[ksr985]

Q1. why do we add the extra KI around the lid of the iodine flask, why not add it to the soln. itself?
Q2. vogel says  starch cannot be added in the beginning of the titration since it gives an insoluble ppt. in presence of extra iodine. why exactly is this a problem?

[Borek]

Please post in the appropriate forums, this is Analytical Chemistry Question.

Dr. HMX, please move this thread to my forum

Now to the answers: I don't recal I have ever meet with the idea from Q1. Colud you elaborate a little bit?

Q2: if there is insoluble precipitate part of the iodide is removed from the solution in the inactive form and the determination will be not precise - there will be a negative error.

[ksr985]

Since we are actually performing the titration with the iodine liberated due to the reaction with whatever we are estimating, say copper sulphate, how does the precipation of iodide in the solution affec our titration? the iodine liberated will still be the same, on account of the excess of KI added.One reason that i can think of is that the indicator, starch, will itself go out of solution.

Also, referring to Q1, we add some KI to the solution, cap the flask, then add a little KI around the neck. The flask is then kept in the dark, and then the KI on the neck washed into the soln with distilled water. I dont understand the need for this procedure.

[Borek]

Sorry, typo - not iodide is removed from the solution, but iodine.

Q1 - I don't think it makes sense, but the the idea seems to be like that: iodine is very volatile and it will leave the bottle no matter how tight it is capped. KI is there to catch the iodine in the form of KI3, when it is washed to the solution part of the iodine is recovered.

[ksr985]

Does this mean that it doesnt matter how much KI we keep on the neck, as long as it is enough to not ge saturated by the escaping iodine? We can keep 1 mL, 2 mL, any amount?

[Borek]

It seems so, but I doub't it really works. Some of the iodine will be recovered, but not quantitatively IMHO.

[kevins]

In the presence of excess of iodide, the volatility is decreased markedly through the formation of tri-iodide ion (I3-), at room temp the loss of iodine by volatilisation from a solution containing at least 4% of KI is negligible provided the titration is not prolonged.

[pradeep]

Q1. why do we add the extra KI around the lid of the iodine flask, why not add it to the soln. itself?
Q2. vogel says  starch cannot be added in the beginning of the titration since it gives an insoluble ppt. in presence of extra iodine. why exactly is this a problem?

1. Iodine is volatile & it forms tri-iodide with iodide which is soluble. This prevents some losses. generaly it is sufficient to add some water while opening flask lid.
2. Starch solution is colloid & posses adsorption property. I2 may adsorb on starch & prevented from rxn.