SI Analytics-Application report
Titration
Determination of Selenite SeO32-
Description
The determination of the Selenium - content is done by reaction of the Selenite with Potassium iodide and
titration with Sodium thiosulfate solution 0.1 mol/l.
2- -
SeO3 + 4 I Se + 2 I2
This determination is not always easy because the elemental selenium precipitates out of the solution and
can pollute the electrode. For this reason, polyvinyl alcohol must be added as a dispersant. The calculation
is done as% selenium.
Instruments
Titrator TL 7000 or higher
Electrode Pt 62 oder Pt 61
Cable L1A
Stirrer Magnetic stirrer TM 235 or similar
Lab accessory Glass beaker 150 ml
Magnetic stirrer bar 30 mm
Reagents
1 Sodium thiosulfate 0.1 mol/l
2 Potassium Iodide
3 Hydrochloric acid ~25%
4 Polyvinylalkohol – solution 0.5%
5 Distilled Water
6 Electrolyt solution L300
All reagents should be of analytical grade or better.
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Titration procedure
Reagents
The titer determination of the Na2S2O3 - solution is carried out as described in the application report "Titer
determination of Na2S2O3".
Polyvinyl alcohol - solution 0.5%
0.5 g of polyvinyl alcohol are dissolved in 100 ml of distilled water with heating.
Cleaning of the electrode
The electrode is rinsed with distilled water. The electrolyte solution L300 is suitable for storage.
Sample preparation
The sample is weighed into a 150 ml beaker and made up to about 70 ml with distilled water and 5 ml of
HCl 25% are added. For basic samples, more HCl may be needed. After dissolving the sample, 1 ml of the
polyvinyl alcohol solution and 1-1.5 g of KI are added. The solution turns reddish-brown and becomes
cloudy. Then it is titrated with sodium thiosulfate 0.1 mol / l. The consumption should be about 10 - 40 ml.
The amount of sample should be adjusted to the selenium content.
Selenium content [% Se] Sample [g]
~ 50% 0,1 – 0,15
~ 5% 0,8-1,2g
For inhomogeneous samples, e.g. mixtures of solid Sodium selenite with fillers, it may be necessary to
dissolve a larger amount of sample and titrate an aliquot of it.
If the electrode is rapidly polluted by precipitated selenium and the titration curve becomes more noisy, it
may be better to dilute the sample more strongly.
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Titration parameter
Sample titration
Default method -
Method type Automatic titration
Modus Dynamic
Measured value mV
Measuring speed / drift User defined Minimum holding time 5s
Maximum holding time 15 s
Measuring time 3s
Drift 8 mV/min
Initial waiting time 0s
Dynamic average Max step size 1.0 ml
Slope max ml 10
Min. step size 0.02 ml
Slope min. ml 120
Damping none Titration direction increase
Pretitration off Delay time 0s
End value off
EQ On (1) Slope value 700
Max. titration volume 50 ml
Dosing speed 100% Filling speed 30 s
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Calculation:
(𝐵 − 𝐸𝑄1) ∗ 𝑇 ∗ 𝑀 ∗ 𝐹1
𝑅𝑒𝑠𝑢𝑙𝑡 [% 𝑆𝑒] =
𝑊 ∗ 𝐹2
B 0 Blank value
EQ1 Consumption of titrant at first Equivalence point
T WA Actual concentration of the titrant
M 78.97 Molecular weight
W man sample weight in g
F1 0.1 Conversion factor
F2 4 Conversion factor
Any questions? Please contact the application team:
Xylem Analytics Germany Sales GmbH & Co. KG, SI Analytics
Hattenbergstraße 10
D-55122 Mainz, Germany
Telefon: + 49 6131 66 5126
Fax: + 49 6131 66 5101
E-Mail: titration@si-analytics.com
Xylem Analytics Germany Sales GmbH & Co. KG · Hattenbergstr. 10 · D-55122 Mainz ·
Germany Telefon: +49 6131.66. 5111 · E-Mail: Info.si-analytics@Xyleminc.com · www.si-
analytics.com
Alle Namen sind eingetragene Handelsnamen oder Warenzeichen der Xylem Inc. oder eines seiner
Tochterunternehmen. Technische Änderungen vorbehalten.
© 2018 Xylem Analytics Germany Sales GmbH & Co. KG.
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