Blue, Alkali Benzidine

Procedure. The distillation can be carried out in the apparatus described on page 40 (Fig. 8). A porcelain crucible, paraffined inside, serves as receiver and should contain about 1 ml of reagent solution. A drop of the test solution (plus a drop of alkali if the solution is acid) is placed in the distillation apparatus, and evaporated to dryness. Five drops of concentrated sulfuric acid and 5 drops of pure methyl alcohol are then added, and, after stoppering the apparatus, it is heated to about 80 C in a water-bath. The methyl borate distils into the receiver and undergoes saponification. The resulting alkali causes the formation of a black precipitate. For very small amounts, it is advisable to add a few drops of an acetic acid solution of benzidine to the micro crucible. Traces of manganese dioxide are revealed by the blue color (see page 301). 

Sixty milliliters of a solution containing Ce and Fe (as sulfates) were treated with alkali until a permanent precipitate formed. Then a few drops of hydrofluoric acid were added, the mixture boiled, and filtered. The precipitate was washed once with water, covered with dilute alkali, and tested with an acetic acid solution of benzidine. In this way the blue color caused by 70 y cerium can be discerned in the presence of 227 mg iron. This corresponds to a ratio Ce Fe = 1 3230. 

Benzidine blue is produced by oxidation when even very dilute solutions of copperii united with benzidine and alkali cyanide (see page 348)... 

Gold salts oxidize acetic acid solutions of benzidine to benzidine blue which is a quinoneimine type of dyestuff (compare page 283). Colored gold hydrosols are produced at the same time. Considerable quantities of heavy metals or alkali salts flocculate the reaction products and so prevent the formation of the sols and consequently impair the test. Platinum salts and oxidants which act on benzidine must be absent. 

This test for lead is interfered with by cerium, manganese, bismuth, cobalt, nickel, silver and thallium salts. Under the same conditions, these metal ions also give higher oxides capable of oxidizing benzidine to benzidine blue. If, however, the lead test is carried out in an alkaline extract (plumbite solution), only thallium offers interference. All the other metal ions are precipitated as hydrous metal oxides and so do not enter the alkaline extract. When only bismuth salts are present, it is sufiftcient, before the addition of bromine water, simply to heat with the alkali, BiO(OH) is formed which is not converted into the higher bismuth oxide by hypobromite. 

Procedure. A drop of the test solution is placed on filter paper and treated successively with 2 N alkali and saturated bromine water. After the last drop has soaked into the paper, 2 drops of a 1 1 solution of ammonia are added, allowed to spread, and the excess ammonia removed by waving the paper over a small flame. The solution of benzidine in acetic acid is then added and, according to the amount of lead present, a light to deep blue fleck is formed. The blue color fades if only very small amounts of lead are present. 

Lead can be detected in even greater dilutions by the following procedure The test solution is treated with 3 ml of 3 iV alkali and 2 ml of bromine water, boiled a few times, and filtered through quantitative filter paper. After washing with hot ammonia (1 1) followed by hot water, a drop of an acetic acid solution of benzidine is placed on the paper. In the presence of lOy of lead in 10 ml test solution, which implies a dilution of 1 1,000,000, there is an appreciable blue coloration. 

Neutral or weakly acetic acid solutions of alkali persulfates turn blue on addition of benzidine acetate due to the formation of a quinoidal oxidation product of benzidine (see page 283). Alkali peroxides, perborates, and hydrogen peroxide do not react with benzidine under these conditions. 

When solutions of lead or bismuth salts are treated with alkali hypo-bromite, lead peroxide and higher oxides of bismuth are formed. These products react with benzidine acetate to give benzidine blue (seepage 282). However, if bismuth salts are heated alone with alkali hydroxides, a yellow precipitate forms, [probably BiO(OH)] and, when once formed, is not converted into higher bismuth oxides on treatment with h3q)obromite. This resistance is applied in the detection of lead in bismuth salts. The following procedure is recommended in the examination of pharmaceutical bismuth preparations (e.g., bismuth sub-gallate, -tannate, -salicylate). 

Amounts smaller than 0.15 y manganese per drop may be detected 100 - 150 ml water is treated with a few drops of sodium hydroxide solution, boiled, and filtered. The calcium and magnesium carbonates thus precipitated by the carbonate in the alkali coprecipitate any manganese dioxide formed. The mixture is filtered through a quantitative paper and tested on the paper with benzidine solution. A blue color indicates manganese. As little as 1.2 y manganese (dilution 1 25,000,000) may be detected. 

The oxidation of benzidine to benzidine blue has been used for peroxidase tests by Madelung (234), Begemann (58), Zinn el al. (382), and Farrell (145). But this reaction can hardly be regarded specific and reliable since numerous compounds give the blue color, such as Cu salts (245), alkali metal halides (302), benzoyl peroxide (119), and others. A compound closely related to benzidine, 2,7-diaminofluorene, has been suggested as a donor by Schmidt and Hinderer (306) and is said to be even more sensitive than benzidine itself. The structures of the two compounds are ... 

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