Lesson 12 of 13 · 9 min
Quantitative analysis: estimating elements
NCERT § "Quantitative Analysis"
Knowing that vinegar's acid contains carbon, hydrogen and oxygen is not enough. How much of each? Burn exactly 0.30 g of it, weigh what comes out, and the formula CH₃COOH follows from arithmetic.
The lesson in notes
In short
Carbon and hydrogen are estimated by burning a known mass in oxygen and weighing the CO₂ (absorbed in KOH) and water (absorbed in anhydrous CaCl₂) produced.
Dumas method: the compound is heated with copper(II) oxide in CO₂ atmosphere; the nitrogen gas released is collected over KOH solution and its volume measured.
Kjeldahl's method: the compound is heated with concentrated sulphuric acid so that its nitrogen becomes ammonium sulphate; the ammonia released with NaOH is absorbed in a known volume of standard acid and the excess acid is titrated.
Kjeldahl's method gives wrong results for nitro compounds, azo compounds and ring nitrogen (such as pyridine): digestion with sulphuric acid fails to turn that nitrogen into ammonium sulphate.
Carius method for halogens: the compound is heated with fuming nitric acid and silver nitrate in a sealed tube, and the silver halide formed is weighed.
Sulphur is estimated by heating with fuming nitric acid to form sulphuric acid, which is precipitated and weighed as barium sulphate.
Phosphorus is converted to phosphoric acid and weighed either as ammonium phosphomolybdate or as Mg₂P₂O₇.
Oxygen is usually found by difference: subtract the combined percentages of every other element from 100.