1. pertinent chemical equations and stoichiometry in the standardization of Na2S2O3?

1 mmol IO3- = 6 mmols S2O3 2-

2. What is the purpose of adding H2SO4 and excess KI during standardization and

why the acid was added before KI?

Sulfuric acid in particular was added due to the need for an acidic environment

for the reaction with iodate (IO3-) to take place.

3. Discuss the pertinent chemical equations and stoichiometry in the sample anal

ysis

1 mole of O2 2 moles of MnO(OH)2 2 mole of I2 4 mole of S2O32

4. Explain stepwise how I3 was produced from the dissolved O2 in the water sampl

e. Explain briefly why the reagents are added in definite sequence.

5. what is the reason why starch was used as indicator in the analysis and why i

t was added towards the end of titration?

6. How is the analysis (an iodometric process) different from an iodimetric one?

7. From the calculated ppm O2, identify the degree of water pollution and abilit

y of the water sample source to sustain aquatic life.

8. Predict the effect, if any, of wach of the following on the DO oxygen content

obtained.

a. The water sample is made to stand overnight before analysis.

b. MnSO4 is added and the solution made to stand for an hour before the alkaline

KI solution is added.

9. Discuss the possible sources of errors and their effect on calculated paramet

ers.

The success of this method is critically dependent upon the manner in which the

sample is manipulated. At all stages, steps must be taken to ensure that oxygen

is neither introduced to nor lost from the sample. Furthermore, the water sample

must be free of any solutes that will oxidize or reduce iodine.

A Winkler bottle is a piece of laboratory glassware specifically made for carryi

ng out the Winkler test. These bottles have conical tops and a close fitting sto

pper to aid in the exclusion of air bubbles when the top is sealed. This is impo

rtant because oxygen in trapped air would be included in the measurement and wou

ld affect the accuracy of the test.

10. Discuss how the result of your group correlate to those obtained by other gr

oups.