Molybdic acid method, silica determination

Instead of an alkaline solution, a dilute solution of NaF and HCl to generate HF was employed by Goto (216a) to measure the specific surface area of colloidal silica from the rate of dissolution in this acid medium. The samples from the reaction mi.x-ture were removed, the reaction was stopped by adding aluminum salt to combine with the fluoride, and the dissolved silica was then determined by the molybdic acid method. 

The term active silica has sometimes been used in referring to polysilicic acid. For example, a distinction has been made by Rule (160) between active silica and other forms of polymeric or colloidal silica. Active silica is defined as any silica in molecular or colloidal aqueous solution, in such a state of polymerization that when diluted with sodium hydroxide solution to a pH of 12, and concentration of about 0.02 percent SiOj, at 30 C, the silica will be depolymerized substantially completely to monomer in not more than 100 minutes. The monomer is determined by the molybdic acid method. 

Where the ionization constant and solubility are not known, as for example at some unusual pH, Van Lier (114) has shown how the ion concentration and solubility can be calculated easily from data on pH and total soluble silica, which includes both monomer and ionic silica as determined by the molybdic acid reagent. In general terms their method is as follows. 

In a patent issued to Balthis (212b) a method is described whereby a sample of deionized silica sol is put into an excess of 0.01 IV NaOH solution at 30 C and over a period of 90 min samples of the solution are removed and the amount of monomeric silica is determined by reaction with the molybdic acid reagent. The rate of dissolution of the silica to form monomeric silicate in the alkaline solution was related to A, the specific surface area of the silica (in square meters per gram) determined by other means, by the following equation ... 

Although this reaction was discovered in 1898 by Jolles and Neurath (298) and used for many years, it was only in 1952 that Strickland (299) showed that variations in the extinction coefficient under some conditions were due to the existence of two forms of molybdic acid, alpha and beta, that react with silica to give yellow silicomolybdic acids having, different extinction coefficients and absorption peaks. The beta form is obtained at lower pH and is used in the yellow method, but unless conditions are optimum, it changes with time to the alpha form, which is less colored but more stable. The beta form is the only one that is used for determining monosilicic acid in the presence of polymers because it is formed at once, and its color is more intense. 

In contrast to the above, Andersson (301) reported that at a wavelength of 325 nm, at pH 1.5, alpha silicomolybdate had a very low extinction coefficient as compared with the molybdic acid blanks, and for this reason he used 400 hm. However, his method involves a heating step to convert the beta to alpha form, and so is time-consuming and inconvenient for studying silica polymerization. It is useful for accurate determination of total silica with a standard deviation of less than 0.5%. 

Her has carried out a hitherto unpublished investigation of the rates at which extremely small particles of colloidal silica depolymerize to monomer both directly in the molybdic acid reagent and in dilute alkali in which the monomer is determined on separate samples by reaction with molybdic acid. The size of the particles was estimated from the specific surface area, which was determined by the Sears alkali titration method corrected for monomer. The measurements were made as the particles grew in size at pH 8.5 and also as they became aggregated at pH 5.9. 

The method may be standardised, if desired, with pure potassium dihydrogen-orthophosphate (see below) sufficient 1 1 hydrochloric acid must be present to prevent precipitation of quinoline molybdate the molybdophosphate complex is readily formed at a concentration of 20 mL of concentrated hydrochloric acid per 100 mL of solution especially when warm, and precipitation of the quinoline salt should take place slowly from boiling solution. A blank determination should always be made it is mostly due to silica. 

Silica. The silica content of natural waters is usually 10 to x 10 ) M. Its presence is considered imdesirable for some industrial purposes because of the formation of silica and silicate scales. The heteropoly-blue method is used for the measurement of silica. The sample reacts with ammonium molybdate at pH 1.2, and oxalic acid is added to reduce any molybdophosphoric acid produced. The yellow molyb do silicic acid is then reduced with l-amino-2-naphthol-4-sulfonic 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 silica to the reactive form. Silica can also be determined by atomic... 

Soluble silica as determined by the molybdate method is not necessarily present as Si(OH)4. Bogdanova (40) reported that in natural waters that contained only about 5 ppm total silica, 4-9% of the silica was polymeric but was converted to monomer by acid. It is most likely that the polymeric" silica was actually very small colloidal particles of aluminum silicate that liberated monomer when acidified.. ... 

A turbidometric method based on the extreme insolubility of the quinine salt of silicomolybdic acid was developed by Defasse (331). The reagent is a mixture of quinine sulfate, ammonium molybdate, HjSO, and HNO3, and owing to the large mass of precipitate formed by a small amount of silica, the turbidity permits rapid determination of silica down to 1 ppm in water as long as phosphorus is not present at concentration greater than I ppm. 

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