Heteropoly blue method

Silica. The siUca content of natural waters is usually 10 to (5 x lO " ) M. Its presence is considered undesirable for some industrial purposes because of the formation of siUca and siUcate scales. The heteropoly-blue method is used for the measurement of siUca. The sample reacts with ammonium molybdate at pH 1.2, and oxaUc acid is added to reduce any molybdophosphoric acid produced. The yellow molybdosiUcic acid is then reduced with l-amino-2-naphthol-4-sulfoiiic acid and sodium sulfite to heteropoly blue. Color, turbidity, sulfide, and large amounts of iron are possible interferences. A digestion step involving NaHCO can be used to convert any molybdate-unreactive siUca to the reactive form. SiUca can also be deterrnined by atomic... 

The chloride was added as an interference in the nitrate determination and the pH was adjusted to 1.5 with sulfuric acid to preserve the samples. In the determination of silica, the participants were to analyze the samples by either the colorimetric molybdosilicate method or the heteropoly blue method. Phosphates were to have been determined by... 

Spectrophotometric methods based on an enhancement of the blue color produced on reduction of 12-molybdophosphate (arsenate) in the presence of antimony(III) are widely used for the determination of phosphoms(V) or arsenic(V). However, nature of heteropoly blue, their spectra, mechanism of the reaction are obscure. In addition, mixed POMs were shown as very efficient analytical forms for the determination of P(V) and As(V). 

A. Molybdenum blue method Discussion. Orthophosphate and molybdate ions condense in acidic solution to give molybdophosphoric acid (phosphomolybdic acid), which upon selective reduction (say, with hydrazinium sulphate) produces a blue colour, due to molybdenum blue of uncertain composition. The intensity of the blue colour is proportional to the amount of phosphate initially incorporated in the heteropoly acid. If the acidity at the time of reduction is 0.5M in sulphuric acid and hydrazinium sulphate is the reductant, the resulting blue complex exhibits maximum absorption at 820-830 nm. 

Chloride does not interfere in the procedure. Phosphorus interferes in the micro method by formation of a heteropoly blue similar to that formed by pentavalent arsenic. 

The title, "The Determination of Phosphorus, for example, would interest the present author, but at once questions arise. In what kind of material is the phosphorus determined Is a separation of the element necessary If so, what kind of method is applicable Is a colorimetric method of measurement used If so, what is the color-forming reagent If it is ammonium molybdate, is the system measured molybdophosphoric acid, molybdovanadophosphoric acid, or a heteropoly blue If the latter, what are the reductant and the conditions of reduction To the extent that the title does not answer these and other similar questions, the summary should. 

In 1998, Baker and Kirby conducted a 31P NMR investigation of electron exchange in the two-electron reduced heteropoly blue complex of [(P2Wi706i)2Th]18 (which contains an equilibrium mixture of oxidized, two-electron- and four-electron-reduced species) as a function of alkali metal counterion, concentration, and temperature. They interpreted their data in terms of Equation (8) in which the more strongly pairing alkali metals (M in Equation (8) = K+, Rb+, and Cs+ but not Li+) form an ion bridge between the two defect HPA units in the syn isomer. This interaction stabilizes the syn isomer and drives an apparent syn-anti equilibrium, Equation (8), to the left.118 The change in chemical shifts and other features of the 31P NMR spectra of these Th sandwich POM complexes as a function of the counterion defined a qualitative method to estimate the association of monocations with POM polyanions ... 

Interfering species in the determination of phosphorus by the phosphomolybdenum blue method are As(V), Si, and Ge, which also react with molybdate to form the corresponding acids which are reduced to the respective heteropoly blues. Arsenic(V) does not interfere when reduced to As(III) using sulphite or thiourea. In the presence of vanadium(V), molybdovanadophosphoric acid is produced. Large amounts of vanadium(V) are reduced with Mohr s salt to V(IV) before the molybdate is added. The difference in the rates of formation of the phosphomolybdenum- and silicomolybdenum- blues has been utilized for the determination of phosphorus in the presence of silicon [29]. The interference of silicon can be prevented by the use of a sufficiently acidic medium [30]. ... 

The new method reverses the order of the transfer/reduction procedure of the heteropoly complex. Instead of transferring the oxidized TMS HPT into nonpolar solvent and then reducing it in that solvent, we decided to reduce the TMS HPT in aqueous solution first, and transfer the reduced HPT (heteropoly blue) into a nonpolar solvent (toluene). 

Heteropoly Blue Complexes. Evaluation by a Modification of Evans s Susceptibility Method, 7. Am. 

Colorimetric Techniques The most familiar colorimetric technique is the molybdate blue method (Koch and Koch-Dedic 1964). Phosphate ion (P04 ), when in solution, reacts with a molybdate ion to form a heteropoly acid complex, H7[P(Mo207)5], that after use of suitable reduction agent is reduced to a phosphoro-molybdate blue complex, with an absorption maximum at 820-830 nm. The most popular reduction agents are chlorostannous (tin II dichloride) acid reductant, sodium bisulfite (monosodium, monohydrogen sulfate (IV)) and ascorbic acid. 

Inlerference in the phosphomolybdenum blue method comes primarily from As, Si and Ge, which also react with molybdate to form the corresponding isostructural acids which are reduced to heteropoly blues. All of these have absorption maxima close to each other (Figure 14.1). 

Principle Soluble silica species react with molybdate at 37°C and pH of 1.2 to form a yellow silicomolybdate complex. This complex is subsequently reduced with stannous chloride to form a heteropoly blue complex, which has an absorbance maximum at 820 nm. The intensity of the color is proportional to the concentration of molybdate-reactive silica. Though the method is written for brackish and seawater, it is also applicable to nonsaline sample matrixes. The method is calibrated using standards prepared in deionized water. Once calibrated, samples of varying salinities (0 to 35 ppt) may be analyzed. The determination of background absorbance is necessary only for samples, which have color absorbing at 820 nm. 

A standard Lowry-based protein assay has been adjusted to the special conditions encountered with skin [126], Basically, proteins reduce an alkaline solution of Cu(II)-tartrate to Cu(I) in a concentration-dependent manner. Then, the formation of a blue complex between Folin-Ciocalteau reagent (a solution of complex polymeric ions formed from phosphomolybdic and phosphotungstic heteropoly acids) and Cu(I) can be measured spectrophotometrically at 750 nm. A calibration curve can be obtained by dissolving known amounts of stratum corneum in 1 M sodium hydroxide. A piece of tape that has not been in contact with skin is subjected to an identical procedure and serves as negative control. The method was recently adapted to a 96-well plate format, notably reducing analysis times [129],... 

Ingle and Crouch described a diflerential kinetic method for silicate and phosphate based on the faster rate of formation of heteropoly molybdenum blue from the yellow heteropoly acids in the presence of phosphate than in the presence of silicate. They found that silicon in the range of 1 to 10 ppm could be determined with 3% accuracy in the presence of 10 ppm of phosphorus, and phosphorus in the range of 1 to 10 ppm with 1% accuracy in the presence of 50 ppm of silicon. This system was also automated, with the analyses of mixtures being performed in less than 5 min. 

In the spectrophotometric determination of Si, Ge, P(V), As(V), and V(V) the yellow heteropoly acids occurring in acid solutions in the presence of an excess of molybdate or tungstate are important. The yellow heteropoly acids are the basis of less sensitive spectrophotometric methods, but the blue reduction products (e.g., phosphomolybdenum blue) are the basis of very sensitive spectrophotometric methods for determining these elements. The conditions for formation and extraction of these compounds have been investigated [133-135]. 

Germanium(rV) forms heteropoly acids with molybdate and other ions. The method for determining germanium, based on yellow germanomolybdic acid [35-37] is insensitive (e = 2.0-10 at 430 nm), but reduction of the heteropoly acid to germano-molybdenum blue [38,39] considerably increases the sensitivity (e = 1.0-10 at 800 nm). 

Silicon is determined spectrophotometrically as the yellow heteropoly molybdosilicic acid (less sensitive method) or, after reduction, as silicomolybdenum blue. Very sensitive methods, based on ion-association complexes with basic dyes, are becoming increasingly important. 

Folin-Denis method Reduction of complex polymeric ions formed from phosphomolybdic and phospholungslic heteropoly acids to complex molybdenum-tungsten blue. detection wavelength 725 - 770 nm recommended for uniformity 765 nm complexes and reagent are unstable in alkaline solution, formation of precipitates, controlled sequence and timing of the addition of reagents (reproducibility ), deviation from Beer-Lambert law (high phenol contents), reaction is stoichiometrically predictable 105,106,110... 

Spectrophotometric methods are usually preferred for routine analysis of this parameter, most of them relying on the reaction between orthophosphate ions and molybdate in acidic medium in order to form a heteropoly acid. Color formation can be enhanced by adding vanadate to obtain the yellow vanadomolybdate complex (vanadomolybdophosphoric acid method) or by reducing the molybdo-phosphoric acid to yield strongly colored phosphomolybdenum blue species. 

All methods for the determination of inorganic phosphate in seawater are based on the reaction of the ions with an acidified molybdate reagent to yield a phosphomolybdate heteropoly acid, which is then reduced to a highly coloured blue compound. In early work, tin(//) chloride was used as the reductant in flow-analysis (Hager et al, 1968). However, this reductant has several disadvantages, including the appreciable temperature dependence of the reduction rate and the pronounced salt error. 

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. 

Silicate, arsenate, and germanate also form heteropoly acids, which on reduction yield molybdenum blue species with similar absorption maxima [97]. This positive interference in the determination of phosphate is particularly pronounced for silicate because of its relatively high concentration in many waters. However, the formation of silicomolyb-date may be suppressed by the addition of tartaric or oxalic acid to the molybdate reagent [98]. If, however, the organic acid is added after the formation of the heteropoly acid, the phosphomolybdate is destroyed, and this is used as the basis for determination of silicate in the presence of phosphate. Kinetic discrimination between phosphate and silicate, arsenate and germanate is also possible because of the faster rate of formation of phosphomolybdate. Thus, the widely adopted Murphy and Riley method employs a reagent mixture of acidic molybdate and antimonyl tartrate [83] at concentrations which are known to enhance the kinetics of phosphomolybdate and suppress the formation of silicomolybdate. 

Silicic acid may be detected by a non-specific method beginning by forming a stable heteropoly acid, yellow H4SiMoi204o The unknown is treated with a neutral solution of ammonium molybdate in a test tube, and the mixture is slightly acidified, forming the complex then a few drops of SnCb are added, producing a deep-blue Molybdenum Blue if more than 0.01-mM silicic acid is present. 

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