YSI P 700 IQ Analyzer - Manual (Page 8)

8 ba76133e01 09/2013
P 700 IQ
1.2 Metrological basics
Phosphate The salts of the phosphoric acid are called phosphate. With simple
phosphoric acid (orthophosphoric acid, H
3
PO
4
) this is orthophosphate
(anion PO
4
3-
).
Measuring method The P 700 IQ phosphate analyzer measures the concentration of
orthophosphate in an aqueous solution with the aid of the vanadate
molybdate method (yellow method).
The reagent contains an aqueous solution of ammonium metavana-
date NH
4
VO
3
and ammonium heptamolybdate (NH
4
)
6
Mo
7
O
24
with an
addition of sulfuric acid H
2
SO
4
. In an acidic environment, the chemical
reaction takes place according to the following molecular formula:
PO
4
3-
+ 2 VO
3
-
+ 10 MoO
4
2-
+ 20 H
+
→ [PV
2
Mo
10
O
40
]
5-
+ 10 H
2
O
The originally pale yellow reagent will turn a deep yellow. The change
of absorbance is photometrically measured at a wavelength of 420 nm.
From this, the concentration of orthophosphate is calculated.
Citation forms Phosphate concentration is quoted in milligrams per liter (mg/l). This
value can either refer to all orthophosphate ions or only to the phospho-
rus atom included. The values can be converted as follows:
• 1 mg P = 3.066 mg PO4
• 1 mg PO4 = 0.3261 mg P
Concentration values referring to the phosphorus atom are indicated by
the addition PO4-P (citation form).