Interferences with silicate determination phosphate

Seasalt also may cause errors in the spectrophotometric signal. These salt effects are either a suppression of the analyte absorbance (e.g., in the determination of silicate and phosphate) by the ions of seawater or an effect of the buffer capacity of seawater (e.g., shifts in the reaction pH interfere with the determination of ammonia). 

Metal salts may be used in the treatment of wool. Flame methods for the determination of aluminium [185], barium, chromium, copper, mercury, strontium, tin, zinc [186] and zirconium [187] in wool have been published. Standard additions to wool cleaned by soaking and washing it with disodium EDTA (800 ml of 0.5 M for 30g wool with soaking for 3 days and double washing) was used as the calibration technique. This compensated for interferences from hydrochloric acid and amino-acids. The samples were equilibrated to a constant humidity for 24 h and then 0.3 g sealed with 5 ml of constant boiling point hydrochloric acid in a glass tube. The tubes were placed in an oven at 110UC for 20 h. The nitrous oxide/acetylene flame was used for the determination of aluminium and zirconium. Sulphate, phosphate, citrate and silicate have been found to interfere in the determination of titanium and zirconium in fire-proofed wool [188], These flame... 

Chromium was determined by Williams et cH. (W7) in animal feces samples to study pasture intakes. In the air-acetylene flame the sensitivity limit was 0.15 ppm. Of a variety of substances tested individually, only calcium, silicate, and phosphate depressed chromium absorption. However, when interferences were studied following treatment of solutions with phosphoric acid, manganese sulfate, and potassium bromide, depression was caused by silicon and aluminum, but calcium and magnesium enhanced absorption. Calcium was also capable of abolishing the effect of silicon and aluminum. 

Protective agents prevent interference by preferentially forming stable but volatile species with the analyte. Three common reagents for this purpose are EDTA, 8-hydroxyquinoline, and APDC (the ammonium salt of 1-pyrrolidine-carbodithioc acid). For example, the presence of EDTA has been shown to minimize or eliminate interferences by silicate, phosphate, and sulfate in the determination of calcium. 

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. 

Brewer and Spencer [428] have described a method for the determination of manganese in anoxic seawaters based on the formulation of a chromophor with formaldoxine to produce a complex with an adsorption maximum at 450 nm. Sulfide (50 xg/l), iron, phosphate (8 ig/l), and silicate (100pg/l) do not interfere in this procedure. The detection limit is 10 pg/1 manganese. 

A prerequisite for the molybdenum blue method is that all the arsenic has to be present as arsenate. After digestion with oxidizing acids, such as nitric acid, all the arsenic is converted into arsenate when appropriate heating time and temperatures are applied. The principle of this determination is the reaction of arsenate with ammonium molybdate in acidic medium to form an arsenate containing molybdenum heteropolyacid that can be reduced to molybdenum blue with stannous chloride, hydrazine, or ascorbic acid. Best results are obtained with hydrazine sulfate. The absorption maximum of the blue solution is between 840-860 nm (15). The most severe interferences for this method derive from phosphates and silicates. To remove interfering ions, distillation of arsenic as AsCb or AsBrs is often recommended (12,15). 

Typical calibration graphs are presented in Figure 98. Sulfate has been determined by titrating with magnesium ions (Figure 98A). The sulfate ion concentration may vary between 1 and 20/i.gl . Phosphate and silicate interfere, and these ions should be removed before the determination. 

Silicate is determined by reaction with a solution of molybdate in an acidic medium to produce a complex molybdosilicic acid with a yellow color (400 nm). More accurate determinations are obtained by reduction of the complex to molybdenum blue (815 nm). Phosphates, arsenates, and german-ates cause interference because they react with molybdate to produce complexes of the same color. 

An SIA system for the simultaneous determination of phosphate and silicate in waste-water is proposed. The method is based on the formation of yellow vanadomolybdopho-sphate and molybdosilicate, respectively, in addition to the use of large sample volumes. The mutual interference between both analytes was eliminated by selection of the appropriate acidity and by sample segmentation with oxalic acid. The calibration graph for phosphate and silicate is linear up to 12 mg/L P and 30 mg/L Si, respectively. The detection limits are 0.2 mg/L P and 0.9 mg/L Si. The method provides a throughput of 23 samples/h with a relative standard deviation <1.4% for phosphate and <4% for silicate. The method was foimd to be suitable for the determination of these species in wastewater samples. 

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