KARL FISCHER METHOD FOR DETERMINATION OF WATER
A plethora of chemical compounds for the determination of small amounts of water present in organic solids, pharmaceutical substances and organic solvents have been devised over a length of time. But unquestionably the most important of these is the one proposed by Karl Fischer (1935), which is considered to be relatively specific for water*. It essentially makes use of the Karl Fischer reagent which is composed of iodine, sulphur dioxide, pyridine and methanol.
Note : Both pyridine and methanol should be anhydrous.
Water present in the analyte reacts with the Karl Fischer reagent in a two-stage process as shown below :
From Eq. (a) step l, it is obvious that the oxidation of sulphur dioxide takes place by iodine to yield sulphur trioxide and hydrogen iodide thereby consuming one mole of water. In other words, each one molecule of iodine disappears against each molecule of water present in the given sample. It is pertinent to mention here that in the presence of a large excess of pyridine (C5H5N), all reactants as well as the resulting products of reaction mostly exist as complexes as evident from Eqs. (a) and (b).
Stability of the Reagent : The stability of the original Karl Fischer reagent initially prepared with an excess of methanol was found to be fairly poor and hence, evidently needed frequent standardization. However, it was estabtished subsequently that the stability could be improved significantly by replacing the methanol by 2-methoxyethanol.
It has been observed that the titer of the Karl Fischer reagent, which stands at 3.5 mg of water per milliliter of reagent, falls rapidly upon standing with the passage of time. Hence, the following precautions must be observed rigidly using the Karl Fischer reagent, namely :
(a) Always prepare the reagent a day or two before it is to be used,
(b) Great care must be taken to prevent and check any possible contamination either of the reagent or the sample by atmospheric moisture,
(c) All glassware(s) must be thoroughly dried before use,
(d) Standard solution should be stored out of contact with air, and
(e) Essential to minimise contact between the atmosphere and the solution during the course of titration.
End-point Detection : The end-point of the Karl Fischer titration may be determined quite easily by adopting the electrometric technique employing the dead-stop end-point method. When a small quantum of e.m.f. is applied across two platinum electrodes immersed in the reaction mixture, a current shall tend to flow till free iodine exists, to remove hydrogen and ultimately depolarize the cathode. A situation will soon arise when practically all the traces of iodine have reacted completely thereby setting the current to almost zero or very close to zero or attain the end-point.
Limitations of Karl Fischer Titration : The Karl Fischer titration has a number of serious limitations due to possible interferences tantamount to erroneous results, namely :
(i) Oxidizing agents, for instance : chromates, Cu(II), Fe(III), Cr2O72–, peroxides, salts, higher oxides,
MnO2 + 4C5H5NH+ + 2I– → Mn2+ + 4C5H5N + I2 + H2O
(ii) Reducing agents, such as : Sn(II) salts, sulphides, and S2O32–, and
(iii) Compounds that have a tendency to form water with the ingredients of the Karl Fischer reagent, for instance :
(a) basic oxides : e.g., ZnO ;
Example : ZnO + 2C5H5NH+ → Zn2+ + C5H5N + H2O
(b) salts of weak oxy-acids e.g., NaHCO3 ;
Example : NaHCO3 + C5H5NH+ → Na+ + H2O + CO2 + C5H5N
Note : As H2CO3, carbonic acid, is very unstable ; hence it splits up to yield a mole each of water and CO2.