HIRANUMA APPLICATION DATA

Automatic Titrator

Data No.

J2Apr. 5,
2019

Purity determination ofphosphoric acid

Inorganic acids &Mixed acids
1. Abstract

Phosphoric acid is important reagent as a raw material of chemical industrial reagents and fertilizers. Its
production amount is large and it is used in a broad range of fields. The determination method of phosphoricacid purity is described in “JIS (Japanese Industrial Standards) K9005:2006 Phosphoric acid (reagent)”; thepurity of phosphoric acid is determined by potentiometric titration with sodium hydroxide standard solution.
This report introduces an example of the purity determination of phosphoric acid by this method.
Phosphoric acid reacts with sodium hydroxide as described in the reaction formula (1) ~ (3) because
phosphoric acid is triacid. The titration curve of neutralization titration for phosphoric acid theoretically has
three inflection points, but practically it shows two inflection points. This is because third acid dissociation
constant (pKa) of phosphoric acid is 12.35, the pH at the third inflection point is undetectable value by thetitration with strongly basic titrant. The purity of phosphoric acid is determined by the titration to the second
inflection point in this measurement.
＋NaOH

→

NaH2PO4＋

H2 O・・・(1)

NaH2PO4 ＋NaOH→

Na2HPO4＋

H2 O・・・(2)Na2HPO4 ＋NaOH→

Na3HPO4＋

H2 O・・・(3)H3PO4

2. Configuration of instruments and reagents(1) Configuration of instruments
Main unit:

Hiranuma Automatic Titrator COM series

Electrodes:

Glass electrodeGE-101B

Reference electrodeRE-201Z*Instead of above electrode, the following electrodes are usable.
・Glass reference combination electrode GR-501BZ…Fixed sleeve type・Glass reference combination electrode GR-511BZ…Moveable sleeve type(2) Reagents
Titrant:

1 mol/L Sodium hydroxide standard solution

3. Measurement procedure(1) Take about 1.5 g of sample into a 100 mL beaker and weigh accurately.
(2) Add 40 mL of DI water without carbon dioxide.
(3) Immerse electrodes and start titration with 1 mol/L sodium hydroxide standard solution. Perform theblank test with the same procedure of sample measurement.

4. Measurement conditions and results
Examples of titration conditions
Measurement of blankCndt No.
MethodBuret No.
Amp No.
D. Unit
S-Timer
C.P. mLT Timer
D.P. mLEnd Sens
Over mL
Max.Vol.

1Auto

11pH
100002500.11sec
mLsec
mL
mL
mL

ConstantNo.
SizeBlank
Molarity
FactorKL
Unit
Formula

100
1.000
1.005
00mL
D

Decimal Places
Auto In Pram.

3Non

mL
mL
mol/L

Mode No.
Pre Int
Del K
Del Sens
Int Time
Int Sens
Brt Speed
Pulse

140005328

Mode No.
Pre Int
Del K
Del Sens
Int Time
Int Sens
Brt Speed
Pulse

409033240sec
mVsec
mVMeasurement of sample
Cndt No.
MethodBuret No.
Amp No.
D. Unit
S-Timer
C.P. mLT Timer
D.P. mLEnd Sens
Over mL
Max.Vol.

2Auto

1
1pH
100020250
0.2
40sec
mL
sec
mL
mL
mL

ConstantNo.
SizeBlank
Molarity
FactorKL
Unit
FormulaDecimal Places
Auto In Pram.

200.01501.000
1.005
49.000
0%g
mL
mol/L
*sec
mV
sec
mV

(D-B)*K*F*M/(S*10)
3Non*”49 (Parameter K)” comes from “98 (H3PO4)/2” because titrant volume will be twice according to reaction (1) and (2).

Measurement results
Measurement results of blank

Measurement results of sample

Number ofSize

Titrant

Number ofSize

Titrant

Concentration

Measurement(g)

Volume (mL)

Measurement(g)

Volume (mL)

(%)

1-

0.015

1

1.669329.09485.784

2-

0.015

2

1.674329.14285.6690.015 mL3

1.679729.27785.790Avg.
(Blank)Statistic
calculation

Avg.

85.75 %

SD

0.07 %

RSD

0.08 %

Measurement of blank

Measurement of sample

Examples of titration curves

5. Note(1) Collection of sample
The sample should be collected directly to 100 mL beaker and weighed accurately. The accuracy of sample
collection influences the measurement accuracy. Please note that the sample should be carefully taken andaccurately weighed.
(2) Control of titrant
The concentrated sodium hydroxide standard solution is used as titrant in this report. The carbon dioxide
gas absorber (soda lime) on reagent bottle has to be regularly exchanged because sodium hydroxide readily
absorbs carbon dioxide gas in the air (formula (4)).
2NaOH+CO2→

Na2CO3+

H2 O・・・(4)(3) Endpoint detection and calculation of concentration
The titration curve with strong base (sodium hydroxide) shows two inflection points as described in theabstract. The first inflection point at around pH 4 is not detected (“D.P. mL” is set to “20 mL”) and thesecond inflection point is detected as endpoint. The titrant volume at second inflection point is used for thecalculation of concentration.
(4) Reduction of measurement time
The titrant volume was 25 mL or more, the measurement time was about 8.5 min on this measurement. Thesodium hydroxide standard solution can be added continuously up to 20 mL by the function “C.P. mL”, it
reduces the measurement time. The measurement time was about 4.5 min when using the function “C.P.
mL”.
Keywords: Phosphoric acid, Neutralization titration, Purity*Some measurement would not be possible depending on optional configuration of system.