Potentiometric Determination of Vitamin C

Potentiometric Determination of Vitamin C

Equipment
Potentiometer = 751 GPD Titrino / Titroprocessor
Electrodes = 6.0309.100 double Pt sheet electrode; input Pol
polarized Ipol = 1 mA
Voltametric indication

Reagents
c(DPIP) = 0.001 mol/L; D0
2,6-Dichlorophenol indophenol

Sample
5 mL sample solution
Dissolve 1 effervescent vitamin C tablet in 1 L dist. water
15 mL oxalic acid solution 1 g/L
1 mL w(sodium acetate) = 0.10 (10%)
10 mL dist. Water

Remarks
· Calculations:

Vit.C=EP1*C01*C02*C03;0;mg/pc
Vit.C=EP1*C01*C02/C00;4;g/l

Vit.C = content of Vitamin C in mg/pc (tablet)
Vit.C = content of Vitamin C in g/L
C01 = mg Vitamin C / 1 mL titrating agent (0.088)
C02 = factor of titrating agent (1.00)
C03 = dilution factor (200)

. Answer

Vit.C = 15.291 x 0.088 x 1.00 x 200 = 269 mg/pc
Vit.C = 15.291 x 0.088 x 1.00 / 5.00 = 0.2691 g/L

EP1 = 15.291 mL
C00 = 5.00 mL

· Titrating agent:
Dissolve 295 mg 2,6-Dichlorophenol indophenol with vigorous agitation in 1 L dist. water, then filter and mix with 100 mg sodium bicarbonate.
This solution can be stored in the refrigerator for about 1 month, the factor should be checked daily with standard ascorbic acid. (As a titrating agent, the more readily soluble sodium salt may be used instead.)

· Standard solution:
r (Vitamin C) = 500 mg/L
Dissolve 50 mg ascorbic acid in oxalic acid solution (1 g/L) and make up to 100 mL. This solution should be freshly prepared daily.

· Sample preparation:
Place dist. water, oxalic acid solution and sodium acetate buffer in the titration vessel and deaerate by passing a stream of nitrogen for 3...5 min. Then add a quantity of sample or standard solution containing about 0.05...0.5 mg vitamin C. Now titrate under nitrogen with titrating agent.
· Store electrode in acidified Na2S2O3 solution.
· Select the appropriate formula. The other may be deleted.
· The 6.0431.100 Pt Titrode may be used together with mode MET U.

Literature
· Metrohm Application Bulletin No. 98: Determination of ascorbic acid
(Vitamin C) and its compounds.

Method documentation

'pa
751 GPD Titrino 15215 751.0010
MET Ipol… Vit.C
parameters
>titration parameters
V step …0.10 ml
dos.rate max…. ml/min
signal drift …OFF mV/min
equilibr.time …15 s
start V: …OFF
pause …30 s
dos.element:… internal D0
I(pol) …1 mA
electrode test:… OFF
temperature …25.0 °C
>stop conditions
stop V: …abs.
stop V …20 ml
stop U… OFF mV
stop EP …9
filling rate max…. ml/min
>statistics
status: …OFF
>evaluation
EPC …30 mV
EP recognition: all
fix EP1 at U …OFF mV
>preselections
req.ident: …OFF
req.smpl size: …OFF
activate pulse: …OFF
------------
'fm
751 GPD Titrino 15215 751.0010
MET Ipol Vit.C
>calculations

Vit.C=EP1*C01*C02*C03;0;mg/pc
Vit.C=EP1*C01*C02/C00;4;g/l

C00= 5.0
C01= 0.008
C02= 1.00
C03= 200
------------
'fr

751 GPD Titrino 15215 751.0010
U(init) 121 mV MET Ipol Vit.Csmpl size 5.0 ml

EP1 …15.291 ml…. 122 mV
Vit.C… 269 mg/pc
Vit.C ….0.2691 g/l
stop V reached
============
'cu
751 GPD Titrino 15215 751.0010
start V 12.000 ml MET Ipol Vit.C
1.0 ml/div dU=50.0 mV/div

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