites

The nitrite solution is prepared from dry chemically pure sodium nitrite with a content of 1 mg of nitrite ion in 1 ml. The solution of phenothiazine in 96% alcohol has a concentration of 0.4 mg of reagent in 1 ml.

The calibration curve was constructed from measurements of the optical density of extracts in isoamyl alcohol, brought to a total volume of 10 ml. Colorimetric measurements were carried out on an FEK-2 photocolorimeter with a green light filter.

The data from measurements of the optical density of the solutions are summarized in Table 1.

Table 1

From the data in Table 1 a calibration curve was constructed (Fig. 1).

Fig. 1

The maximum color intensity of the solutions was reached 15 minutes after extraction and remained stable for 24 hours.

The colored solution obeys Beer’s law. To determine the dependence of the optical density on an excess of phenothiazine, it was measured in extraction solutions after interaction of 1 ml of nitrite solution (0.1 mg of nitrite ion in 1 ml) with 3, 4, 5, and 6 ml of the alcoholic solution

phenothiazine (0.4 mg of reagent in 1 ml). In all four cases the extinction value was 0.43, which corresponds to 63% absorption.

Thus, the optimal ratio of nitrite concentration to reagent concentration is 1:1.5. An excess of reagent has no effect.

Dependence of optical density on the acidity of the solution

To determine this dependence, buffer mixtures in the pH range from 1 to 8, prepared according to the prescription of A. K. Babko and A. T. Pilipenko [7], were used. With constant amounts of nitrite ion (1 ml of a solution containing 1 mg of nitrite ion per ml), reagent (0.16 mg in 4 ml of alcohol), and buffer mixture (10 ml), the optical-density values of the extraction solutions were measured as in constructing the calibration curve. The measurement data are presented in Table 2.

From the data of Table 2 a graph was plotted of the dependence of the optical density of the solutions on the pH of the medium during the interaction of nitrites with phenothiazine (Fig. 2).

Fig. 2

Table 2

From the data of the graph (Fig. 2) it follows that the optimal pH value in determining nitrites with phenothiazine should be in the range 3–4.2.

On the basis of the foregoing, the determination of nitrites in solution should be carried out as follows: to the solution being analyzed, with a volume up to 10 ml, placed in a separatory funnel of 25–30 ml capacity, add 5–10 ml of buffer solution of pH 3.5 and 2–2 ml of an alcoholic solution containing 0.4 mg/ml phenothiazine.

After 5 minutes, the precipitate that has separated is extracted with 3–4 ml of isoamyl alcohol; the alcoholic extract is separated into a 10-ml volumetric flask. Extraction with 2–3 ml portions of isoamyl alcohol is repeated se-

several times; the extracts obtained are added to the first, the flask is brought up to the mark, mixed, and the optical density of the solution is determined on a photocolorimeter with a green light filter.

The nitrite concentration is calculated from a calibration curve, which is constructed by the procedure indicated above, using standard nitrite solutions. The sensitivity of the method is 0.001 mg in 10 ml of final volume, or 1 mg/l of final volume for this type of colorimeter, with a layer thickness of 0.5 cm in a cylindrical cuvette of 1.8 ml capacity.

Conclusions

On the basis of the nitrosation reaction of phenothiazine, it is possible to carry out a qualitative test and a quantitative photocolorimetric determination of the nitrite ion in samples containing nitrites at concentrations above 1 mg/l, and in colored and turbid solutions where application of the Griess reaction is difficult.

Literature

1. Treadwell F. P., Hall W. T., Course of Analytical Chemistry, 1, 381, 1945.
2. Zimarev V. N., Factory Laboratory, 7, 555, 1938.
3. Mikhailov B. M., Ocherovich R. E., Factory Laboratory, 9, 836, 1940.
4. Kristalev P. V., Kristaleva L. B., Proceedings of TSU, 145, 73, 1957.
5. Shnaiderman S. Ya., Proceedings of the Kiev Polytechnic Institute, 24, 152, 1954.
6. Vanino L., Präparative Chemie, II Bände, 821, 1923.
7. Babko A. K. and Pilipenko A. T., Colorimetric Analysis, GKhI, 387, 1951.