OBJECTIVE: In this lab experiment, you will determine the solubility product constant of a slightly soluble salt.
THEORY: In order to determine the maximum solubility of a salt, you need to have the salt in the presence of excess solid. The magnitude of the solubility product constant (K sp
) gives you an idea of the maximum solubility of the salt. Most values of K sp
are small such as the K sp
for AgCl which is equal to 4 x 10
-11
. In this lab, you will determine the K sp
for calcium iodate, Ca(IO
3
)
2
through a starch titration with sodium thiosulfate that will determine the concentration of the iodate ion. The reaction that describes the equilibrium of Ca(IO
3
)
2
with its ions is as follows
Ca(IO
3
)
2
(s) <==> Ca +2 (aq) + 2 IO
3–
(aq) and the solubility product expression is
K sp
= [Ca
+2
] [IO
3
–
]
2
The ionic equations for the titration reactions are:
2IO
3
-
+ 10I
-
+12H
+
6I
2
+ 6H
2
O
I
2
+ 2S
2
O
3
2-
2I
-
+ S
4
O
6
2deep blue colorless
with starch
In the presence of starch, the I
2
forms a dark blue complex. To determine the concentration of iodate in the unknown solution you will add the starch indicator and then titrate with Na
2
S
2
O
3
until the dark blue color disappears.
Preparation of a Saturated Solution of Ca(IO
3
)
2
1. Combine 100.0 mL of 0.20 M KIO
3
with 40.0 mL of 1.00 M Ca(NO
3
)
2
in a 150 mL beaker. Stir the mixture completely. A white precipitate of Ca(IO
3
)
2
should form.
2. Let the mixture stand a few minutes, then filter it through a dry filter into a dry flask, using gravity filtration.
Prepare 0.1 M sodium thiosulfate:
1. Heat 750 mL of distilled water to boiling for at least five minutes. ALLOW TO COOL and then add
18.75 grams Na
2
S
2
O
3 x 5H
2
O and 0.07 grams Na
2
CO
3
. Stir until solution is complete. MIX
THOROUGHLY!! Transfer to a clean, glass-stoppered bottle. Store in the DARK for 20 hours.
Prepare 1.5 x 10
-2
M potassium iodate (standard solution):
1. Weigh 0.80 to 0.85 grams (~0.1 mg) of dried KIO
3
into a 250 mL beaker. Quantitatively transfer to a 250 mL volumetric flask using distilled water. Dilute to the mark and MIX THOROUGHLY!! Store in the dark.

Standardization of Thiosulfate Solution
1. Set up a buret and fill it with the thiosulfate (Na
2
S
2
O
3
) solution that you made last week.
2. Pipet four 25.00 mL aliquots of the KIO
3
solution into four 250 mL erlenmeyer flasks.
*** Completely finish one trial before beginning the next (store the samples which are not being used in the dark). Read all of the next paragraph before beginning your titrations.***
3. Add 2 grams NaI (iodate free).
4. When solution is complete, add 10 mL 1.0 M HCl.
5. TITRATE IMMEDIATELY with the thiosulfate solution until the color becomes pale yellow.
6. Add 5 mL of the starch solution and titrate to the disappearance of the starch-blue/black color.
HINT: the dark blue/black solution will start to become slightly transparent as you near the endpoint. The solution should remain COLORLESS for 15 seconds.
7. Once again, add the thiosulfate solution slowly near the equivalence point. You should be able to measure this end point to within 1/2 drop of the true equivalence point.
8. Repeat the titration two more times.
Determination of [IO
3
–
] in the Saturated Solution of Ca(IO
3
)
2
1. Pipet 10.00 mL of the filtrate from last week into a clean Erlenmeyer flask.
2. Add about 20 mL of deionized water to the flask.
3. Dissolve about 2 g of solid KI in the solution. Add 20 drops of 2 M HCl and mix thoroughly. The solution should take on a brown color.
4. Titrate the mixture with YOUR standardized Na
2
S
2
O
3
until the brown color changes to yellow.
5. Add several drops of starch to the solution, enough to turn the solution blue/black.
6. Continue titrating with YOUR standardized Na
2
S
2
O
3
until the blue/black color just disappears.
7. Repeat the titration two more times.
CALCULATIONS:
1. Using the known concentration of sodium thiosulfate, calculate the [IO
3
–
] in the filtrate.
2. From the [IO
3
–
], calculate the solubility of [Ca
+2
], and then the average K sp
of Ca(IO
3
)
2
along with the standard deviation.