Updated: Question in GATE 2012 Civil Engineering Paper with Solution

A question in GATE 2012 Civil Engineering Paper with Solution has been updated:

A sample of domestic sewage is digested with silver sulphate, sulphuric acid, potassium dischromate and mercuric sulphate in chemical oxygen demand (COD) test. The digested sample is then titrated with standard ferrous ammonium sulphate (FAS) to determine the un-reacted amount of

1. Sulphuric acid
2. Mercuric sulphate
3. Silver sulphate
4. Potassium dichromate