Gasometric assays

-Depends upon two type of operational concepts:

i)measurement of the volume of a gas liberated

under the conditions that are described in the

assay.

ii)Decrease in volume of a gas when a suitable

reagent is used to remove one of the gases

present.

-Apparatus: gas burette:

-These are provided with two(or three)stopcocks & two –way outlet.

-Properly connected with a balancing tube.

Assay of carbon dioxide:

-Limit: Minimum 99.5 per cent V/V of CO2 in the gaseous phase.

-The sample is absorbed in 50% potassium hydroxide & the volume of the residual gas is measured.

-It is carried out by the use of hempel apparatus:

-It is composed of burette & absorption pipette.

-Hempel burette is fitted with three-way tap.

-Measuring tube & leveling tubes are connected by a rubber tube(about 3 feet).

-The three way tap is turned so as to connect the measuring tube to gas air.

-The top tap is also opened.

-A stream of the gas from the supply is passed through the measuring tube for about two minutes.

-Both taps are closed.

-The top of the measuring tube is connected to the absorption pipette(via a bent capillary & rubber tubes).

-The lower bulb of the pipette is

filled with 50%w/v solution of

potassium hydroxide.

-The upper bulb is partly filled.

-The connection is made such a way

that the connecting tube becomes

completely filled with the liquid.

-The top tap of the burette is opened.

-The leveling tube is(filled with

water) raised with so as to drive

the whole of the gas into the lower

bulb of the pipette.

-Gentle rocking of the pipette is

carried out(2-3 minutes).

-It ensures complete absorption of

carbon dioxide.

-Then the leveling tube is lowered.

-The small volume of the residual

gas is drawn back into the burette.

-The tap is turned off as soon as the

capillary has become filled with

liquid.

-The levels in the two burette are

adjusted & the volume of

unabsorbed gas is read.

Assay of oxygen:

-Limit: not less than 99.0 per cent v/v of O2.

-The gas sample is exposed to the action of an ammonical copper solution/alkaline pyrogallol.

-The solution reacts with oxygen.

-The residual gas volume is a measure of the impurities present.(Should not more than 1% of the original).

4NH4Cl+8CuNH3Cl+8NH3+2H2O+O2 = cuprous amimno-chloride 4CuCl,CuCl2,4NH3+4NH4OH.

-Either Hempel or Franke burette can be used for measurement.

-For rapid assay Franke burette is used.

-The dry burette is filled with the gas by passing a stream through for two minutes as appropriate.

-Then, taps are closed.

-The plug is removed & receiver is

filled with ammonical copper

solution/pyrogallol(alkaline).

-The plug is replaced & the tap is

closed.

-By central tap the solution is

allowed to flow into the burette.

-The burette is gently shaken to

ensure the complete absorption of

oxygen.

-The burette is then inverted & the

central is opened.

-All the absorbent is flowed back

into the receiver.

-The central tap is closed.

-The burette is turned right way up.

-The burette is lowered into a

cylinder of water.

-The bottom tap is opened.

-Water is adjusted to the same level

inside & outside the burette.

-The volume of unabsorbed gas is

read on the scale.

Assay of cyclopropane:Limit: not less than 99.0 per cent v/v of .

-The sample is treated with Conc. sulphuric acid in a mercury-charged nitrometer.

-Cyclopropane is absorbed by ring fission & formation of propyl hydrogen sulphate.

CH2

| CH2 + H2SO4→ CH3.CH2 .CH2.HSO4 CH2