Purity determination of oxalic acid | Autotitrator COM-A19

HIRANUMA APPLICATION DATA

Automatic Titrator

Data No.

Jul. 14,2020

Organic acid

Purity determination of oxalic acid

1. Abstract

Oxalic acid has two carboxy groups (-COOH) in the molecule , it is called dicarboxylic acid. There are two crystallization water molecules in the molecule. Oxalic acid is ortho acid and forms the following structure. It is readily oxidized by the acid stronger than formic acid.

Oxalic acid is used as raw material of organic compound. In addition, sodium salt of oxalic acid is used as standard substance of quantitative analysis because it quantitatively reacts with oxidizing agent like potassium permanganate and is stably conservable. The determination method for oxalic acid is prescribed in JIS K8519, it is determined by potassium permanganate standard solution. This report introduces an example of the purity determination for oxalic acid using redox titration as described below (formula 1) and neutralization titration with sodium hydroxide titrant (formula 2).

2. Configuration of instruments and Reagents

(1) Redox titration with potassium permanganate

(i)	Configuration of instruments
	Main unit	:	Hiranuma Automatic Titrator COM series
	Electrode	:	Platinum electrode PT-301 Reference electrode RE-201Z *Instead of the above electrodes, the following electrodes are usable. ・Platinum reference combination electrode PR-701BZ
(ii)	Reagents
	Titrant	:	0. 02 mol/L Potassium permanganate standard solution
	Additive	:	Diluted sulfuric acid (1:1, [v/v])

(2) Neutralization titration with sodium hydroxide

(i)	Configuration of instruments
	Main unit	:	Hiranuma Automatic Titrator COM series
	Electrode	:	Glass electrode GE-101B Reference electrode RE-201Z *Instead of the above electrodes, the following electrodes are usable. ・Glass reference combination electrode GR-501BZ…Fixed sleeve type ・Glass reference combination electrode GR-511BZ…Moveable sleeve type
(ii)	Reagents
	Titrant	:	1 mol/L Sodium hydroxide standard solution

3. Measurement procedure

(1)	Redox titration with potassium permanganate
	(i) Take 0.2 g of sample into a 300 mL beaker and weigh accurately with 0.1 mg digit.
	(ii) Add stirring bar and 200 mL of DI water and 20 mL of diluted sulfuric acid.
	(iii) Dispense 30 mL of 0.02 mol/L potassium permanganate with stirring solution.
	(vi) Heat the beaker at about 60 °C.
	(v) Immerse electrodes and start titration with 0.02 mol/L potassium permanganate standard solution. 　 Perform the blank test in the same procedure without sample and procedure (iii).

(2)	Neutralization titration with sodium hydroxide
	(i) Take 1.0 g of sample into a 100 mL beaker and weigh accurately with 0.1 mg digit.
	(ii) Add stirring bar and 50 mL of DI water. Dissolve the sample by stirring.
	(iii) Immerse electrodes and start titration with 1 mol/L sodium hydroxide standard solution.

4. Measurement conditions and results

(1) Redox titration with potassium permanganate standard solution

Examples of titration conditions

(i) Measurement of blank

Cndt No	1
Method	Auto
Buret No.	1
Amp No.	2
D. Unit	mV
S-Timer	5	sec
C.P. mL	0	mL
T Timer	0	sec
D.P. mL	0	mL
End Sens	100
Over mL	0.3	mL
Max Vol.	20	mL

Constant No.	1
Size	0	g
Blank	0	mL
Molarity	0.02	mol/L
Factor	1.003
K	0
L	0
Unit	mL
Formula	D

Decimal Places	3
Auto input Param.		Non

Mode No.	18
Pre Int	0	sec
Del K	0
Del Sens	0	mV
Int Time	5	sec
Int Sens	5	mV
Brt Speed	2
Pulse	40

(ii) Dispense 0.02 mol/L potassium permanganate standard solution.

Cndt No	2
Method	Disp
Buret No.	1
S-Timer	5	sec
Disp Vol.	30	mL

(iii) Measurement of oxalic acid dihydrate

Cndt No	3
Method	Auto
Buret No.	1
Amp No.	2
D. Unit	mV
S-Timer	10	sec
C.P. mL	0	mL
T Timer	0	sec
D.P. mL	0.1	mL
End Sens	500
Over mL	1	mL
Max Vol.	20	mL

Constant No.	3
Size	0.2	g
Blank	0.074	mL
Molarity	0.02	mol/L
Factor	1.003
K	126.07
L	2.5
Unit	%
Formula
	(D+30-B)KFML/(S*10)
Decimal Places	3
Auto input Param.		Non

Mode No.	22
Pre Int	0	sec
Del K	0
Del Sens	0	mV
Int Time	5	sec
Int Sens	5	mV
Brt Speed	2
Pulse	40

* Enter the blank value to “B”, the molecular weight of oxalic acid dehydrate to “K”, and the reaction ratio between oxalic acid dihydrate and potassium permanganate to “L”.

Measurement results

Number of measurement	Size (g)	Titrant volume(mL)
1	–	0.073
2	–	0.074
Avg.(Blank)		0.074 mL

Examples of titration curves

Measurement results

Number of measurement	Size (g)	Titrant volume(mL)	Purity (%)
1	0.2007	1.779	99.876
2	0.2017	1.976	99.999
3	0.2012	1.876	99.933
Statistic calculation	Avg.		99.94 %
	SD		0.0616 %
	RSD		0.0616 %

Examples of titration curves

(2) Neutralization titration with 1 mol/L sodium hydroxide standard solution

Examples of titration conditions

(i) Measurement of oxalic acid dihydrate

Cndt No	4
Method	Auto
Buret No.	1
Amp No.	1
D. Unit	pH
S-Timer	5	sec
C.P. mL	0	mL
T Timer	0	sec
D.P. mL	0	mL
End Sens	1000
Over mL	0.3	mL
Max Vol.	20	mL

Constant No.	4
Size	1	g
Blank	0	mL
Molarity	1	mol/L
Factor	1.005
K	126.07
L	2
Unit	%
Formula
	(D-B)KFM/(S10*L)
Decimal Places	3
Auto input Param.		Non

Mode No.	8
Pre Int	0	sec
Del K	5
Del Sens	0	mV
Int Time	5	sec
Int Sens	3	mV
Brt Speed	2
Pulse	40

* Enter the molecular weight of oxalic acid dihydrate to “K”, and the valence of oxalic acid dihydrate to “L”.

Measurement results

Number of measurement	Size (g)	Titrant volume(mL)	Purity (%)
1	1.0067	15.874	99.893
2	1.0024	15.822	99.993
3	1.0001	15.775	99.925
Statistic calculation	Avg.		99.94 %
	SD		0.0511 %
	RSD		0.0511 %

Examples of titration curves

5. Note

(1) Measurement result
The purity of oxalic acid was determined by the following two method: redox titration with potassium permanganate and neutralization titration with sodium hydroxide. There is no difference between the results on these methods, the titration was possible without problem in each method. The neutralization titration is particularly easy because of no pretreatment and the less waste solution.

(2) Collection of sample
The sample is collected directly to the beaker and weighed accurately. The accuracy of sample collection influences the measurement accuracy. Please note that the sample should be carefully taken and accurately weighed.

(3) 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 (3)). The standard solution of sodium hydroxide that has absorbed carbon dioxide contains sodium carbonate, and the inflection point on titration curve may be unclear due to buffer capacity of sodium hydrogen carbonate generated in the reaction with an acidic sample (formula (4)).