Phosphotungstic acid reagent

Lowry method uses a combination of the Biuret copper-based reagent and the Folin-Ciocalteau reagent, which contains phosphomolybdic-phosphotungstic acid. Reagents react with protein, yielding a blue colour that displays an absorbance maximum at 750 nm... 

Singleton, V. L. and Rossi, J. A. 1965. Colorimetry of total phenolics with phosphomo-lybdic-phosphotungstic acid reagents. Am. J. Enol. Vitic. 3 144-58. 

The Folin-Denis assay is used as a procedure for the quantification of total phenolics in plant materials, food, and beverages. Reduction of phosphomolybdic-phosphotungstic acid (Folin-Denis reagent) to a blue-colored complex in an alkaline solution occurs in the presence of phenolic compounds (Folin and Denis 1912). 

For small peptides, the amount of color increases with the size of the peptide. The presence of any of five amino acid residues (tyrosine, tryptophan, cysteine, histidine, and asparagine) in the peptide or protein backbone further enhance the amount of color produced, because they contribute additional reducing equivalents for further reduction of the phos-phomolybdic/phosphotungstic acid complex. With the exception of tyrosine and tryptophan, free amino acids will not produce a colored product with the Lowry reagent however, most dipeptides can be detected. In the absence of any of the five amino acids listed above, proteins containing proline residues have a lower... 

Phosphotungstic acid finds use in the precipitation of proteins, alkaloids, and certain amino acids. The solubility of sodium phosphotungstate, contrasted with the comparative insolubility of the potassium and ammonium salts, has suggested its use as a reagent in qualitative analysis. 

Phosphoric acid white precipitate of phosphotungstic acid, H3[P04 (Wl2036)].5H20, soluble in excess of the reagent. 

In addition to concerns over the incomplete solubility of some of the H-coal vacuum-still bottom materials in toluene, the precipitation experiments just discussed are subject to problems associated with two-phase reactions in general. Incomplete contact between the organic compounds in toluene solution and the aqueous acids can lead to inconsistent precipitation results. With this in mind, a different approach was developed in which various H-coal vacuum-still bottoms were treated with phosphotungstic acid, which is soluble in tetrahydrofuran. This reagent allows precipitation to be accomplished from an initially one-phase system, and gives rise to crisp, easily collected salts. 

III. The Ordinary Alkaloidal Reagents,—The reagents which precipitate alkaloids from their solutions react also with proteins. Among these may be mentioned phosphotungstic acid, phosphomolybdic acid, tannic acid, picric acid, potassium mercuric iodide, and potassium bismuth iodide. The alkaloidal reagents precipitate the majority of the proteins in acid solution only. 

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