Determined, with pyknometer

In density determination the volume of fluid, displaced by a known weight of powder, is determined. Since weight can be measured accurately, the problem is that of accurate determination of volume. With pyknometers (or density bottles) the fluid is a liquid, usually water with surfactant, unless the powder is water miscible. With gas pyknometers the fluid is usually dry air or helium. 

Determination of Specific Gravity.—A simple method for determining the specific gravity of liquids is as follows I. pyknometer, or small glass bottle, is used of about 20 to 30 c.c. capacity, with narrow neck, upon which a mark is etched and which is closed by a ground glass stopper (Fig. 45). 

The densities of polymers can be determined by the pyknometer technique or by the flotation method. In the pyknometer technique the liquid volume displaced by the polymer sample is determined by weighing. Most polymers have a density larger than that of water, which can, therefore, be used as the liquid. Polymers in the form of powders or pressed discs tend to adsorb or occlude air bubbles, which can lead to serious errors. This can be largely prevented by careful degassing of the pyknometer and polymer sample under vacuum before filling with liquid, and/or by addition of a small amount (0.1%) of commercial detergent to lower the surface tension of the water. 

The density of the liquid mixture can be determined using a pyknometer or it can be derived from a previously determined calibration curve. For the determination of densities less than unity (e.g., polyethylene), ethanol/water mixtures are suitable for densities larger than unity one may use mixtures of water with aqueous salt solutions (40% CaCl2 solution = 1.40 g/ml 72% ZnCl2 solution = 1.95 g/ml). The density gradient method, which is an elegant var-... 

Quoted density values in standard reference works are of the materials true density. If density is determined using a gas pyknometer, the volume measured would include closed pores but exclude open pores i.e. the measured density would be the apparent density. If the suspending liquid penetrates all the cracks and fissures on the particle surface, the measured volume would be the same as that determined by gas pyknometry but the total mass would be greater due to the included liquid that will remain with the particle as it falls in the liquid, hence its sedimentation density will be intermediate between the apparent density and the true density and greater than the effective density. These differences are usually not highly significant for coarse particles unless they are highly porous. 

As it is not always possible to find known powders of the same shape as the test powder, Abrahamsen and Geldart11 introduced an experimentally-determined factor k which varied from 0.82 to 1.22 for conversion from spherical to angular or the other way round. They tested the method using non-porous powders and compared it with liquid pyknometer results, and found an excellent agreement. 

Volume shrinkage. The volume shrinkage was determined by density Increase. Density measurements were done with a pyknometer for liquid and a density scale for solid state. The volume shrinkage is 4.5Z in case of the unfilled resin and decreases according to the decreasing volume portion of the reactive part of the mixture. The values are included in Table I. 

The simplest instrument for measuring liquid density is the hydrometer, a float with a graduated stem. To approach reasonable accuracy, however, it is essential to make the measurement at the particular calibration temperature marked on the hydrometer. Densities may be determined more accurately by the specific gravity bottle method, or with a pyknometer (BS 733, 1983) or Westphal balanee, details of which may be found in most textbooks of practical physics. 

The aggregates are then removed and the pyknometer is refilled with water, dried on the outside and weighed with its cover (M3). The temperature of water is again recorded. The two temperatures during determining mass M2 and M3 shall not differ by more than 2°C. 

According to CEN EN 1097-7 (2008), a small amount of dry filler, namely, 10 1 g, is placed in the 50 ml pyknometer along with a suitable liquid. The suitability of liquid is determined by its ability to detach fine grains. Liquids such as water, denatured ethanol, redistilled kerosene or toluene have been found to be suitable for many types of fillers. Significant attention during the test should be paid to the determination of the volume of the pyknometer with liquid, as well as to the absence of enclosed air in the mass of filler located inside the pyknometer. 

The degree of crystallinity was estimated by a flotation method. Water and tert-butanol were used for sPS as a mixed solvent. In the case of sPPMS, solvents with large molar volume such as 3-ethyl-3-pentanol and dimeth)rsulfoxide (DMSO) were used as a mixed solvent. We confirmed by infrared spectroscopy that these solvents were not absorbed in the pol3rmer films. The density was determined by a Lipkin-t)rpe pyknometer at 25 °C. The measurements were carried out three times and the average value was taken to estimate the crystallinity. The following equation was used to determine the crystallinity (xc). 

The gas comparison pyknometer is used for skeletal density measurement and it is found that, when used with air, erroneous results are obtained and sometimes even negative volumes are found. This indicates that air is being adsorbed and the amount adsorbed can be determined by comparison with helium density measurements. As nitrogen and oxygen, at room temperature, obey Henry s law the... 

This rate can be controlled with the help pinch cock attached to a rubber tube at the upper end of the stalagmometer. The density of the liquid is separately determined by means of a pyknometer. The surface tension is calculated from equation (5). 

After having determined the weight of the pyknometer filled with water, the pyknometer is once more dried out and filled w ith the liquid the density of which is to be determined. It is placed as before in the bath at constant temperature, the liquid is adjusted to the mark, the pyknometer dried with a cloth in the same manner as before, and weighed. 

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