SOLUBILITY PRODUCT CONSTANT FOR CALCIUM IODATE
The purpose of this experiment is to determine the value of K sp
, the solubility product constant, for calcium iodate, Ca(IO
3
)
2
, and to examine how the presence of KIO
3
affects the solubility of Ca(IO
3
)
2
.
Saturated solutions of Ca(IO
3
)
2
can be purchased from G Frederick Smith Chemical Co. or can be prepared. Ca(IO
3
)
2
in 0.0100 M KIO
3
have already been prepared. You will determine the concentration of IO
3
1– ions in each solution by oxidizing IO
3
IO
3
1–
+ 5 I
1–
3 I
2
+ 6 S
2
1– to I
2
, then titrating the I
2
with thiosulfate ion, S
2
O
3
2– :
+ 6 H
O
3
2–
1+ 
3 I
2
+ 3 H
2
O

6 I
1–
+ 3 S
4
O
6
2–
The thoisulfate solution has not been standardized, so you will need to do that first by using it to titrate a known amount of KIO
3
. The iodate ion is mixed with iodide ion and acid to produce I
2
, which is then titrated as above. Near the endpoint, you will add starch to act as an indicator. The starch is deep blue in the presence of I
2
, and turns abruptly colorless when the I
2
is consumed.
Procedure
1. Standardization of the thiosulfate solution.
Fill your buret with sodium thiosulfate solution. Put 25 mL deionized water, 2 g KI, and 10 mL 1 M
HCl into your titration flask and swirl to dissolve the solid. Dispense about 10 mL standard 0.0100 M
KIO
3
solution into your titration flask and record the exact volume. The solution should turn dark brown.
Titrate the brown solution until the brown color (I
2
) is mostly gone and the solution has turned pale yellow (not golden). Add 5 mL 0.1% starch indicator (made by dissolving 2.00g starch in 1L); the solution should turn dark blue. Titrate carefully to the colorless endpoint.
2. Titration of iodate in saturated Ca(IO
3
)
2
.
Repeat the procedure in step 1, using 10 mL saturated Ca(IO
3
)
2
in place of standard KIO
3
.
3. Titration of iodate in saturated Ca(IO
3
)
2
in 0.0100 M KIO
3
.
Repeat the procedure in step 1, using 10 mL saturated Ca(IO
3
)
2
in 0.0100 M KIO
3
in place of standard
KIO
3
.
Analysis and Discussion
1. Calculate [S
2
O
3
2–
] and record the result on the class data sheet. Calculate [IO
3
1–
] in each saturated solution, using the class average [S
2
O
3
2–
].
2. For the saturated Ca(IO
3
)
2
: Write the chemical equation and K sp
expression for saturated Ca(IO
3
)
2
.
From the measured concentration of iodate ions, deduce the concentration of Ca
2+
ions and calculate the value for K sp
in your experiment. Calculate the molar solubility of Ca(IO
3
)
2
.
3. For the saturated Ca(IO
3
)
2
in 0.0100 M KIO
3
: From the measured total concentration of iodate ions, subtract the iodate ions from KIO
3
to determine the concentration of iodate ions from Ca(IO
3
)
2
.
Deduce the concentration of Ca 2+ ions and calculate the value for K sp
in your experiment (use the total
[IO
3
1–
] and the calculated [Ca
2+
]). Calculate the molar solubility of Ca(IO
3
)
2
in 0.0100 M KIO
3
.
4. Gather class results. Discuss the agreement between your value and the average, the consistency of class results, and how many sigfigs the overall results justify. Do you expect the values of K sp
to be

the same in both solutions? Do you expect the solubility of Ca(IO
3
)
2
to be the same in both solutions?
Explain.
Notes:
Preparation of Ca(IO
3
)
2
* 6H
2
O
Measure 0.5mole 112g of KIO
3
in 600 mL hot water. Slowly add 0.25 mole Ca(NO
3
)
2
dissolved in
200 mL water. Allow to cool. Filter. Wash with 3-50ml portions of water.
Saturated solution is prepared by adding 2.14 g KIO3 and 4.00g Ca(IO
3
)
2
* 6H
2
O stir for 24hours. Allow to stand for 48 hours before use.
0.0100 M KIO
3
made by dissolving 2.14 g KIO
3
in 1.00L of water
1 M HCl made by dissolving 83.33ml of 12.00 M HCl in 1.00 L of water
0.1% starch indicator made by dissolving 2.00g starch in 1.00L of water and heat for 1 hour