Estimation and Determination

Determination of Molecular Mass of Acids by Silver Salt Method

Determination of Molecular Mass of Acids by Silver Salt Method

Determination of Molecular Weight of a Base by H₂PtCl₆ Method

Determination of Molecular Mass of Base by Chloroplatinate Method

Detection of Nitrogen

Sodalime Test

The given substance is mixed with double the amount of sodalime and heated in a test tube. The vapour of ammonia evolved show the presence of nitrogen.

Sodium Test (Lassaigne's Test)

This is a golden test for the detection of nitrogen in all classes of nitrogenous compounds. It involves the following steps:
The substance is heated strongly with Na metal
            Na + C + N → NaCN
The water extract of the fused mass is boiled with ferrous sulphate solution.
            FeSO₄ + 2NaOH → Fe(OH)₂ + Na₂SO₄
            6NaCN + Fe(OH)₂ → Na₄[Fe(CN)₆] + 2NaOH
To the cooled solution is then added a little ferric chloride solution and excess of concentrated HCl.
            Na₄[Fe(CN)₆] + 4FeCl₃ → Fe₄[Fe(CN)₆]₃ + 12NaCl
                                                        prussian blue
The formation of prussian blue or green coloration confirms the presence of nitrogen in given organic compound.

Detection of Halogens:

Sodium Test:

Upon fussion with sodium, the halogens in the organic compound are converted to the corresponding sodium halides. Thus,
          Cl + Na → NaCl
          Br + Na → NaBr
          I + Na → NaI
Acidify a portion of sodium extract with dilute HNO₃ and add to it AgNO₃ solution.
White ppt. soluble in ammonia indicates Chlorine
Yellowish ppt. sparingly soluble in ammonia indicates Bromine
Yellow ppt. insoluble in ammonia indicates Iodine.

Detection of Sulphur

Sodalime Test:

If sulphur is present in the given organic compound, upon fussion with Na reacts to form sodium sulphide.
          2Na + S → Na₂S
Thus, the sodium extract obtained from the fused mass may be tested as:
A. To a portion, add freshly prepared sodium nitropruside solution. A deep violet coloration indicates the presence of sulphur.
B. Acidify a second portion of the extract with acetic acid and then add lead acetate solution. A black ppt. of lead sulphide confirm the presence of sulphur.
          Pb(CH₃COO)₂ + N₂S → PbS + 2CH₃COONa
            Lead Acetate               Black ppt.

Detection of Phosphorus

The solid substance is heated with an oxidizing agent such as concentrated nitric acid or a mixture of sodium carbonate and potassium nitrate. The phosphorus present in the substance is thus oxidized to phosphate. the residue is extracted with water, boiled with some nitric acid and then a hot solution of ammonium molybdate is added to it in excess. A yellow coloration or ppt. indiccates the presence of phosphurs.

Estimation of Nitrogen

Kjeldahl Method

This method is based on the fact that when an organic compound containing nitrogen is heated with concentrated sulphuric acid, the nitrogen in it is quantitatively converted into ammonium sulphate. The resultant liquid is then treated with excess of alkali and the liberated ammonia gas absorbed in excess of standard acid. The amount of ammonia is determined by finding the ammount of acid neutralized by back titration with some statndard alkali.
Calculation
Let the weight of organic substance be x gm.
V volume of N HCl is required for complete neutralization of NH₃ evolved.
V ml. N HCl ≡ V ml. N NH₃
1000ml N NH₃ contains 17 gm of NH₃ or 14 gm N
Ammount of N present in V mol of N NH₃ = (14/1000). V . N gm
% of nitrogen = wt. of nitrogen (100/wt. of substance)
                       = (14/1000).V.N.(100/x) = 1.4VN/x
where: N = Normality and V = Volume of the acid used.

Estimation of Sulphur

Carius Method

Known weight of organic compound is heated with fuming nitric acid in carius tube, sulphur present in organic compound oxidized to sulphuric acid. The whole solution is then transfered in a beaker and BaCl₂ solution in excess is added to it for complete ppt. The ppt. of BaSO₄ is then washed, dried and ignited to obtain the constant wt. of BaSO₄. Now by knowing the weight of organic compound and BaSO₄, the percentage of sulphur in the organic compound is easily calculated.
          Organic Compound + HNO₃ → CO₂ + H₂ + H₂SO₄
          H₂SO₄ + BaCl₂ → BaSO₄ + 2HCl
Let wt. of an organic compound = a gm
wt. of BaSO₄ = b gm

Calculation:
molecular weight of BaSO₄ = 233
∵ 233 gm BaSO₄ contains 32 gm S.
∴ b gm BaSO₄ contains (32 X b)/233 gm S.
Again,
∵ a gm of organic compound contains (32 X b)/233 gm S.
∴ 100 gm of organic compound contains (32 X b X 100)/233 X a gm S.
∴ % of Sulphur in organic compound = (32 X b X 100)/233 X a