QUANTITATIVE ANALYSIS

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QUANTITATIVE ANALYSIS:This analysis gives the percentage composition of elements present in an organic compound.A known mass of an organic compound is burnt in the presence of excess of oxygen and copper II oxide.Carbon and hydrogen in the compound are oxidised to carbon dioxide and water respectively. The mass of water produced is determined by passing the mixture through a weighed U-tube containing anhydrous calcium chloride. Carbon dioxide is absorbed in another U-tube containing concentrated solution of potassium hydroxide. These tubes are connected in series. The increase in masses of calcium chloride and potassium hydroxide gives the amounts ofwater and carbon dioxide from which the percentages of carbon and hydrogen are calculated.Let the mass of organic compound be m g, mass of water and carbon dioxide produced be m1 and m2 g respectively;Percentage of carbon=12x m2 x100/ 44 x m.Percentage of hydrogen = 2 x m1 x 100/ 18 x m.
Nitrogen:There are two methods for estimation of nitrogen: 1. Dumas method and 2. Kjeldahl's method.
1. Dumas method:The nitrogen containing organic compound, when heated with copper oxide in an atmosphere of carbon dioxide, yields free nitrogen in addition to carbon dioxide and water.The mixture of gases so produced is collected over an aqueous solution of potassium hydroxide which absorbs carbon dioxide. Nitrogen is collected in the upper part of the graduated tube.Let the mass of organic compound = m g. Volume of nitrogen collected = V1 ml. Room temperature = T1K. Volume of nitrogen at STP= T1K. Volume of nitrogen at STP=p1 v1 x 273 / 760 x T1. Where p1and V1are the pressure and volume of nitrogen, value of p1 is obtained by the relation:p1= Atmospheric pressure - Aqueous tension. 22400 ml N2 at STP weighs 28 g Vml N2 at STP weighs =28 x V /22400 grams. Percentage of nitrogen =28 x 100/2240xM.
2. Kjeldahl's method:The compoundcontaining nitrogen is heated withconcentrated sulphuric acid. Nitrogen in thecompound gets converted to ammoniumsulphate. The resulting acidmixture is then heated with excess of sodium hydroxide.The amount of ammonia produced is determined by estimating the amount of sulphuric acid consumed in thereaction.It is done by estimating unreacted sulphuric acid left after the absorption of ammonia by titrating it with standard alkali solution. The difference between the initial amount of acid taken and that left after the reaction gives the amount of acid reacted with ammonia.Let the mass of organic compound taken = m g. Volume of H2SO4 of molarity, M,taken = V mL.Volume of NaOH of molarity, M, used for titration of excess of H2SO4 = V1 mL.V1mL of NaOH of molarity M = V1 /2 mL of H2SO4 of molarity M. Volume of H2SO4 of molarity M unused = V - V1/2mL.V- V1/2 mL of H2SO4 of molarity M = 2 V-V1/2 mL of NH3 solution of molarity M.