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Gas Eudiometry

Gas eudiometry involves determining the composition of gaseous mixtures, molecular formula for gases and gaseous hydrocarbons, and establishing volume–volume relationship among gases.
The fundamental principle involved in gases eudiometry is the Avogadro’s law. According to this law, the volume of the gas is proportional to the number of moles of gas at constant temperature and pressure, i.e., the volume ratio among gases is same as the mole ratio at constant temperature and pressure.
In eudiometry, gaseous reactions are studied in a closed graduated tube open at one end and closed at other end. The tube is provided with platinum terminals required to produce electric spark through the mixture of gases. Such graduated tube is known as eudiometer tube, and hence the term gas eudiometry is also used for gas analysis. During eudiometry, the graduated tube is first filled with mercury. Then a known volume of the gas or gaseous mixture (which is to be studied either for molecular formula determination or composition determination) is inserted into the eudiometer tube, which displaces equivalent amount of mercury. Now a certain volume of O2 (in excess) is introduced and the electric spark is produced, when the combustible material gets oxidized. In general, all elements on oxidation with O2 form oxide with element in the highest oxidation state unless and until mentioned in the problem. The volume of gases before combustion and after combustion is noted. Soon after the combustion, the reaction mixture is brought to room temperature when H2O(g) condenses to H2O(l). This leads to contraction in volume. This contraction, called the first contraction in volume, is noted. The volumes of CO2, other gaseous products, and remaining O2 are determined by absorbing them in suitable reagents. For example, the volume of CO2 is determined by inserting KOH (or any other alkali) in the eudiometer tube, which absorbs all the CO2 present. The volume of remaining O2 is determined by introducing alkaline pyrogallol, which absorbs all the left over oxygen. There is contraction in volume noted by introducing KOH, which is referred to as the second contraction in volume, and contraction in volume by inserting alkaline pyrogallol is termed as the third contraction in volume. From the various contractions in volume obtained, the requisite information can be derived.
  1. During eudiometry, the reaction mixture is brought to the room conditions after combustion in all cases. This means that if H2O is the product, it will always exist in liquid phase.
  2. If reaction mixture contains nitrogen, it is not oxidized by O2 even in the presence of electric spark. This implies that in eudiometry, nitrogen is considered inert toward oxidation.
  3. The various reagents required for absorbing different gases are as follows:
    O2: absorbed by alkaline pyrogallol
    O3: absorbed by turpentine oil
    NO: absorbed by FeSO4 solution
    CO: absorbed by ammoniacal Cu2Cl2 solution
    H2O vapor: absorbed by conc. H2SO4, fused CaCl2 (required when reaction mixture is not brought to room temperature)
    Cl2: water or alkali solution
    NH3: acid solution or CuSO4 solution
    CO2 and SO2: alkali solution [NaOH, KOH, Ca(OH)2, Ba(OH)2, or ethanolamine (HOCH2CH2NH2)].

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