Interferences with silicate determination metals

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... 

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. 

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. 

Silicate is determined spectrophotometrically with ammonium molybdate and ammonium vanadate. The pH of the sample must be adjusted to 7-8. Potassium cyanide is added to prevent interference of heavy metals. Oxalic acid is added to destroy mo-lybdophosphate and vanadophosphate and to bind aluminum in a complex. As in all spectrophotometric determinations, high and variable optical absorption of the sample (due to color or turbidity) at the wavelengths of investigation causes errors tannin, iron, and sulfide also interfere. To avoid contamination, all contact surfaces should be of polyethene. It is also possible to determine silicate using FAAS, in which case nitrous acid and ethyne must be used as flame gases. As silicates are present in colloidal form, the sample must be introduced into the AAS equipment using an ultrasonic nebulizer. Such a nebulizer is also used when silicate is measured by ICP-AES. 

---