PYCNOMETRY ( SPECIFIC GRAVITY BOTTLES):- A pycnometer is a vessel with a precisely known volume. Although a pycnometer is used to determine density ρ or specific gravity, it measures volume V; a balance is used to determine mass m. Manual pycnometers (glassware) typically are used to determine the density or specific gravity of liquids by filling the vessel, then weighing. Density is calculated by ρ = m/V and specific gravity by the same equation and dividing both sides by the density of water with reference to temperature. First the object containing the void is weighed empty. It is then filled with a liquid of known density and reweighed. The weight difference ∆m is the weight of the liquid and from these data, volume can be calculated by V = ∆m/ρ. As will be explained, this process is used to ‘calibrate’ sample cells used in mercury porosimetry. Another pycnometer method is to place a quantity of a dry, pre-weighed solid sample in the pycnometer and fill the rest of the pycnometer with a liquid of known density (typically water), the weight of the pycnometer filled only with the liquid having previously been established. The density of the sample can be determined from the known density of the water, the weight of the pycnometer filled only with the liquid, the weight of the pycnometer containing both sample and liquid, and the weight of the sample. This is a common method used in characterizing soil samples.
HYDROSTATIC WEIGHING ( DISPLACEMENT METHOD):- By this method, the volume of a solid sample is determined by comparing the weight of the sample in air to the weight of the sample immersed in a liquid of known density. The volume of the sample is equal to the difference in the two weights divided by the density of the liquid. Conversely, if the volume of a solid object is accurately known, the density of the liquid can be determined by the loss of weight of the immersed object.This is the basis for the hydrometer method. If the sample is porous, one must determine if the pores are to be included or excluded from the volume. If they are to be included or the sample will react with the displacement medium, a sealing coating can be applied. If pore volume is to be excluded, the liquid must displace the air and completely fill the pores. Various pretreatment methods are used including evacuation and boiling. When determining volume by directly measuring the displaced volume, liquids, fine particles or gases can be used as the displacement medium. If the sample material is porous, fine particles will not penetrate into the smaller pores that water can enter. Mercury, being a non-wetting liquid, also will not penetrate pores under ambient pressure as will wetting liquids. Gases, Helium in particular, will penetrate readily into very fine pores.
HYDROMETERS:- A hydrometer is a vertical float that measures the density or specific gravity of a liquid or liquid/solid suspension (slurry). The hydrometer, inscribed with a graduated scale along its length, sinks into the liquid until it has displaced a volume of liquid equal in weight to that of the float. Specific gravity or density is read directly from the inscribed scale at the liquid surface after buoyancy and gravitational forces equalize.
FLOAT-SINK OR SUSPENSION (BUOYANCY) METHOD:-This method requires a liquid of known and adjustable density in which the sample is placed. The density of the liquid is adjusted until the sample either begins to sink or float , or is suspended at neutral density in the liquid.The density of the object is then equated to that of the liquid. This method also is used to separate materials by their density.
DENSITY GRADIENT COLUMN:- A density gradient column is a column of liquid that varies in density with height. A sample is placed in the liquid and observed to determine at what vertical level in the column the sample is suspended. The density of the liquid at that level is the density of the sample, and that value is determined by standards of known density.
TAP DENSITY AND VIBRATORY PACKING DENSITY:- These are very similar methods for determining the bulk density of a collection of particles under specific conditions of packing. In the former case, packing is achieved by tapping the container and in the latter by vibrating the container. The particles under test should not break up under test conditions.
BULK/ENVELOPE VOLUME BY COATING:- Coating the sample allows determination of bulk volume or apparent volume of solids while preventing absorption or reaction with suspension liquids. Penetration of the coating into the open pores of the sample must be considered. Following the referenced method the mass of the sample is obtained. The sample is dipped into molten wax of known density. After withdrawal, any air bubbles in the wax coating are pressed out, and the coated sample is weighed. The difference in weight before and after coating is the weight of the wax, and dividing this number by the density of the wax provides the volume of wax composing the coating. The volume of the coated sample is determined by hydrostatic weighing. From this volume, the volume of wax (or other coating) is subtracted, yielding the bulk (or envelope) volume of the sample.
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