Question

One of the most popular methods for determine trace amounts of water is a coulometric titration, called the Karl Fischer. Measuring trace amounts of water is one of the most challenging analytical methods.

a) What would be another method that would measure larger amounts of water contained in a sample, and how would you do it?

b) The Karl Fischer method is a secondary coulometric analysis that utilizies the generation of I2/I3 with starch, just like you did in lab, generated from the Iodide ion. This iodine molecular species then reacts with water in a complicated Redox, involving stinky pyridine....hense, the advantage of doing this by remote control under a hood. The actual limiting reagent is WATER: One mole of water reacts with one mole of generated I3.

The goal of your problem is to find the ppm of water in a 250 mL sample of pure ethanol, that has a density of 0.7893 g/mL. A current of 55.5 milliamps was run for 4 minutes 23 seconds, until the blue/black color of the starch was observed. However, the current generator box was only 89.3 % current efficient, losing some current. The electrochemical production of the tri-iodide ion reagent was the same oxidation reaction as in lab. What is the ppm of WATER contamination the 250 mL of pure ethanol?

Answer 1

a) If the water content in a sample is large one of the ideal ways to measure water content is by gravimetric measurement where the sample is weight first and subsequently it can be heated in an oven at 100^oC to measure loss of water. The weight loss on loss of water is measured and % loss can be estimated.

Another method would be to record the thermogramimetric analysis and measure water loss as the loss around 100 ^oC as a percentage of total loss.

Another method if we are working with low volatile organic liquids could be to treat the liquid with drying agents like Calcium hydride or molecular seives and measure water loss. This type of measure ment however will lead to loss of the material also by adsorption on the drying material.

b) The reaction in the Karl Fischer reaction is

I₂ + 2e- 2I^-

I₂ + I^- I₃^-

1 mole of Iodine reacts with 1 mole of electrons to give 1 mole of I₃^-

55.5 x 10^-3 amperes x 263 sec =   14.59 coulombs      (55,5 milliamperes is 55.5 x 10^-3 amperes; 4minites and 23 sec is 263 sec)

14.59 x 1F/96485C = 1.51 x 10^-4 F

1.51 x 10^-4 moles of electrons

1.51 x 10^-4 moles of I₃^-

This reacts with 1.51 x 10^-4 moles of water

MW of water is 18g/mol

So water in the sample is 18 g/mol x 1.51 x 10^-4 moles = 0.0027 g

250 mL ethanol with density of 0.7893 g/mL = 197.32 g

197.32 g of ethanol has 0.0027 g of water

1000 g of ethanol has 0.0138 g of water

1 ppm is 1 mg/Kg or 1mg/1000 g

0.0138 g of water is 13.8 mg of water

13.8 mg of water is there in 1000g of ethanol

So the water content in ethanol is 13.8 ppm