Potentiometric Determination of the Phospat

Potentiometric Determination of the Phosphate

Equipment
Potentiometer = 751 GPD Titrino / Titroprocessor
Electrodes = 6.0232.100 combined pH glass electrode; input 1

Reagents
c(NaOH) = 0.1 mol/L; D0

Sample
5 mL c(NaH2PO4) ...0.1 mol/L
40 mL dist. water
adjust the pH value to 4.2 with dilute NaOH or H2SO4
10 mL c(La(NO3)3) = 0.1 mol/L, pH = 4.2

Remarks
Determination reaction:

NaH2PO4 + La(NO3)3 .....LaPO4() + 2 HNO3 + NaNO3

2 HNO3 + 2 NaOH .......2 NaNO3 + 2 H2O

Calculations:

P=EP1*C01*C02/C00;2;%
P2O5=EP1*C01*C03/C00;2;%
PO4=EP1*C01*C04/C00;2;%

P = content of P in %
P2O5 = content of P2O5 in %
PO4 = content of PO4 in %
C01 = factor for %, 0.1
C02 = 1 mL c(NaOH) = 0.1 mol/L = 1.5487 mg P
C03 = 1 mL c(NaOH) = 0.1 mol/L = 3.5486 mg P2O5
C04 = 1 mL c(NaOH) = 0.1 mol/L = 4.7486 mg PO4

P = 9.992 x 0.1 x 1.5487 / 0.07798 = 19.84%
P₂O₅ = 9.992 x 0.1 x 3.5486 / 0.07798 = 45.47%
PO4 = 9.992 x 0.1 x 4.7486 / 0.07798 = 60.85%

Sample preparation:
Adjust the pH of your sample aliquot to 4.2 by adding either NaOH or H2SO4. Add 10 mL c(La(NO3)3 = 0.1 mol/L, pH = 4.2, to this sample solution and titrate.
Select the appropriate formula. The others may be deleted.
For automatic curve output add in , >report “curve“.

Literature

Metrohm Application Bulletin No. 129: Potentiometric determination
of ortho-, meta- and polyphosphates.

Method documentation

'pa
751 GPD Titrino 15215 751.0010
SET pH… Phosphat
parameters
>SET1
EP at pH ….4.20
dynamics …1
max.rate …10 ml/min
min.rate …25 ml/min
stop crit: ….drift
stop drift …20 ml/min
>SET2
EP at pH ….OFF
>titration parameters
titr.direction:… +
pause …1 0 s
start V: …OFF
pause ….2 0 s
extr.time…. 0 s
dos.element:…. internal D0
meas.input: …1
temperature …25.0 C
time interval …2 s
>stop conditions
stop V: …abs.
stop V ….99.99 ml
filling rate max…. ml/min
>statistics
status: ….OFF
>preselections
conditioning:…. OFF
req.ident: ….OFF
req.smpl size:…. value
activate pulse:… OFF
------------
fm
751 GPD Titrino 15215 751.0010
SET pH Phosphat
>calculations

P=EP1*C01*C02/C00;2;%
P2O5=EP1*C01*C03/C00;2;%
PO4=EP1*C01*C04/C00;2;%

C00= 0.07798
C01= 0.1
C02= 1.5487
C03= 3.5486
C04= 4.7486
------------
'fr
751 GPD Titrino 15215 751.0010
pHc(init) 1.94 SET pH Phosphat
smpl size… 0.07798 g
EP1….. 9.992 ml…. 4.21mV
P… 19.84 %
P2O5… 45.47 %
PO4 ….60.85 %
============
'cu
751 GPD Titrino 15215 751.0010
SET pH Phosphat
10.0 s/div dV=2.0 ml/div