Pycnometer

Pycnometer, an instrument for determining the density of liquids and solids, consisting of a small glass vessel with a narrow tube, fitted with a mark up to which the liquid is poured. There are P.'s of various authors (Mendeleyev, Sprengel, etc.), differing from each other in shape, presence of a thermometer, and other details. When determining density, one first weighs the empty P. (weight = P), then filled with water to the mark (weight = P₁), and (after drying) filled with the liquid being tested to the same mark (weight = P₂). If corrections are neglected, the density d equals P₂ - P₁ / P₁ - P. In such measurements, the weights are reduced to vacuum and the difference in temperature of the liquid being tested and water is taken into account. The determination of the density of solids is somewhat more complex (see figure). In this case, the density d equals (P₃ - P₂) / (P₁ - P₃), where Pₓ is the weight of the P. filled with water to the mark, P₂ is the weight of the substance being tested, and P₃ is the weight of the P. containing the substance being tested and the remaining volume of water (the water displaced by the substance is removed so that its level is at the mark). The determination of the density of a solid body becomes complicated when the substance being tested dissolves in water (another liquid is used, the density of which is also determined separately) or is lighter than water (a piece of substance heavier than water is attached). Due to the accuracy of the results obtained with the P., its relative portability, and simplicity, the P. is used in physical and chemical laboratories.