1. Standardization of Sodium Thiosulfate
Yevgenny Strashnov
CHEM 222, Spring ‘07

2. Introduction
Sodium Thiosulfate (Na2S2O3) is used in Iodometry to quantify the amount of iodine in solution, in the form of Triiodide (I3-)
Since Sodium Thiosulfate is rarely/never available as a primary standard, Na2S2O3 solutions must be standardized between preparation and use
To standardize an Na2S2O3 solution, KIO3 is usually employed as the primary standard

3. Stoichiometry
Two chemical equilibrium equations are involved in the process:
IO I H+  3I H2O
I S2O3 2-  3I- + S4O6 2-

4. Preparation of (unstandardized) Sodium Thiosulfate Soln.
The first step is to prepare the unstandardized Sodium Thiosulfate
solution:
We do this by dissolving approximately 25g of Na2S2O3*5H2O,
and .1g of Na2CO3* in 1L of freshly boiled** dH2O.
* Addition of Sodium Carbonate serves to buffer the pH at the optimal level for stability.
** The boiling expels CO2, which can make the solution acidic and promote unwanted side reactions.

5. Preparation of Primary Standard
The primary standard (KIO3) is accurately weighed, dissolved in dH2O,
and chemically treated to obtain I3- in solution, according to Eq. (1):
IO I H+  3I H2O

6. Preparation of Primary Standard
The primary standard (KIO3) is accurately weighed, dissolved in dH2O,
and chemically treated to obtain I3- in solution, according to Eq. (1):
IO I H+  3I H2O
A slight excess of I- is supplied
by dissolving solid KI…

7. Preparation of Primary Standard
The primary standard (KIO3) is accurately weighed, dissolved in dH2O,
and chemically treated to obtain I3- in solution, according to Eq. (1):
IO I H+  3I H2O
A slight excess of I- is supplied
by dissolving solid KI…
The H+ can be supplied by adding
a non-reactive acid, such as H2SO4…

8. Preparation of Primary Standard
The primary standard (KIO3) is accurately weighed, dissolved in dH2O,
and chemically treated to obtain I3- in solution, according to Eq. (1):
IO I H+  3I H2O
A slight excess of I- is supplied
by dissolving solid KI…
The H+ can be supplied by adding
a non-reactive acid, such as H2SO4…
In this manner, 3 moles of I3- can be obtained in the solution
for every mole of KIO3 dissolved…

9. Standardization of Sodium Thiosulfate Solution
The standardization of Na2S2O3 occurs according to Eq. (2):
(2) I S2O3 2-  3I- + S4O6 2-

10. Standardization of Sodium Thiosulfate Solution
The standardization of Na2S2O3 occurs according to Eq. (2):
(2) I S2O3 2-  3I- + S4O6 2-
An exact amount of the treated
primary standard soln. is placed
in an Erlenmeyer flask…

11. Standardization of Sodium Thiosulfate Solution
The standardization of Na2S2O3 occurs according to Eq. (2):
(2) I S2O3 2-  3I- + S4O6 2-
An exact amount of the treated
primary standard soln. is placed
in an Erlenmeyer flask…
The unstandardized thiosulfate soln.
is placed in the buret…

12. Standardization of Sodium Thiosulfate Solution
The standardization of Na2S2O3 occurs according to Eq. (2):
(2) I S2O3 2-  3I- + S4O6 2-
Starch indicator is
used to accentuate
the endpoint…
An exact amount of the treated
primary standard soln. is placed
in an Erlenmeyer flask…
The unstandardized thiosulfate soln.
is placed in the buret…

13. Standardization of Sodium Thiosulfate Solution
The standardization of Na2S2O3 occurs according to Eq. (2):
(2) I S2O3 2-  3I- + S4O6 2-
Starch indicator is
used to accentuate
the endpoint…
An exact amount of the treated
primary standard soln. is placed
in an Erlenmeyer flask…
The unstandardized thiosulfate soln.
is placed in the buret…
Thus, 2 moles of S2O3 2- are required to neutralize each mole of
I3-…

14. Sample Calculations 1. Obtain exact molarity of primary std.:
(1.000g / g/mol) / 500.0mL = 9.346E-6 mol/mL
2. Obtain average equivalence volume of Na2S2O3 (for 50.0
mL aliquots of primary std.):
(1/3)*( )mL = mL
3. Determine how many moles of Na2S2O3 must be in this average volume as follows:
50.0mL*(9.346E-6 mol IO3-/mL)*(3mol I3-/mol IO3-)
*(2mol S2O3 2-/mol I3-) = 2.80E-3 mol S2O3 2-
4. Finally, calculate the molarity of the (now standardized) Na2S2O3 soln:
(2.80E-3 mol S2O3 2- / mL)*(1000mL/1L)
= M

15. Pop Quiz !!! ;)
1. What ion does S2O3 2- (the thiosulfate ion that comes from Na2S2O3) react with during standardization? What is the stoichiometric ratio?

16. Pop Quiz !!! ;)
1. What ion does S2O3 2- (the thiosulfate ion that comes from Na2S2O3) react with during standardization? What is the stoichiometric ratio?
A: S2O3 2- reacts with I3- in a 2:1 ratio.

17. Pop Quiz !!! ;)
1. What ion does S2O3 2- (the thiosulfate ion that comes from Na2S2O3) react with during standardization? What is the stoichiometric ratio?
A: S2O3 2- reacts with I3- in a 2:1 ratio.
2. How is the I3- ion obtained from our primary standard (KIO3)? In what ratio?

18. Pop Quiz !!! ;)
1. What ion does S2O3 2- (the thiosulfate ion that comes from Na2S2O3) react with during standardization? What is the stoichiometric ratio?
A: S2O3 2- reacts with I3- in a 2:1 ratio.
2. How is the I3- ion obtained from our primary standard (KIO3)? In what ratio?
A: The I3- ion is obtained by reacting IO3- with an excess of I- and H+… 3 I3- ions are obtained for every IO3- ion (3:1 ratio)