Interferences with silicate determination

The primary use of hydrofluoric acid is for the decomposition of silicate rocks and minerals in the determination of species other than silica. In this treatment, silicon is evolved as the tetrafluoride. After decomposition is complete, the excess hydrofluoric acid is driven off by evaporation with sulfuric acid or perchloric acid. Complete removal is often essential to the success of an analysis because fluoride ion reacts with several cations to form extraordinarily stable complexes that interfere with the determination of the cations. For example, precipitation of aluminum (as AI2O3 XH2O) with ammonia is incomplete if fluoride is present even in small amounts. Frequently, it is so difficult and time-consuming to remove the last traces of fluoride ion from a sample that the attractive features of hydrofluoric acid as a solvent are negated. 

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... 

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. 

Clean-up procedures were applied to remove the bulk of the co-extracted lipid and other organic compounds that would interfere with the chromatographic determination of CBs, either by masking the chromatographic response or by degrading the performance of the column. Different techniques were used, e.g. decomposition with concentrated sulphuric acid (in some cases loaded on silica gel), absorption chromatography (e.g. alumina column), separation with silica gel, Florisil or potassium silicate [32]. 

A detailed description of the methods used is given in the certification report [33], including the internal standards used, GC conditions, calibration techniques etc. Clean-up procedures were based on well proven techniques to remove the bulk of the mineral oil and other organic compounds which would interfere with the chromatographic determination, e.g. decomposition with concentrated H2SO4, adsorption chromatography, separation on silica gel, or separation with Florisil or potassium silicate. For the separation, each CB was identified and confirmed using at least two capillary columns coated with different stationary phases to compare the relative retention times or... 

The pH of the silicate solutions with R — 2.0 and R = 1.5 are given in Table 1. The listed values are corrected for cation interference. Accurate pH determinations for Li silicate solutions and silicate solutions for which R = 0.4 could not be made because the error due to cation interference was on the order of 2 pH units. 

Sodium silicate is somev at more difficult to analyze than many other materials because of the formation of the relatively long lived radionuclide Na whose emissions interfere with the detection of other elements. Nevertheless we were able to determine, in a sample of sodium silicate, that many heavy elements of toxicological concern were undetectable down to the ppm to ppb level in the undiluted silicate (13), An XRF spectrometer can be configured to perform sequential multi-elemental analyses. It is less sensitive to the elements of lower atomic number. Also, since the X-rays penetrate only to a depth of about 10 urn, the sample must be homogeneous. Solid samples must be presented to the X-ray beam with a flat surface. However, the relative ease of sample preparation and the ability to run glasses and solutions with only minor dilution make X-ray fluorescence a useful technique where analysis for a wide range of impurities is required,... 

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 number of techniques have been developed for the trace analysis of silicones in environmental samples. In these analyses, care must be taken to avoid contamination of the samples because of the ubiquitous presence of silicones, particularly in a laboratory environment. Depending on the method of detection, interference from inorganic silicate can also be problematic, hence nonsilica-based vessels are often used in these determinations. Silicones have been extracted from environmental samples with solvents such as hexane, diethyl ether, methyl isobutylketone, ethyl acetate, and tetrahydrofuran (THF)... 

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. 

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... 

Silicate is determined as a reduced silicomolybdate dye 14). The method is sensitive to the reductant used, either stannous chloride or ascorbic acid. The reaction kinetics with stannous chloride are much faster than with ascorbic acid. The analysis with stannous chloride is more sensitive, therefore, and has a detection limit of 0.5 pM compared to 1.0 pM when ascorbic acid is used. Eighty determinations can be made per hour at these detection limits. A detection limit of 0.1 pM can be obtained if the sampling rate is decreased to 50 per hour 14). The analysis with stannous chloride has a smaller salt error than with ascorbic acid, but the interference due to... 

An example of the first approach (matrix assimilation) would be to match the acid content in the standards with the acid content in the samples. Matrix assimilation is only effective provided that the interference is not severe and the sample matrix is relatively simple. For more marked interferences, a second cation can act as a release agent. As an example, lanthanum [as La(N03)3] can be added to solutions in which Ca is to be determined in the presence of P04 , silicate or aluminate in an air/C2H2 flame. An example of the third approach would be to add a strong complexing agent (such as EDTA) to both samples and standards. Many metals have an appreciable tendency to hydrolyse in aqueous media moreover the hydroxides can be sparingly soluble yet precipitates can be difficidt to detect visually in dilute solutions. To limit this process, samples are customarily prepared in acidic media. 

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). 

Probably the most commonly used instruments for cation impurity analysis of silicates are flame atomic absorption spectrophotometers and ion selective electrodes. In most cases, separation of silica is required to reduce interferences. The sample may also have to be diluted to bring the analyte concentration within the linear operating range. For cations, the atomic absorption spectrophotometer is more versatile than ion specific electrodes. If the analyst is concerned with the presence of heavy metals, then accessories such as a hydride system for the elements that form high vapor pressure compounds, e.g., Sb, and a mercury vapor cold trap are useful. If a large number of elements are to be determined, a substantial investment in hollow cathode and electrode discharge lamps must be made. Several gas mixtures will also be required. 

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. 

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