Name: Qualitative Analysis of Organic Compounds
Uploaded: 2019-07-19
Duration: 18 min 19 s
Description: Can one learn about the elements present in any compound? Let's discuss a method citing examples of carbon and hydrogen by which the identification can be made.

Question

Describe Carius method for the estimation of Phosphorus.

Accepted Answer

Carius method for the estimation of phosphorus:

An organic compound is heated with fuming nitric acid in Carius tube. Phosphorus present in the organic compound is oxidized to phosphoric acid $(H_{3} {PO}_{4})$ . It is precipitated as magnesium ammonium phosphate by the addition of magnesia mixture $({MgCl}_{2} + {NH}_{4} Cl + {NH}_{4} OH)$ . The precipitate is ignited to obtain magnesium pyrophosphate.

$\underset{Magnesium ammonium phosphate}{2 {MgNH}_{4} {PO}_{4}} \to \underset{Magnesium pyrophosphate}{{Mg}_{2} P_{2} O_{7}} + 2 {NH}_{3} + H_{2} O$

Magnesium pyrophosphate is weighed and the percentage of phosphorus is calculated.

Calculations:

Weight of organic compound taken = $w$ g

Weight of magnesium pyrophosphate = $x$ g

Molecular weight of magnesium pyrophosphate $= 24 \times 2 + 31 \times 2 + 16 \times 7 = 222$

Hence, $222 g$ of magnesium pyrophosphate contains phosphorus $= 62 g$

$x$ g of magnesium pyrophosphate contains phosphorus $= \frac{62 x}{222} g$

Thus, $w$ g of the compound contains phosphorus $= \frac{62 x}{222} g$

$100 g$ of the compound contains phosphorus $= \frac{62}{222} \times \frac{x}{w} \times 100$ = percentage of phosphorus.

Describe Carius method for the estimation of Phosphorus.

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