New titration method for analysing phosphate in fertilisers is fast, accurate and reproducable and may replace classical gravimetric analysis
The spectrophotometric determination of phosphate has very wide application.
However, it does suffer from a number of disadvantages.
It is subject to interferences, requires a considerable level of analyst skill, is relatively slow, and in common with all spectrophotometric methods there are limits on the precision that may be achieved.
Spectrophotometric procedures may be called logarithmic methods, as the concentration of the analyte is proportional to the logarithm of the transmittance. Such methods are great for measuring over a wide concentration range, but less satisfactory for determining small differences in concentration.
In contrast, titrimetric methods are linear, because analyte concentration is directly proportional to the amount of titrant consumed.
Relative standard deviations of around 0.1% are common in titrimetry.
Multitrator has converted a classical (and very slow) gravimetric procedure for orthophosphate into a rapid, accurate, and highly precise application.
Phosphate may be determined gravimetrically by reaction with magnesium ions in an ammoniacal environment to form insoluble magnesium ammonium phosphate, MgNH4PO4.6H2O.
By titrating phosphate with Mg2+ in the presence of an NH3/NH4Cl buffer, exceptionally sharp and reproducible endpoints are obtained.
The titration only takes a few minutes, and is suitable for use by semi-skilled laboratory labour.
In the determination of soluble phosphate in diammonium phosphate (DAP) fertilizer, an analytical precision of 0.04% RSD was obtained.
The Mg2+ solution is standardised against pure diammonium phosphate.
Copies of the standard methods for phosphate and the DAP standardisation may be obtained by e-mail by contacting Multitrator.
