Determination of phosphates

The carrier stream contains Hg(SCN)2 and Fe(III). The chloride of the injected sample reacts with Hg(SCN)2, liberating SCN , which in turn forms with Fe(III) the red-colored complex ion Fe(SCN)2, the intensity of which is measured spectrophotometrically at 480 nm. The height of the recorded absorbance peak is then proportional to the concentration of chloride in the sample (see Fig. 2.1fe). Besides Fe(SCN)2, other (higher) complex ions might be formed thus the calibration curve cannot be expected to be linear over wide range of concentrations. 

The reproducibility of the procedure might be estimated by calculating the standard deviation on the peaks obtained by injecting one of the standards 10 times. Note that the coil length is the same as that used in the exercise in Section 6.3, experiment B that is, the dispersion coefficient )max estimated directly from the previous exercise, although the 

Reagents The carrier stream consists of a mixture of two solutions pumped at equal rates that is, (a) 0.005 M ammonium heptamolybdate (6.1793 g/liter) in 0.4 M nitric acid and (b) 0.7% (w/v) aqueous solution of ascorbic acid to which is added 1% (v/v) glycerine (to minimize precipitation of reaction products on the walls of the flow cell). 

Standard Solutions Standard solutions in the range 0-40 ppm P-PO4 is prepared by successive dilutions of a 100 ppm phosphate stock solution (0.4390 g of anhydrous potassium dihydrogen phosphate per liter). 

Exercise The analytical procedure is based on the following reactions  

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Bello, M. A. Gustavo Gonzalez, A. Determination of Phosphate in Gola Beverages Using Nonsuppressed Ion Ghromatography, /. Chem. Educ. 1996, 73, 1174-1176. 

Representative Method Although each FIA method has its own unique considerations, the determination of phosphate described in the following method provides an instructive example of a typical procedure. 

Description of Method. The FIA determination of phosphate is an adaptation of a standard spectrophotometric analysis for phosphate. In the presence of add, phosphate reacts with molybdate to form a yellow-colored complex in which molybdenum is present as Mo(VI). 

SPECTROPHOTOMETRIC DETERMINATION OF PHOSPHATE AND ARSENATE IONS BY MEANS OF IONIC ASSOCIATES OF CYANINE DYES WITH POLYOXOMETALATES... 

EXTRACTION-SPECTROPHOTOMETRIC DETERMINATION OF PHOSPHATE AND ARSENATE USING IONIC ASSOCIATES OF POLYOXOMETALATES WITH BASIC DYES... 

ELUCIDATION OF THE HETEROPOLYBLUE NATURE AND EXTRACTION-SPECTROPHOTOMETRIC DETERMINATION OF PHOSPHATE USING MIXED HETEROPOLYANIONS... 

Determination of phosphate as ammonium molybdophosphate. This may be readily effected by precipitation with excess of ammonium molybdate in warm nitric acid solution arsenic, vanadium, titanium, zirconium, silica and excessive amounts of ammonium salts interfere. The yellow precipitate obtained may be weighed as either ammonium molybdophosphate, (NH4)3[PMo12O40], after drying at 200-400 °C, or as P205,24Mo03, after heating at 800-825 °C for about 30 minutes. 

Apart from other control activities, such as the determination of phosphate, it is always necessary to closely monitor the FW sodium (Na) content. 

Confirmation by GC/MS is recommended. Recovery is acceptable (82%). Tributyl phosphate and triphenyl phosphate contamination was reported in procedural blanks and may prevent determination of phosphate esters at levels near the detection limit (1 ng/g) (LeBel and Williams 1983 1986). 

Figure 2.5. Analytical manifolds for the determination of phosphate by flow injection analysis (a) and reverse flow injection (b). The symbols S, M, and A are the seawater, mixed reagent, and the ascorbic acid solutions. The pump injection valve and detector are represented by P, I, and D, respectively. W = waste. From [177]... 

Johnson and Petty [177] adapted reverse flow injection analysis to the well-known Murphy and Riley [178] colorimetric phosphomolybdate reduction method for the determination of phosphate. 

Figure 2.5a and b show flow sheets for the determination of phosphate by flow injection analysis and reversed flow injection analysis, respectively. 

Spencer and Brewer [111] have reviewed methods for the determination of phosphate in seawater. Earlier methods for the determination of phosphate in seawater are subject to interferences, particularly by nitrate. In one early... 

Isaeva [181] described a phosphomolybdate method for the determination of phosphate in turbid seawater. Molybdenum titration methods are subject to extensive interferences and are not considered to be reliable when compared with more recently developed methods based on solvent extraction [182-187], such as solvent-extraction spectrophotometric determination of phosphate using molybdate and malachite green [188]. In this method the ion pair formed between malachite green and phosphomolybdate is extracted from the seawater sample with an organic solvent. This extraction achieves a useful 20-fold increase in the concentration of the phosphate in the extract. The detection limit is about 0.1 ig/l, standard deviation 0.05 ng-1 (4.3 xg/l in tap water), and relative standard deviation 1.1%. Most cations and anions found in non-saline waters do not interfere, but arsenic (V) causes large positive errors. 

A commonly used procedure for the determination of phosphate in seawater and estuarine waters uses the formation of the molybdenum blue complex at 35-40 °C in an autoanalyser and spectrophotometric evaluation of the resulting colour. Unfortunately, when applied to seawater samples, depending on the chloride content of the sample, peak distortion or even negative peaks occur which make it impossible to obtain reliable phosphate values (Fig. 2.7). This effect can be overcome by the replacement of the distilled water-wash solution used in such methods by a solution of sodium chloride of an appropriate concentration related to the chloride concentration of the sample. The chloride content of the wash solution need not be exactly equal to that of the sample. For chloride contents in the sample up to 18 000 mg/1 (i.e., seawater),... 

Figure 2.7. Interference by chloride in the autoanalyser determination of phosphate... 

Airey et al. [195] have described a method for the removal of sulfide prior to the determination of phosphate in anoxic estuarine waters. Mercury (II) chloride was used to precipitate free sulfide from samples of anoxic water. The sulfite-free supernatant liquid was used to estimate sulfide by measuring the concentration of unreacted mercury (II), as well as to determine phosphate by the spectrophotometric method in which sulfide interferes. The detection limit for phosphate was 1 ig/l. 

Tyree and Bynum [132] have described an ion chromatographic method for the determination of phosphate and nitrate in seawater. 

Johnson and Pilson [229] have described a spectrophotometric molybdenum blue method for the determination of phosphate, arsenate, and arsenite in estuary water and sea water. A reducing reagent is used to lower the oxidation state of any arsenic present to +3, which eliminates any absorbance caused by molybdoarsenate, since arsenite will not form the molybdenum complex. This results in an absorbance value for phosphate only. 

Particularly in autoanalyser methods this wide variation in chloride content of the sample can lead to serious salt errors and, indeed, in the extreme case, can lead to negative peaks in samples that are known to contain ammonia. Salt errors originate because of the changes of pH, ionic strength and optical properties with salinity. This phenomenon is not limited to ammonia determination by autoanalyser methods it has, as will be discussed later, also been observed in the automated determination of phosphate in estuarine samples by molybdenum blue methods. 

CL emission. The system allows a simple determination of phosphate in 3 min with a linear range of 4.8-160 pM. Owing to its sensitivity, this method could be satisfactorily applied to the analysis of maximum permissible phosphate concentrations in natural waters [42-44], Also, the maltose-phosphorylase, mutar-ose, and glucose oxidase (MP-MUT-GOD) reaction system combined with an ARP-luminol reaction system has been used in a highly sensitive CL-FIA sensor [45], In this system, MP-MUT-GOD is immobilized on A-hydroxysuccinimide beads and packed in a column. A linear range of 10 nM-30 pM and a measuring time of 3 min were provided, yielding a limit of detection of 1.0 pM as well as a satisfactory application in the analysis of river water. 

Several investigators have described the indirect determination of orthophosphate by extraction of the phosphomolybdic acid complex and the measuring the molybdenum extracted. Zaugg and Knox 2921 first applied this technique to the determination of phosphate in urine. A protein-free filtrate was formed and the complex was extracted into 2-octanol. More recently, Devoto 293) determined 0 to 25 pg of phosphate in 50 ml of urine by extracting the complex from acidified urine into isobutyl acetate. 

Add 1.5 g of air-dry soil to a 125-mL Erlenmeyer flask. Add 40 mL of 0.5N NaHC03, pH 8.5, and shake on a mechanical shaker for 30 minutes. Filter through a Whatman No. 2 filter paper. Refilter through the same filter paper if the filtrate is not clear. A 5 mL aliquot of this filtrate is taken for colorimetric determination of phosphate (adapted from Reference 10). 

An excellent example of this type of analysis involves the determination of phosphate in soil extracts. Soil is extracted with an appropriate extractant and added to a solution of acid molybdate, with which the phosphate reacts to produce a purple- or blue-colored solution of phosphomolybdate. Standard phosphate solutions are prepared, reacted with acid molybdate, and the intensity of the phosphomolybdate color produced is measured. A standard curve (also called a calibration curve) is prepared (see Section 14.10) from which the intensity of the color is directly related to the concentration of phosphate in the extract. 

J. P., A modified single solution method for the determination of phosphate in natural waters , pp. 31-6, copyright 1962, with permission from Elsevier. 

Bickford and Willett [61] have reported that the filtration of soil extract solutions through a Gelman GA6 0.45pm membrane which contains a wetting agent caused interference in spectrophotometric methods for the determination of phosphate. This was due to the release of some substance from the membrane. It is recommended that low extractable membranes such as Gelman CM-450 are used for this purpose. 

In the method for extractable phosphorus [62, 64-66] the phosphorus is extracted from the soil at 20 1°C with sodium bicarbonate solution at pH8.5. After filtration and release of carbon dioxide the extracts are introduced into a flow-injection system for the determination of phosphate. Phosphate is determined by reaction with vanadomolybdate and the yellow colour evaluated at 410nm. Between 20 and lOOOmg kg-1 phosphorus in soil has been determined using this method. 

The American Public Health Authority has published a standard method for the determination of phosphates in sediments [89]. 

Solyom has conducted an intercalibration of methods used for the determination of phosphorus in sludges [37]. The methods used to determine phosphorus were that of Koroleff [83] in which the sample is digested with potassium peroxydisulphate and phosphate determined spectro-photometrically. Alternatively a reducing Kjeldahl digestion was used followed by determination of phosphate using molybdate and ascorbic acid. The former method gives somewhat low results. The reducing Kjeldahl method is therefore recommended. 

Tecator Ltd., Sweden. (1983) Application Note No. AN 60/83. Determination of Phosphate Stannous Chloride Method. 

L-Glutamate synthetase 6.3.1.2 L-Glutamate/ATP Phosphate Determination of phosphate... 

Cell Culture Analysis Kit for the determination of phosphate and organic anions in cell cultures... 

We studied the feasibility of phosphate determination in a low water-soluble API hydrochloride salt. The aim of the work was quantification of phosphate below 0.1% and evaluation of the effect of the high chloride counterion on the determination of phosphate. 

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