Question

Which of the following is determined by the oxidation of acidified potassium dichromate?

• A. DO
• B. BOD
• C. COD
• D. TLV

Answer 1

Answer: (C)

COD

The minimum level of toxic pollutants present in atmosphere which affects a person adversely when he is exposed to 7 to 8 hours in a day is called TLV.
BOD $=$ Number of mg of $O_2$ required / number of litres of water sample.
BOD $=$ Parts of oxygen / million parts of water sample.
Chemical oxygen demand (COD) test is commonly used to indirectly measure the amount of organic compounds in water.
In COD, Most commonly, a 0.25N solution of potassium dichromate is used for COD determination, although for samples with COD below 50 mg/L, a lower concentration of potassium dichromate is preferred.
The reaction of potassium dichromate with organic compounds is given by

$C_n{H}_a{O}_b{N}_c+dC{r}_2{O}_7^{2-}+(8d+c)H^{+}\to nC{O}_2+\frac{a+8d-3c}{2}{H}_{2}O+cN{H}_4^{+}+2dC{r}^{3+}$

where $d=\frac{2n}{3}+\frac{a}{6}-\frac{b}{6}-\frac{c}{2}$