2. Gas Pycnometry
“Pycnometry”
• Is a Greek word “puknos” meana to measure the density and volume.
• It is Reliable and absolute method of measuring.
• Based on Boyle’s law (volume-pressure relationship).
• Regularly shaped, granular, powder, porous and non-porous samples can
be used.
• Gas pycnometers also called Helium pycnometers.
• A gas displacement method for measuring the volume.
• It can be performed in less than 20 minutes.
3. Gas Pycnometry
Principle of operation:
1. valve a opens and
2. Inert gas flows into the sample chamber
3. On seeing equilibrium is reached
4. valve b opens and
Gas flows into second
chamber for volume measurement
4. Again equilibrium is reached
5. Sample weight divided by Volume determines the density.
6. Then valve c is openend and pressure vented off to open atmosphere.
4. Gas Pycnometry
Kinds of pycnometers:
1. Constant-volume pycnometer
2. Variable-volume pycnometer
3. Comparative pycnometer
• Constant-volume pycnometer
1. Valves are opened, sample is placed into the sample chamber, and
pycnometer is filled with gas.
2. There is a absolute pressure transducer for measuring the initial gas
pressure in the pycnometer (Pi), while closing the valve “M”.
5. Gas Pycnometry
3. To separate the sample chamber from tank valve “Z” is closed and valve
“M”
is opened and some gas is sent into the tank.
4. Now valve “M” is closed and gas pressure in the
tank is measured (Pj); and
5. On opening the valve “Z” gas
will expand from the tank to the sample chamber (or vice versa), when
the gas expansion is completed, then final gas pressure is measured (Pf).
Vs= V ◦c + V◦t τ τ =Pf − Pj /Pf − Pi
7. Gas Pycnometry
• Variable-volume pycnometer
1. Now sample is placed in the sample chamber, and piston is
placed at an initial position
2. Opening the valve, and fill the pycnometer with gas.
3. Close the valve and measure the initial gas pressure in the
pycnometer (Pi) with the help of absolute pressure transducer.
4. For measuring the final pressure (Pf), the piston is moved to
another position.
Vs= V ◦c + V◦pτ τ = - [Pmin / Pmax – Pmin]
9. Gas Pycnometry
• Comparative pycnometer
1. The piston chamber and the volume controller are set such as their
internal volume is minimum, the sample is placed in the chamber,
open the valve, and fill the pycnometer with gas.
2. Close the valve so that there is no difference in the gas pressures in
the sample chamber and the tank.
3. Pulled the piston is to a maximum position.
4. And then adjust the volume controller, again to see that there is no
difference in the gas pressures of sample chamber and the tank.
10. Gas Pycnometry
5. The internal volume of the volume controller (Vr) can be determined.
Vs = Vc − (Vt /Vp)Vr
12. Gas Pycnometry
Advantages
• Simple method
• Relatively inexpensive instrument
• Fastness
• Less manpower necessary
• Adjustable gas flow rates
• Easy to read digital display
• Non-destructive technique
13. Gas Pycnometry
Disadvantages
• In some cases, they are expensive. (on industrial scale)
• Not suitable for expensive samples because large sample volumes are
required
• Difficult to clean and dry
Applications
• Used for characterizing catalysts, carbons, metal powders, soils,
ceramics, pharmaceutical ingredients, petroleum coke, cement,
cosmetics etc.
• Density of viscous materials can also be determined.
14. Gas Pycnometry
Article:- Use of gas pycnometry for estimating the iron content in mineral
samples.
• Hematite iron ore
• Helium gas pycnometry (The helium gas pycnometer is typically consists
of two chambers of known volumes that are connected by an expansion
valve. One chamber is used where sample is placed, and the other is
used where the gas expansion happens.)
• Vca = Sample chamber volume, Vce = Expansion chamber volume, and Vs =
Volume of the solid(sample).
15. Gas Pycnometry
• Only hematite (Fe2O3) and quartz (SiO2) were assumed to be present
in the sample, rest of components (especially Al2O3, CaO, K2O, and
MgO) represented less than 5% by weight of most of the studied
samples.
• Thus, the mass of hematite (mhe) plus the mass of quartz (msi) results
in the solid mass of the sample (ms) as follows:
mhe+msi = ms
• The total volume of the solid sample is a volumetric fraction ϕ
16. Gas Pycnometry
• Because the hematite mass fraction (fhe), which is fhe = mhe/ms
• m=ρ∗V so,
• specific gravities of the pure constituents (ρhe = 5.2 g/cm3 and
ρsi = 2.65 g/cm3) and the mean density of the sample obtained
experimentally by helium gas pycnometry.
• Fe(%)=0.70⋅fh⋅100
• The factor 0.70 shows the Fe fraction present in hematite ore.
17. Gas Pycnometry
Sample ρs (g/cm3)
Fe2O3 (%) –
Pic Fe (%) – Pic Fe (%) – CA
Absolute
difference
1 4.1916 75.0 52.5 48.6 −3.9
2 3.6079 54.1 37.9 34.1 −3.8
3 3.9019 65.4 45.8 43.90 −1.9
4 3.5671 52.4 36.7 33.8 −2.9
5 4.2244 76.0 53.2 50.4 −2.8
6 4.3911 80.9 56.6 53.90 −2.7
7 3.0446 26.4 18.5 17.20 −1.3
8 2.9473 20.6 14.4 12.2 −2.2
9 3.0495 26.7 18.7 16.2 −2.5
10 2.7303 6.0 4.2 2.49 −1.7
Table 1.
Comparison of the Fe contents estimated by pycnometry [Fe(%) – Pic] and those obtained by chemical analysis [Fe(%) – CA].
