Measurement of gas volume contributes to studies on reaction rates and commercial viability of chemical and biochemical processes in which a gas Is a by-product. Additionally measurement of volumes of vapours are also required for determination of molecular weights of volatile liquids by Victor – Meyer method.
Measurement of volumes of liquids in comparison is simpler as a liquid can be measured using a graduated cylinder, a volumetric flask, burette, pipette or a graduated vial. This is possible because liquids acquire the shape of the measuring vessel and the meniscus level can be noted accurately without difficulty. Liquid volumes are influenced insignificantly by changes in pressure but temperature changes have a significant effect so It becomes essential to specify the temperature of measurement.
The volume measurements of gases on the other hand are not as simple as majority of gases are invisible and their volumes depend significantly on both temperature and pressure. Volume of gas reported without specifying both these parameters carries no meaning.
Measurement of gas volumes
The simplest method Is displacement of water inside an Inverted graduated measuring cylinder or graduated test tube placed inside a water bath. This method Is suitable for gases such as Hydrogen or Nitrogen but does not work for water soluble gases such as ammonia or chlorine.
Syringe Barrel Displacement
The liberated gas in a lab-scale reaction Is led to a syringe connected to the reaction vessel. Due to an increase in pressure the gas begins to displace the plunger of the gas tight syringe. On completion of the reaction the syringe barrel gets displaced to Its final position and the volume of liberated gas can be noted. Correction is applied for temperature and pressure which are conventionally reported under NTP(Normal temperature and pressure conditions,ie, 20 degrees centigrade and 1 atmosphere pressure).
An evacuated volumetric flask or bulb is weighed on an accurate balance and then filled with the gas. The weight is recorded again accurately to 4 decimal place or more and the temperature also recorded. The density is calculated by dividing the weight of gas by volume of measuring vessel. The volume can be converted to the Normal Temperature and Pressure conditions using the standard conversion equations
Volume from flow rate measurements
At times you may be required to record the volumetric flow rate of gases. This can be achieved by making use of commercially available gas flow meters. The flow rates can be converted to the flow volumes by recording the temperature and pressure.