Supporting Information

 

                                           Helium Pycnometer

      Here the name "helium pycnometer" appeared first time ever in a scientific journal.

 

 

"Air and Helium Pycnometer”, Powder Technology, 3 , No. 3, 179-180, 1969.  By Edward Yun Ho Keng

 

 

                               Air and Helium Pycnometer

 

The volume and density of powders and porous and irregularly shaped solids can be obtained rapidly and accurately by means of a newly designed gaseous pycnometer. The pycnometer has been successfully used with either air or helium. Air is employed for inert low-surface materials on which negligible adsorption of gas occurs. For fine powders or porous materials, helium, an inert gas, is recommended if a precise measurement is to be attained. Helium must be used with activated samples or those having a high surface area.

 

Apparatus and procedure

 

The design of the pycnometer is shown schematically in Fig. 1. The sample holder permits rapid sample addition and removal. The four-way valve permits the system to be connected to a vacuum pump, a helium source, the atmosphere or the system may be isolated off for measurements.

 

The most critical part of the pycnometer is the pressure detector. It consists essentially of a metal bellows and a signal light which indicates when the bellows is at its final position. The bellows is provided with stops in both directions so that evacuation or moderate overpressuring will not damage it. The pressure in the bellows can be arbitrarily set, but pressures between 400 and

500 mm

Hg were found to give the most satisfactory results. The bellows rests on a stop until the pressure in the sample holder is reduced to a preset value. The bellows then expands and makes electrical contact, turning on the signal light. The precise point at which contact is made should be carefully determined with the hand wheel. The pressure detector described will detect changes within the sample holder of less than

0.1mm

Hg.

 

Samples should be dried before measurements are begun. If the sample is out-gassed, weighing is best performed immediately after volume measurement. To operate the pycnometer, the piston is withdrawn from a pre-set position. The final position of the piston should be that at which the signal light is barely on. The difference between the final readings of the counter dial with and without a sample is directly proportional to the sample volume. Temperature changes can affect the validity of the measurements, however, since only about 2-5 min is required for a determination, this correction is negligible. The difference between the final and initial readings, after multiplying by a volume parameter, is the true volume of the sample. The volume parameter can be obtained simply by measuring a standard of known volume. A stainless steel ball of known volume is suitable.

Theory of operation

 

When the gas pressure in the pycnometer system is reduced from an initial pressure P1 to a final pressure P2, the gas in the system expands from the initial volume V1 to a volume of V2 as is shown by Fig. 2(a). If a sample of volume Vs is now placed in the system, the initial gas volume is reduced to V1s. When P1 is now reduced to P2 with the sample in the system, the volume of gas expands from V1s to V2s as is shown in Fig. 2(b). At constant temperature and relatively low pressure, the perfect gas law is adequate and the change in volume △Vs due to the presence of the sample volume Vs is

• Vs = (  P1-P2  )/P2  x Vs           (1)

   

 

If the initial pressure P1 and the final pressure P2 are known, the volume of the sample, Vs, can be calculated from a measurement of △Vs. Experimentally, it is simpler to obtain Vs by a volume parameter σ which is expressed by

 

σ =  Vs  = (    P2    )    △Vs     (2)

      △M       P1-P2      △M

 

Where △M  is the difference between the final dial readings with and without a sample. The value of σ is readily determined by measuring an object of known volume such as a steel ball. Once the value of σ is obtained, the volume of an unknown sample can be calculated simply by

 

                       Vs =σ△M

 

Results and discussion

 

Since helium is essentially nonadsorbing on most solids at room temperature, results with helium are usually reliable if previously adsorbed gases are removed. If the outgassing is not sufficient or the system is not air-tight, the signal light will not remain on at any one dial setting. If unstable operation of the signal light is noted when air is used, it usually indicates that the material is not suitable for use with air. Helium should be used if this condition exists.

 

Material                                      Handbook                                    Densities measured with

                                                     density                                            ————————————

                                                     (g/㎝³)                                                      air          helium

                                                                                                                   (g/㎝³)        (g/㎝³)

_____________________________________________________________________

Clay                                           1.8-2.6                                                     2.58          2.58

Salt                                               2.16                                                      2.15          2.15

 

Galena

                                                   7.3-7.6                                                    7.45          7.41

Amorphous Silica*                 2.1-2.3                                                    2.42          2.11

Amorphous Silica**               2.1-2.3                                                    2.85          2.10

Carbon Black                          1.8-1.9                                                     >10          1.85

Activated Charcoal                 2.1-2.3                                                     >10          2.28

_____________________________________________________________________

*   Surface area = 54.7 ㎡/g

**  Surface area = 295.2 ㎡/g

 

 

Typical densities determined with both air and helium are presented in Table 1. A comparison of results for air and helium indicates that air is acceptable for inert and low specific surface area materials such as clay, salt, and galena. With materials of high specific surface area such as the amorphous silicas, the results obtained with air are somewhat higher than their actual values due to adsorption. For carbonaceous materials such as carbon black and activated charcoal the results obtained with air are always much greater than the true values. With the pycnometer describe here, however, helium gives results that compare favorably with accepted density values for all these materials. It should be noted, however, that for some higher density carbon, a correction must be made for adsorption and surface effect.

 

 E. Y. H. KENG

 Georgia Institute of Technology,

 Atlanta, Ga. ( U.S.A.)  

 Received July 20, 1969