G. Abril 2000, modified by M. Hesselsøe 2001, A.H. Nielsen 2007
Oxygen measurement by Winkler titration In 1888, the Hungarian chemist Lajos Winkler proposed a titremetric method to measure dissolved oxygen (DO) in waters. Since that time, membrane electrodes have been developed, but Winkler titrations are still used to calibrate electrodes, and when very accurate measurements are needed. Winkler titrations can give oxygen values at about ± 0.05 mg/l (a few µM), whereas membrane electrodes have a precision of approximately 0.1 mg/l and often drift with time. However, in some particular waters, Winkler titrations are not possible (because of high concentrations of suspended solids that unable colorimetric titration, high concentrations of organic matter that interfere, etc…).
Principle of the Winkler method
The method consists in adding simultaneously a strong base solution containing iodide (Solution 1 = NaI 3M / NaOH 8M) and a solution of divalent manganese (Solution 2 = MnCl 2 3M) which leads to the precipitation of manganese hydroxide (Mn(OH) 2) according to the reaction 1a. The oxygen dissolved in the water then oxidizes an equivalent quantity of manganese hydroxide into a manganese hydroxide of higher valence state: MnO(OH) 2, according to reaction 1b. By adding a -
strong acid, this manganese hydroxide complex is destabilized and, in presence of the iodide (I ) introduced with the solution 1, this leads to the formation of a quantity of Iodine (I 2) equivalent to the original DO content (Reaction 2). Iodine is then titrated with a standard solution of thiosulfate (Na2S2O3 ≈ 0,0040 M) according to reaction 3.
Reaction 1a :
MnCl 2 + 2 NaOH → Mn(OH) 2 + 2 Na + + 2 Cl-
Reaction 1b :
½ O 2 + Mn(OH) 2 → MnO(OH)2
Reaction 2 :
MnO(OH)2 + H 2SO 4 + 2NaI → Mn(OH) 2 + NaSO 4 + H 2O + I 2
Reaction 3 :
I 2 + 2 Na 2S2O3 → Na 2S4O6 + 2 NaI
I.e. : each molecule of oxygen (O 2) corresponds to 4 molecules of thiosulfate (Na 2S2O3). Standardization of thiosulfate: The molarity of the thiosulfate may vary with time so
standardization is necessary. For that, we use a solution of KIO 3. We first add the three reagents to the water sample, but the other way around (acid > solution 2 > solution 1) so no complex is
formed and no iodine (I 2) is produced. We then add a well-known quantity of KIO 3 (8 ml 0.0017M) and I2 is produced by the reaction:
KIO3 + 5 I - + 6 H + → 3 I 2 + 3 H 2O + K +
We can then determine the molarity of the thiosulfate by titration (reaction 3).
Preparation of reagents Solution 1
NaI 3M & NaOH 8M Dissolve 30g NaI into 50 ml deionized water Dissolve 16g NaOH into 50 ml deionized water Let cool down and mix both solutions.
Solution 2
MnCl2:4H2O Dissolve 60g MnCl 2:4H2O in 100 ml deionized water
Concentrated H 2SO4
Use gloves and glasses. Be careful.
Thiosulfate (prepare and subsequently determine the exact concentration by titration)
Dissolve 1 g of Na 2S2O3:5H2O in deionized water. Add 10 ml of a solution of Na 2CO3 (10 g/l) for conservation. Fill until a total volume of 1000 ml.
Starch indicator
Prepare a thin paste of 1 g soluble starch in a few mL deionized water. Bring 100 mL deionized water to a boil, remove from heat and stir in the starch paste.
Standard of potassium iodate
Dissolved 0,3567 of dried K(IO 3) in 200-300 ml deionized water in a 1L volumetric flask. Fill the volumetric flask to 1L with deionized water. The molarity is 0.0017 M KIO 3.
2
Standardization of thiosulfate solution
1. Prepare thiosulfate solution as described previously 2. Add approximately 50 mL deionized water to a titration beaker. 3. Add 250 μL concentrated sulfuric acid (H2SO4) 4. Add 250 μL MnCl2 solution and 250 μL (NaI 3 M + NaOH 8 M) solution 5. Add 8.0 mL 0.0017 M KIO 3. Mix with magnet. 6. Start titration with the thiosulfate solution (approximately 0.0040 M). 7. Continue the titration until a pale yellow color is obtained. Then add 1-2 drops of starch. 8. Continue titration until the first disappearance of the blue color. 9. Note the amount of thiosulfate added. 10. Calculate the molarity (moles per liter) of the thiosulfate on basis of the titrated volume.
Dissolved oxygen measurement
1. Transfer sufficient sample to the Winkler bottle (approximately 50 or 100 mL) for Winkler determination of dissolved O2. 2. Fill the Winkler bottle completely. 3. Add 250 μL MnCl2 solution and 250 μL (NaI 3 M + NaOH 8 M) solution to 50 mL Winkler bottles (twice the volume to 100 mL Winkler bottles), close and mix. Make sure that no air bubbles are trapped inside the Winkler bottle. 4. Let the precipitate settle. 5. Add 250 μL concentrated sulfuric acid (H2SO4) to 50 mL Winkler bottles (500 μL to 100 mL Winkler bottles). Close and mix until all precipitate has dissolved. At this point, the sample can be stored for a few hours for later titration. 6. Transfer 50.0 mL to a titration beaker with a glass pipette. 7. Begin titration with the thiosulfate solution. 8. Continue titration until a pale yellow color is obtained. Then add 1-2 drops of starch. 9. Continue titration until the first disappearance of the blue color. 10. Calculate the concentration of dissolved oxygen based on the titrated volume of thiosulfate.
3
Standardization of Thiosulfate 3
s oc so u on ≡ .
Titration of 8 mL KIO 3 solution: Reaction when adding KIO3 solution: •
KIO 3
+
5 I-
+
6H
+
→
3 I2
+
3 H2 O + K
+
I.e., 3 moles of iodine (I2) is produced per mole of potassium iodate (KIO3). Thus, 8 mL KIO3 produces: 8 mL x 3 (mol I 2/mol S2O32-) x 0.0017 mol S2O32-/L = 0.0408 mmol I 2 Amo un t o f I 2 in titration beaker
Standardization of Thiosulfate osu a e an
o ne reacts : :
I2
+
2 Na 2S 2 O 3
→
Na 2 S 4 O 6
+
2 NaI
Thus, if x mL of Thiosulfate is added during the titration (until change in color) the concentration of Thiosulfate is:
2 •
Thiosulfate concentration =
•
Thiosulfate concentration =
Contains 0.0408 mmol I 2
mol S 2 O 23mol I2
0.0408 mmol I2 M X mL ×
20.0816 mol S 2 O 3 X L
=
0.0816 M X
Titrate with x mL Na 2S2O3 solution
1
Thiosulfate standardization example •
. m osu a e was a e ur ng e s an ar za on, concentration of the Thiosulfate solution is:
•
Thiosulfate concentration =
e
0.0816 M = 0.0039 M = 3.9 m M 21.1
Winkler titration with Thiosulfate : osu ate concentrat on B: Titrated Thiosulfate volume
= = ? mL
.e.,
o
2
3
-
The number of moles of Thiosulfate added during the Titration is: A
•
Mol S2O32-/L x
B
mL x 1/1000 L/mL
(Unit is Mol S2O32-)
According to the reaction schemes below, Thiosulfate and O2 reacts in .
MnCl2 + 2NaOH → Mn(OH)2 + 2Na O 2 + 2 Mn(OH) 2
MnO(OH)2 + H2SO 4 I2
+
+
→
+
+
2 MnO(OH)2
2NaI → Mn(OH)2 + NaSO4
2 Na 2S 2 O 3
→
2 Cl-
Na 2S 4 O 6
+
+
H2O + I2
2 NaI
2
Winkler titration with Thiosulfate e ra on s per orme on m o samp e; us, e be calculated from the amount of Thiosulfate added as:
1 mol O 2 •
O2 Concentration =
4 mol S 2O 32-
A
×
mol S 2O 32-
×
L
2
concen ra on can
B mL
50 mL
If we want the concentration in mg O 2/L, we have to multiply with the molar wei ht of O which is 32 /mol.
1 mol O 2 •
O2 Concentration =
4 mol S 2O 23-
A
×
mol S 2O 32L
×
B mL ×
50 mL
32
g mol
Winkler titration example we a e . m o . osu ate so ut on ur ng t e t trat on, then the O2 concentration of our sample was:
1 mol O 2 O2 Concentration =
4 mol S 2O 23-
×
0.0039
mol S 2O 32L
50 mL
×
12.3 mL ×
32
g mol
O2 Concentration = 0.00768 g O 2/L = 7.68 mg O2/L
3
