Phosphate Sequesterant Feed Control
Water utilities use a variety of phosphate products to control scale and corrosion within their water distribution systems and to sequester specific metal ions which may present a health problem or result in poor water quality.
Products can be classified as either orthophosphates or polyphosphates (molecularly dehydrated phosphate). Although polyphosphates tend to revert to an orthophosphate form in water, the rate of reversion is very slow in pure water. The reversion rate can be increased as pH is lowered and as water temperature is increased, but several hours to days are required for complete reversion at normal water distribution system temperatures and at neutral to high pH.
Typical products used include the following:
Where there is a variable metal ion concentration in water due to background water chemistry characteristics or upstream treatment residuals, flow pacing alone may not produce a consistent phosphate residual at the point of measurement. Also, the effects of polyphosphate reversion to orthophosphate form may be seen at measurement points within the distribution system that are remote from the point of application.
For proper measurement and control it is essential to know the form of phosphate being applied so that the appropriate analysis technique can be used.
ChemScan® Analysis Methods
ChemScan® uses a ferric reagent to detect the available inorganic phosphorous in water. This method will respond to all poly and ortho phosphates present that are available to sequester metal ions.
ChemScan® uses a vanadomolybdate method to detect the orthophosphate fraction of inorganic phosphorous in water. Vanadomolybdate does not respond to polyphosphates.
Thus, subtraction of the ChemScan® results using the vanadomolybdate method from the results using the ferric method can be used to provide a determination of polyphosphate under the following general formula:
Total phosphate - orthophosphate = polyphosphate
A ChemScan® Process Analyzer can be used to detect total phosphate using the ferric method or orthophosphate using the vanadomolybdate method. A ChemScan® analyzer can also detect total, ortho and poly phosphate using a combination of methods.
Although filtration of water samples is not required for analysis, some fraction of phosphate present may be in the form of precipitates or coagulated products from prior treatment steps. Acidification of the sample by ChemScan® prior to analysis will hydrolyze these substances making the bound phosphate available for analysis. Acid hydrolysis may also convert some polyphosphate present into orthophosphate form and should therefore not be performed when the exact form of phosphate is important or if only the amount of phosphate available to sequester metal ions is to be measured.
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Applied Spectrometry Associates, Inc.
2325 Parklawn Drive, Suite I · Waukesha, WI 53186
Phone: 262-717-9500 · Fax: 262-717-9530
Toll Free: 800-665-7133