What is the significance of adding KI before titrating with Na2S2O3 (in the lab experiment for lipids)?

Answer 1

Description
The analysis of household laundry bleach is undertaken by adding potassium iodide to acidified bleach and titrating the iodine formed with a standard solution of sodium thiosulfate.

Safety

Although a common household item, bleach is a dangerous material that causes burns and is toxic. Reacting bleach with acid produces chlorine. Reacting bleach with ammonia produces toxic chloramine. Potassium iodide and iodine are toxic. Wear goggles and apron. Avoid contact with chemicals. Wash spills immediately with large amounts of water. Wash hands after the experiment. Procedure

Both bleach and the thiosulfate must be measured as accurately as possible. Use pipets with a narrow tips. Carefully squeeze individual drops. Count the drops of both bleach and thiosulfate carefully.
Place 20 drops of (diluted) bleach solution in one well of a 24-well plate. Add 20 drops of 3 M acetic acid. Add 12 drops of KI to the bleach. Stir with a toothpick. The solution should have an orange-brown color. While stirring, titrate the solution dropwise with the Na2S2O3 until the solution turns yellow. Be sure to keep track of the number of drops used. When the solution is a light yellow which indicates that most of the I2 has reacted, add one drop of the starch solution; continue stirring. The solution should turn a deep blue black color. Continue titrating dropwise with Na2S2O3 until the solution turns colorless. Continue counting drops. Record the total drops of Na2S2O3 used. Wash hands. Data Analysis

Determine the concentration of bleach in the original solution before dilution.
Questions

Write a balanced equation for the reaction of bleach with potassium iodide. Write a balanced equation for the reaction of thiosulfate ion with triiodide ion.
Describe the effect on bleach of standing in an opened bottle.
Solutions of bleach are very basic. Explain.
Identify the role of starch in the experiment.
The reactions involved are:
ClO- + 3 I- + 2 H+ --> Cl- + I3- + H2O
2 S2O32- + I3- --> S4O62- + 3 I-
Adding these equations together gives:
2 S2O32- + ClO- + 2 H+ --> S4O62- + Cl- + H2O
(M S2O32-) x (V S2O32-) x (1 mol ClO-/2 mol S2O32-)
= (M ClO-) x (V ClO-)
(M ClO-) = (M S2O32-) x (V S2O32-) x
(1 mol ClO-/2 mol S2O32-) x (1/(V ClO-)
= (0.15 ) x (17.5 drops S2O32-) x
(1/2) x (1/20 drops bleach)
= 0.066 M (dilute) = (M ClO-)
M conc x V conc = M dil x V dil
M conc = M dil x ( V dil/V conc )= 0.066 x 10 = 0.66
g bleach/ L = (0.66 mol NaClO /1 L) x
(74 g NaClO/1 mol NaClO)
= 49 g NaClO/L
[In an experiment performed by the instructor, a 10-mL cylinder was tared on a top-loading balance. A 9.63 mL sample had a mass of 10.3 g. From this, the density was determined to be 10.3 g /9.63 mL = 1.07 g/mL.]
% bleach = 100% x (49 g NaClO/L) x
(1 L/1000 mL) x (1 mL/1.07 g)
= 4.6 % NaClO in bleach

Answer 2

Not sure about lipids, but with starches you do a similar process. The iodine from the KI complexes with the starch to turn the solution blue (or purple depending on the concentration) but the sodium thiosulfate, when added, reacts with the complex and takes the iodine, lessening and finally removing the color.

Since lipids are also carbon-based I suppose that the iodine would react similarly. But without the KI, there would be nothing to provoke the color-changing reaction.