The absorptiometric determination of silicon in water. Part I. Formation, stability and reduction of α- and β-molybdosilicic acids

Abstract

The effect of experimental conditions on the formation, stability (in the presence of reagents used for destroying molybdophosphoric acid) and reduction of a-and p-molybdosilicic acids -has been determined. The effectiveness of several reagents for preventing inierference from phosphate has also been investigated. Both cr-molybdosilicic: acid reduced by stannous tin and p-molybdosilicic acid reduced by 1 -amino-2-naphthol-4-sulphonic acid, should be suitable for precise methods of determining "reactive" silicon in water.AT the steam pressures and temperatures used in modern power stations, silicic acid is appreciably soluble in steam, The concentration of silicon in boiler and make-up water is therefore controlled to ensure that the steam contains not more than about 0.02 p.p.m. of silica; above about 0.02 to 0.04 p.p.m. of silica undesirable deposits may be formed on turbine blades.Methods are therefore needed for deteirmining both the "reactive"* and total silicon concentrations in a range of aqueous samples from steamwater circuits in power stations.Thus the methods must be suitable for conde:nsate, feed-water and steam (0-002 to 0-05 p.p.m. of silica), make-up water (0.005 to 0-2 p.p.rrt. of silica) and boiler water (0.2 to 75 p.p.m. of silica).The ratio of phosphate (p.p.m.) to silica (p.p.m.) is usually between 0 and 20, but occasionally ratios as high as 50 may occur.The methods must also be precise, so that they can be used for determining small amounts of "non-reactive" silicon as the difference between the total and "reactive" silicon contents.Absorptiometric methods in which the reduced aand P-molybdosilicic acids are used appear to be the only sufficiently sensitive means of determining extremely small concentrations of silicic acid. They can also be used for determining total silicon provided all forms of silicon are converted to silicate or monomeric silicic acid in an earlier step.This paper reports an investigation of the molybdosilicic acids to determine suitable conditions for each method; Parts 112 and 1113 give details of the methods and the results obtained.Two forms of molybdosilicic acid, a-and p-, are of analytical interest and were first distinguished by S t r i ~k l a n d . ~Each form can be reduced to several different blue compounds with various reducing agents, and Strickland4 has determined the properties of several forms and the conditions for their formation. The relative amounts of ci-and P-molybdosilicic acids produced and their stabilities appear to be affected by factors such as the concentrations of acids, molybdate, neutral salts and reagents such as oxalic or tartaric acid used for destroying molybdophosphoric acid. However, the literature does not give quantitative values for all these effects, and they have, therefore, been investigated.Boiler waters often also contain added phosphate. EXPERIMENTAL APPARATUS, REAGENTS AND TECHNIQUE-All optical-density measurements were made in 4-cm cuvettes with a Hilger Cvispek spectrophotometer against distilled water in the reference cu...

Dispersion behavior and stabilization mechanism of alumina powders in silica sol

“…After 1 ml supernatant was dried on 100 8C water bath, 1 g NaOH and just the right amount of deionized water were added to dissolve the amorphous silica particles to Si(OH) 4 . Then the silica concentration was evaluated by measuring dissolved Si(OH) 4 as a molybdenum blue complex by UV-spectrophotometer (Model S-54, China) at 820 nm [10]. All samples were tested three times each in order to avoid inaccuracy problems.…”

“…The decanted supernatant was centrifuged for further sediment of the alumina particles. Subsequently, the silica concentration of the supernatant after centrifuge was measured through molybdenum blue method [10]. After 1 ml supernatant was dried on 100 8C water bath, 1 g NaOH and just the right amount of deionized water were added to dissolve the amorphous silica particles to Si(OH) 4 .…”

Spectrophotometric determination of traces of silicon in water after collection as silicomolybdenum blue on an organic-solvent-soluble membrane filter

“…Accurate measurements of dissolved silicon may be required also in some biological, geochemical, and marine studies. Spectrophotometric methods for the determination of dissolved silicon in natural water are usually based on the formation of a yellow heteropoly acid (1) with molybdate and the reduction of the yellow acid to the more light-absorbing blue form (2)(3)(4)(5)(6)(7)(8). Many reducíante have been studied for the selective reduction of silicomolybdic acid to molybdenum blue to improve the precision and the detection limit, but the determination of microgram-per-liter levels of silicon in water is difficult without preconcentration.…”

“…Dissolve the filter in 5 mL of DMF, and measure the absorbance of the DMF solution at 740 nm against a reagent blank. l-amino-2-naphthol-4-sulfonic acid (2) ß white precip SnCl4 + L-ascorbic acid (4) a 0.649 SnCl2 (8) ß 0.241 p-(methylamino)phenol sulfate (6) formation of a yellow silicomolybdic acid with molybdate followed by reduction of the yellow acid to the more intensely colored silicomolybdenum blue. Many reducíante have been proposed for the reduction of the yellow acid.…”

An Indirect Spectrophotometric Method for the Determination of Silicon in Serum, Whole Blood and Erythrocytes

“…22,23 Regarding the spectrophotometric method, a silicomolybdenum yellow or blue method based on the reaction of silicon, present as silicate in an aquatic solution, with molybdate has been used. However, in the case that this method is applied for biological samples, a large amount of phosphate 21 and various salts 24,25 interfere seriously. An error from phosphate is caused because it reacts with molybdate and yields yellow or blue phosphomolybdate, similarly to silicate.…”