Silicate spectrophotometric methods

Spencer and Brewer [111] have reviewed methods for the determination of silicate in seawater. Various workers [209-212] have studied the application of molybdosilicate spectrophotometric methods to the determination of silicate in seawater. In general, these methods give anomalous results due, it is believed, to erratic blanks and uncertainty regarding the structure of the silicomolybdate formed. 

The United States Pharmacopoeia 23 [11] and Indonesian Pharmacopoeia IV [9] describe the assay of benzoic acid and salicylic acid in ointments. Two chromatographic columns (20 x 2.5 cm) are used to effect the separation. One transfers a mixture of 1 g siliceous earth and 0.5 mL diluted phosphoric acid (3 in 10) to the first column (A), then packs above this a mixture of 4 g siliceous earth and ferric chloride-urea reagent. A mixture of 4 g siliceous earth and 2mL of sodium bicarbonate solution (1 in 12) is packed into the second column (B). For analysis, column A is mounted directly above column B. The sample solution is inserted onto column A, allowed to pass into the column, and then washed with 2-40 mL of chloroform. Benzoic acid can be eluted from column B by using a 3 in 100 solution of glacial acetic acid in chloroform. The benzoic acid content then can be determined by a spectrophotometric method such as that described earlier (section 4.5). 

Analytical Methods. Temperature, pH, and oxygen were measured in situ by using a combined sensor (Ztillig). Ammonium was determined by flow injection analysis (27), and nitrate and silicate by spectrophotometric methods (Auto-Analyzer) (28). Sulfide was determined by using a H2S-specific electrode (29). 

Many of the UV-VIS spectrophotometric methods (shown in Tables 12.3 and 12.6) have been automated by using flow analyzers. Thus, nitrite and nitrate,50,82 ammonium,50,83 orthophosphate,50,84,85 silicates,50,86 chloride,50,87 cyanide,50,88 and sulfate50,89 are measured by CFA and FIA. Oxygen is measured by iodometric titration51,90 and electrochemical methods91 (Table 12.7). Other dissolved gasses (Table 12.2) are measured by ISE-based gas sensors. 

Colorimetric methods Silicon is determined by the molybdenum blue spectrophotometric method after solubilization in H2O, in alkaline solutions or in concentrated HF. A flow analysis procedure for the measurement of soluble silicon with respect to the total Si concentration is used. The proposed method is applied to samples of rain water and of aerosols on filters [45]. Simultaneous determination of orthophosphate and silicate in brackish water is performed by the same technique. Molybdate/ antimony, ascorbic acid, and oxalic acid reagents are added to the samples and spectra are recorded in the wavelength range 410-820 nm after a total reaction time of 30 min [46]. 

The extraction-spectrophotometric method with the use of BPHA has been applied for determining vanadium in air [95], waters [5], iron ores [96], silicates [97], ilmenite [98], and steel and cast iron [99]. 

Spectrophotometric methods for the determination of silicates in water samples resemble the majority of the methods so far reported as it is cheaper and easier to apply for routine work. The technique has been heavily utilized with modifications that lead to automation and robustness and the introduction of the FIA methodology and application. All the methods reported involve the complexation reaction of the molybdate with silica, a method that has been applied and subjected to different modifications to adapt the different water media and contents. All these modifications are mentioned in the following paragraph with focus and details of the standard and FIA methods. 

The molybdate spectrophotometric method has been applied for the determination of silicates in different water samples. The method has been adapted to the media with minor modifications in some cases, all of which are mentioned in the following paragraphs. 

As mentioned in the historical synopsis (Section 5.1), Levine121 perfected the compendial partition column procedure in which aspirin in chloroform is first trapped in an immobile phase of sodium bicarbonate on a column of siliceous earth (celite) then eluted with a solution of acetic acid in chloroform and measured spectrophotometrically. This has been also used for separation in combination products.80 For the determination of salicylic acid in presence of aspirin by this method, see Section 5.61. Ion exchange columns filled with strongly or weakly basic anion exchange resin in the acetate or chloride cycle have also been used for separation of aspirin in combination products. 122 123/l2lf This has also been adapted for a student experiment.125 A Sepha-dex-G25 column has been used for the separation of aspirin from salicylic acid.126... 

Various methods for the determination of silicon, particularly from biological samples are available. The techniques usually involve decomposition of the compound to give silica or a silicate by either wet combustion or fusion with sodium peroxide. The silicate formed can then be determined colorimetrically, volumetrically, or gravimetrically. Acidified ammonium heptamolybdate reacts only with monomeric Si(OH)4 and not its oligomers to give a yellow sihcomolybdic acid complex that can be determined spectrophotometrically. This method works... 

There exist several indirect methods for the spectrophotometric determination of silicon. After extraction of molybdosilicic acid, the Mo has been determined with phenylfluorone [45], or 2-amino-4-chlorobenzenethiol (e = 2.0-10 ) [46]. When silicic acid is added to a solution which contains hydrofluorotitanic acid and H2O2, a yellow titanium peroxide complex is formed. Chloranilic acid has also been used for determination of silicon [47],... 

In the oxygen flask combustion technique33 the sample is burnt in an oxygen-filled nickel flask. The silica produced is absorbed in a sodium hydroxide solution and the silicate concentration is determined spectrophotometrically at a wavelength of 400 nm34. An accuracy of within 2% is claimed for this method. 

There are several options available for the choice of an analytical technique, although in practice it may be necessary to use more than one technique because of the limitations of the various methods with respect to concentrations of the individual elements, or the method may be determined by the amount of available sample. For example, application of any one of the conventional wet analytical methods (such as for the determination of silicates) may require up to 10 g of sample ash whereas a variety of spectrophotometric techniques may require only micrograms (1 X 10 g) of sample. 

A rapid method for the extraction and spectrophotometric determination of silicate in phosphate-rich samples was developed that uses a flow-injection system, including a suppression column. A molybdate-form anion-exchanger column (20 cm x 2 mm, i.d.) was installed just behind the sample injector, phosphate was removed, then silicate treated with molybdate reagent, and the silicomolybdate was extracted with methyl isobutyl ketone (MIBK). The calibration curve, obtained by measuring the absorbance of silicomolybdate in MIBK phase at 400 ran, was linear for 0.01-1.0 xg Si/mL. Molybdate-reactive Si in fresh urine was determined. 

Flow-injection method for the determination of trace amounts of Si (present as silicate) in boiler water was examined spectrophotometrically using a molybdenum blue coloration reaction. The optimum conditions for the effective formation of molybdenum blue were examined using a two-line flow system coupled with a sandwich method. The lowest determination range was 1 xg/dm, and a calibration graph was linear over the range 1-100 ppb. The relative standard deviations for 1 and 20 ppb silicon were 4.6% and 0.9%, respectively. The method was applied to the determination of silicate in real boiler waters. 

An FIA method for the simultaneous spectrophotometric determination of phosphate and silicate was developed. This method is based on the different reaction rates of heteropolymolybdate formation reactions. Concentrations within 0.026-0.485 mM P/L and 0.125-2.848 mM Si/L ranges were determined at a frequency of 30 samples/h. A relafive standard deviation of 2.1% was obfained for 0.162 mM P/L and of 1.1% for 1.424 mM Si/L. The method was suitable to determine phosphate and silicate in wastewater. 

Titrimetric and gravimetric determination of silicates in water is considered less precise than the spectrophotometric and other automated methods and usually applied for water samples of known high concentrations of silicate [41]. 

Thomsen, J., Johnson, K.S., and Petty, R.L., Determination of reactive silicate in seawater by flow injection analysis. Marine Science Institute of Analytical Chemistry (1983), 55(14), 2378-2382. Mas-Torres, F., Munoz, A., Estela, J.M., and Cerda, V., Simultaneous spectrophotometric determination of phosphate and silicate by a stopped-flow sequential injection method. International Journal of Environmental Analytical Chemistry (2000), 77(3), 185-202. 

Mas-Torres, F., Munoz, A., and Estela, J.M., Simultaneous spectrophotometric determination of phosphate and silicate by a stopped-flow sequential injection method. International Journal of Environmental Analytical Chemistry (2000), 77(3), 185-202. 

Mixtures of phenols may be assayed by the method of White and Vaughan in which the phenols are separated by partition chromatography between cyc/ohexane and sodium carbonate or sodium silicate, the resultant fractions being assayed by a spectrophotometric procedure. This technique is particularly useful for substances such as cresols and xylenols which are mainly isomeric mixtures. 

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