Simultaneous reaction-rate determinations of phosphate and silicate

Kircher, C. C.; Crouch, S. R.

doi:10.1021/ac00253a017

Cited by 17 publications

(13 citation statements)

References 33 publications

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“…The enhancement of the Si signal observed as the concentration of phosphate is increased indicates that the phosphate is influencing the rate at which the silicomolybdate is either forming or being reduced. Crouch et al described a synergistic effect between Si and P with regard to the rate at which their heteropoly complexes form (21). Our present data support and are appropriately explained by these earlier observations.…”

Section: Results and Discussion Reversed Flow Elutionsupporting

confidence: 92%

“…Silicate interferes significantly with the molybdenum blue determination of phosphate and must be eliminated, or accounted for, in order to correctly determine the amount of phosphate present in the sample (19,20). In the past this has been done by examining the differences in the rates of formation of the heteropoly complexes of phosphate and silicate, but the difference in the rate of reduction of the heteropoly complexes to the blue complex was not used due to the problems of longer reaction times, deviations from Beer's law, unstable reagents and products, and high blank absorbances (21). Our goal was to achieve not only preconcentration of the heteropoly complex but also the generation of a detectable species, the blue complex, on the sorbent material itself through on-column chemistry while monitoring this chemistry through optosensing.…”

Section: Introductionmentioning

confidence: 99%

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Enhancement of flow injection optosensing by sorbent extraction and reaction rate measurement

Lacy¹,

Christian²,

Růžička³

1990

Anal. Chem.

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The molybdenum blue reactlon for the determination of phosphate Is used as a model to Illustrate the extension of the use of hydrophoblc sorbents In flow Injection analyds for the preconcentratlon of an anlon and for on-cdumn detec#on, Le., optosenslng. Optosenslng provides for real-time monltorlng of the rate of slgnai change, dA /d T , so that the rate of color devebpment durlng the reductlon step of the analysis can be measured. The synerglstlc relationship between the rates of formation of the phosphate and sHlcate heteropdy complexes is examlned. A kinetic optosenslng method has been developed In which the difference In reduction rate for the heteropoly complexes allows simultaneous determlnatlon of phosphate, In the parts per bllllon range, and slllcate, in the parts per mllllon range. The system used an Inexpensive mlnlspec-20 spectrophotometer wtth a net path length through the sorbent of less than 2 mm. Partlal least-squares analyds was used to analyze the data, and predlctlon errors of approxlmately 10 % were obtained for both components. INTRODUCTIONEnhancement of the selectivity of instrumental methods by flow injection analysis (FIA) has been accomplished by various means, such as the use of solvent extraction ( 1 ) and gas diffusion techniques (2). These may be considered homogeneous techniques in that the chemistry occurs in the homogeneous media of the flowing stream itself. Additionally, sensitivity has been increased by on-line preconcentration using ion exchange (3). The latter technique may be viewed as heterogeneous since isolation of the analyte on a surface has occurred, creating a region of increased concentration within the flowing stream and may be considered similar to solute focusing (4). Only very recently, attention is being given to the concept of sorbent extraction in FIA, which has been used so far only for the preconcentration of metal complexes (5) using hydrophobic interactions on C-18. Further increases in sensitivity can be achieved by flow injection optosensing (6), which is based on the measurement of an analyte retained on the solid sorbent surface. Use of cation exchangers for this purpose has been described by Yoshimura and Waki (7) while Valcarcel and co-workers (8) used an anion exchanger in a detailed study of the iron(II1) thiocyanate complex. This paper deals, for the first time, with the use of sorbent extraction for the optosensing of anionic complexes and exploits in a novel way the differential kinetics of competing species.The present study of differential kinetics allows for the simultaneous determination of two analytes or the determination of one analyte in the presence of an interferent. Thus, while to date the use of sorbents has been driven by the "all or nothing" philosophy that focuses on steady-state chemistry, it is shown in this work how the horizon of applications for sorbent use in FIA can be greatly broadened with the incorporation of kinetic techniques.Previous work in this laboratory has focused on the enhancement of instrumental response at roo...

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Section: Results and Discussion Reversed Flow Elutionsupporting

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Enhancement of flow injection optosensing by sorbent extraction and reaction rate measurement

Lacy¹,

Christian²,

Růžička³

1990

Anal. Chem.

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“…The kinetics of formation of the two complexes were, however, quite different; for example, at room temperature the molybdophosphate was fully formed in <60 s while the molybdosilicate required >1000 s. This appeared to be the basis of a method to determine both silicate and phosphate in a single experiment. A similar concept has been discussed in the spectrophotometric determination of phosphate and silicate [5,6].…”

Section: Timescale Of Electroactive Complex Formationmentioning

confidence: 98%

“…They involve the addition to the sample of an acidic molybdate solution to convert the silicate and phosphate to the Keggin anions, SiMo 12 , and subsequent chemical reduction to a mixed oxidation state, molybdenum complex which is intensely blue. Selectivity between the two analytes remains a significant problem although methods have been suggested based on the differences in the kinetics of the formation of the molybdenum blues [5,6], selective extraction into an organic solvent [1,7,8] or the addition of organic molecules such as tartrate, oxalate, mannitol, acetone or butanone [1,[9][10][11][12][13]. These added organic compounds are variously said to complex the molybdenum(VI) thus preventing or slowing down the formation of a Keggin anion, influence the ratio of isomers formed, stabilise a Keggin anion, change the absorptivity of the molybdenum blue or react with the colored reduced species.…”

Section: Introductionmentioning

confidence: 99%

Microelectrode Procedures for the Analysis of Silicate and Phosphate — Towards Practical Procedures

Hodgson

Pletcher

1998

Electroanalysis

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Amperometric methods using microdisk electrodes and based on the formation of electroactive molybdophosphate and molybdosilicate complexes are described for the determination in water samples of phosphate and silicate. It is demonstrated that it is possible to devise conditions for the analysis of 1-100 mM phosphate or silicate in the presence of a large excess of the other. It is also possible to determine both phosphate and silicate in a single analysis provided they are present in not too dissimilar concentrations.

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“…Other workers, as has been reviewed by Robards et al, 8 have proposed several FIA methods for the simultaneous determination of phosphate and silicate with on-line column separation, [9][10][11] methods based on the different formation rates of the corresponding molybdate heteropolyacid [12][13][14] or by using intermittent flows. 15 In the present work, an SIA method is proposed by using large sample volumes for the simultaneous spectrophotometric determination of phosphate and silicate.…”

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confidence: 99%

Simultaneous Determination of Phosphate and Silicate in Waste Water by Sequential Injection Analysis

Mas-torres¹,

Cerdà²

1997

Analyst

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A sequential injection analysis system for the simultaneous determination of phosphate and silicate in waste water is proposed. The method is based on the formation of yellow vanadomolybdophosphate 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-1 P and 30 mg l-1 Si, respectively. The detection limits are 0.2 mg l-1 P and 0.9 mg l-1 Si. The method provides a throughput of 23 samples h-1 with a relative standard deviation < 1.4% for phosphate and < 4% for silicate. The method was found to be suitable for the determination of these species in waste water samples.

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Simultaneous reaction-rate determinations of phosphate and silicate

Abstract: Phosphate and silicate were determined slmultaneously by stopped-flow reactlon-rate measurements at two different times durlng the formation of the 12-heteropolymolybdates. I n solutlons containing 0.40-0.50 M acld, the formatlon of 12-molybdophosphate reaches equlllbrlum rapidly, whlle p-

Cited by 17 publications

References 33 publications

Enhancement of flow injection optosensing by sorbent extraction and reaction rate measurement

Enhancement of flow injection optosensing by sorbent extraction and reaction rate measurement

Microelectrode Procedures for the Analysis of Silicate and Phosphate — Towards Practical Procedures

Simultaneous Determination of Phosphate and Silicate in Waste Water by Sequential Injection Analysis

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