1. Lecture- 17
Subject – Pharmaceutical Analysis-I
Course – B. Pharm 1st Year
Faculty Name – Umesh Kumar
2. Dichrometry
These are titrations in which, potassium dichromate is
used as in oxidising agent in acidic medium.
The medium is maintained acidic by the use of dilute
sulphuric acid.
The solution of potassium dichromate can be directly
used for titrations.
It is mainly used for the estimation of ferrous salts and
iodides.
3. Conti………
The half reaction for the dichromate system is:
7Cr2O 2- + 14H+ + 6e- → 2Cr3+ + 7H2O
4. Conti……
dichromate titrations require an
indicator.
There are three indicators that may be
used for the titration of Fe2+ with
K2Cr2O7.
These are diphenylamine,
diphenylbenzidine and
diphenylamine sulfonate.
The colour change for all three
indicators is green to violet.
5. Advantages of Potassium Dichromate (K2Cr2O7) as
powerful oxidizing agent;
◦ It is available in pure form and stable form.
◦ The standard solution of exactly known concentration of
K2Cr2O7 can be prepared by weighing out the pure dry salt and
dissolving it in proper volume of water.
◦ Aqueous solution is sufficiently stable on proper storage.
◦ It can be used in acid as well as alkaline solution.
◦ It is stable towards light.
6. Principle
Ferrous ammonium sulphate (FAS) or Mohr’s salt is a
stable double salt with FeSO4 as an active constituent.
Acidic potassium dichromate solution is a strong
oxidizing agent and is rapidly reduced by ferrous ion at
the ordinary temperature to green chromic acid when
added to FAS solution containing dilute sulphuric acid.
In this reaction ferrous sulphate is oxidized to ferric
sulphate while ammonium sulphate remains unreacted.
N-phenyl anthranilic acid is used as an indicator.
7. Conti………
Indicator is not oxidised as long as Fe+2 ions are there in
the solution.
The slight excess amount of dichromate will oxidize the
indicator when all the Fe+2 ions have been converted to
Fe+3 ions resulting in colour change of the solution from
green to purple.
