Silicomolybdate acid complex

Phosphate reacts with ammonium molybdate too, similarly forming yellow phosphomolybdic acid. The presence of phosphate, therefore, interferes in the test. The addition of oxalic acid or citric acid destroys phosphomolybdic acid complex but not silicomolybdic acid complex. The intensity of color developed is proportional to the concentration of sihca in the sample. 

Ammonium molybdate-benzidine test (DANGER THE REAGENT IS CARCINOGENIC.) Silicates react with molybdates in acid solution to form the complex silicomolybdic acid H4[SiMo12O40]. The ammonium salt, unlike the analogous phosphoric acid and arsenic acid compounds, is soluble in water and acids to give a yellow solution. The test depends upon the reaction between silicomolybdic acid and benzidine in acetic acid solution whereby molybdenum blue and a blue quinonoid oxidation compound of benzidine are produced. 

Phosphoric and arsenic acids form compounds analogous to silicomolybdic acid which also react with benzidine with colour formation hence these acids should be removed before applying the test. In the presence of phosphoric acid, the test is carried out as follows. Mix a drop of the test solution with 2 drops of the molybdate reagent in a micro centrifuge tube and centrifuge the mixture. Transfer the supernatant liquid to a micro crucible by means of a capillary tube, warm gently, cool and add 2 drops 05m oxalic acid solution (the latter decomposes the small quantity of residual phosphomolybdate (NH4)3[PMo12O40] but has little action on the silicomolybdic acid complex), then introduce a drop... 

Various methods for the determination of silicon, particularly from biological samples are available. The techniques usually involve decomposition of the compound to give silica or a silicate by either wet combustion or fusion with sodium peroxide. The silicate formed can then be determined colorimetrically, volumetrically, or gravimetrically. Acidified ammonium heptamoiybdate reacts oniy with monomeric Si(OH)4 and not its oiigomers to give a yellow silicomolybdic acid complex that can be determined spectrophotometrically. This method works... 

The silicomolybdate acid complex method is used for in-process monitors for silica content up to 30 ppm in process water. In combination with an automatic sampling and dilution system, such a monitor could assay for silica in a process stream with a precision of 0.5 to 1%, relative. 

Electrochemical method [54] Silicate is determined in sea water by four different electrochemical methods based on the detection of the silicomolybdic complex formed in acidic media by the reaction between silicate and molybdenum salts. The first two methods are based on the addition of molybdate and protons in a seawater sample in an electrochemical cell. A semiautonomous method was developed based on the electrochemical anodic oxidation of molybdenum, the complexation of the oxidation product with silicate and the detection of the complex by cyclic voltammetry. Finally a complete reagent-less method with a precision of 2.6% is described based on the simultaneous formation of the molybdenum salt and protons in a divided electrochemical cell. 

If larger quantities are desired, the wet method of silicomolybdate decomposition is more convenient. The latter complex is treated with barium hydroxide solution saturated at low temperature. (To avoid working with excessive quantities of liquid, part of the Ba(OH)a can be added as the solid.) The amount added must exceed by 20% the amount required to decompose the complex into barium molybdate, barium silicate and RbOH (CsOH). The mixture is then boiled for thirty minutes. Flame gases containing CO3 should not come in contact with the mixture or large amoimts of Ba(OH)a will be converted to worthless BaCOa. The barium molybdate and barium silicate which separate are not filtered off imtil after the reaction mixture has cooled. The filtrate is then saturated with C03 and boiled for fifteen minutes. The BaCOa precipitate is filtered off and the filtrate is evaporated with simultaneous addition of hydrochloric acid. The residue contains RbCl and CsCl free of Mo and Ba. 

Interference by phosphate ion is an especially common problem. Since the phosphate ion reacts like silica to form a yellow phosphomolybdic acid, its interference must be eliminated. Numerous techniques have been proposed, either for separating the silica and phosphorus before analysis or preferentially reducing silicomolybdic acid to molybdenum blue in the presence of the phosphomolybdic acid (311-313). Snell and Snell (314) summarized the possible procedures (a) precipitating and removing phosphate as the calcium salt. (6) adjusting pH so only silica will form the yellow color, (c) destroying the yellow phosphate complex with citric, oxalic, or tartaric acids, and d) preferentially reducing the silicomolybdic acid to molybdenum blue. 

Okura (296) recommended the following procedure to 50 ml of the hot sample in a platinum dish add 4 drops concentrated H SO, 1 ml 10% ammonium molybdate solution, and 20 mg NaF, in that order cool and measure the color. On the other hand, Tarutani (308) stated that F" ion interferes with the silicomolybdic method and must be removed by complexing with boric acid or The difference... 

Silicate. As stated by Murphy and Riley (1962 ) there is no effect of silicon up to 350/imol/L. Koroleff (personal communication) has found this to be valid only if the absorbance is measured after about 5 min. During prolonged standing, a blue silicomolybdic complex is gradually formed. The increase in colour intensity with time is fairly linear during the first hour thereafter the increase is small. The effect of silicate depends also on the acidity of the reaction the increase in colour is somewhat smaller in 0.2 than in 0.1 mol/L sulphuric acid. If the colour is measured after 10 min, there is practically no interference caused by silicate at natural seawater concentrations (up to 200/imol/L). If measurements are performed after about half an hom, 200/imol/L silicate will increase the net absorbance by 0.003 in a 10 cm cell, which still is almost negligible. 

Depending on the pH, the yellow silicomolybdic complex exists in two isomeric forms which differ only in their degree of hydration. The a-isomer is formed at pH 3.5-4.5 and is very stable once formed the jS-silicomolybdic acid is formed rapidly in the pH range 0.8-2.5, but is much less stable. According to Grasshoff 19(A), the molar absorptivity of the p-acid at 390 nm is 1.7 times that of the a-form. 

Since both of the yellow silicomolybdic acids have only low intensity colours, several methods have been developed in which the complexes are reduced to intensely coloured blue complexes. These heteropoly acids are well-defined soluble compounds and not colloidal products as are the blue phospho- and arsenomolybdic complexes. The most conunon reducing agents are metol (p-methylaminophenol sulphate) and sulphite Strickland and Parsons, 1968), and ascorbic acid Koroleff, 1971). The manual method and flow-anal5rsis described below use ascorbic acid as the reductant. 

After a mean reaction time of 10 min, oxahc acid is added for two reasons ( ) to avoid reduction of the excess molybdate and (it) to eliminate the influence of phosphate present in the sample. As the silico complex has only a limited stability in the presence of oxalic acid, the reductant (ascorbic acid) is added immediately after the oxalic acid. The amoimt of ascorbic acid required is one quarter of that in the phosphate determination. No further acid need be added, as is the case in the metol-sulphite reduction. Hie blue silicomolybdic complex is formed within 30 min and is stable for several hours. 

It is possible to detect phosphoric acid in the presence of silicic and arsenic acids by utilizing the fact that, under suitable conditions, the formation of silicomolybdic and arsenimolybdic acids can be prevented by tartaric acid. This masking depends on the formation of a stable complex compound of molybdic and tartaric acids, in which the molybdic acid does not react with arsenic and silicic acids, but is not masked toward phosphoric acid. 

Phosphoric and arsenic acids form complex compounds similar to silicomolybdic acid and these products also give a color reaction with benzidine. These acids must therefore be removed before the test. Regarding other interferences, see page 388. 

Principle Soluble silica species react with molybdate imder acidic conditions to form a yellow silicomolybdate complex. This complex is subsequently reduced with l-amino-2-napthol-4-sulfonic acid (ANSA) and bisulfite to form a heteropoly blue complex, which has an absorbance maximum at 820 nm. 

The seawater sample is allowed to react with molybdate under conditions which result in the formation of the silicomolybdate, phosphomolybdate, and arsenomolyb-date complexes. A reducing solution, containing metol and oxalic acid, is then added which reduces the silicomolybdate complex to give a blue reduction compound and simultaneously decomposes any phosphomolybdate or arsenomolybdate, so that interference from phosphate and arsenate is eliminated. The extinction of the resulting solution is measured using 1- or 10-cm cells. 

The effect of various interfering ions on the molybdate complex must also be considered. Only a very small amount of nitrate can be tolerated, and the permissible chloride concentration depends upon the amount of HCl removed during preparation of the reducing solution (Andersson [1962a]). Molybdate complexes with sulphuric acid when the latter is present in large amounts, and under these conditions, decomposition of the silicomolybdate must occur to a certain extent. Since the heteromolybdates of arsenic and phosphorus are less stable than the corresponding silicomolybdates, interference by these elements can, to some extent, be eliminated by use of polybasic organic acids such as oxalic and tartatic, which form complexes with molybdate. Interference by phosphate, however, cannot be eliminated completely. 

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