ktcd - 23-1-2013 at 03:39
I don't understand... I read they don't want red P cause it reduces I2 to HI. But can't I2 just react with metal and that react with HCl? Below is
iodine left on steel knife
woelen - 23-1-2013 at 04:30
It all depends on the amount of water allowed in the process in which the HI is used. A mix of red phosphorus and iodine can produce very concentrated
HI by adding just some drops of water. Iodine mixed with a metal powder and then acidified with hydrochloric acid gives a dilute solution of HI at
best, which also is mixed with chloride and metal ions.
Nicodem - 23-1-2013 at 08:03
Actually, the addition of HCl(aq) would not change much the state of the iodide anions. The difference in acidity of HCl and HI is of 4 magnitudes and
the pKa of HI is about -11 anyway, which means it is completely dissociated already (except maybe at very high concentrations where there is not
enough water for proper solvation of ions). Acidification by HCl of an aqueous iodide solution would therefore not change anything, except for the pH
of the solution and its chloride content. But if one needs an acidic solution of a iodide, then mixing NaI and HCl would give a "cleaner" result
(application-wise), when compared to transition metal iodides. Needless to say, that would not resemble conc. aqueous HI (application-wise), but might
resemble a diluted aqueous HI for some applications.
woelen - 23-1-2013 at 12:01
Mixing a concentrated solution of NaI and conc. HCl has the added advantage of precipitating NaCl from the solution. NaCl only is sparingly soluble in
solutions of HCl, especially if the concentration of the HCl goes well above 20% by weight.