Determination of phosphate in fertilizers

The spectrophotometric determination of phosphate has very wideapplication.

However, it does suffer from a number ofdisadvantages.

It is subject to interferences, requires aconsiderable level of analyst skill, is relatively slow, and incommon with all spectrophotometric methods there are limits onthe precision that may be achieved.

Spectrophotometric proceduresmay be called logarithmic methods, as the concentration of theanalyte is proportional to the logarithm of the transmittance.Such methods are great for measuring over a wide concentrationrange, but less satisfactory for determining small differences inconcentration.

In contrast, titrimetric methods are linear,because analyte concentration is directly proportional to theamount of titrant consumed.

Relative standard deviations ofaround 0.1% are common in titrimetry.

Multitrator has converted aclassical (and very slow) gravimetric procedure fororthophosphate into a rapid, accurate, and highly preciseapplication.

Phosphate may be determined gravimetrically byreaction with magnesium ions in an ammoniacal environment to forminsoluble magnesium ammonium phosphate, MgNH4PO4.6H2O.

Bytitrating phosphate with Mg2+ in the presence of an NH3/NH4Clbuffer, exceptionally sharp and reproducible endpoints areobtained.

The titration only takes a few minutes, and is suitablefor use by semi-skilled laboratory labour.

In the determinationof soluble phosphate in diammonium phosphate (DAP) fertilizer, ananalytical precision of 0.04% RSD was obtained.

The Mg2+ solutionis standardised against pure diammonium phosphate.

Copies of thestandard methods for phosphate and the DAP standardisation may beobtained by e-mail by contacting Multitrator.