Volume of liquids
The volume of a liquid is measured in liters. The liter has had an unfortunate history. The liter is 1000 cubic centimeters (cm ‘) and, when the standard platinum-iridium . kilogram was constructed in 1889, it was intended to be the mass of I liter of pure water at the temperature of its maximum density, 4 “C. The liter was then officially defined as the volume of I kg of pure water at 4 “C. This is where the trouble began. In 1907, careful experiments showed that a slight error had been made in constructing the standard kilogram. It was found to be the mass of 1000.028 cm ‘ of water. Consequently, as defined above.
I liter = 1000.028 cm
At the time it was decided to leave the matter as it stood and to divide the liter in 1000 equal parts called milliliter (ml) so that, I cm ‘ = 0.999972 ml Scientists thereupon began to use buttresses and other liquid-measuring vessels calibrated in ml instead of cm. At the present time we are almost back where we started. In 1964. the General Conference of Weights and Measures redefined the liter as equal to 1000 cm ‘. This means that the liter is, by its new definition, related directly to the meter and not the kilogram.
Fig. 1.10 shows a selection of graduated vessels in common use. The measuring cylinder is for measuring or pouring out various volumes of liquid; the measuring flask and the pipette for getting fixed ore-determined volumes. The Burnett delivers an required volume up to its total capacity, usually 50 cm-, and is long and thin to increase its sensitivity. Burnett divisions generally represent 0.1 cm ‘, but measuring cylinders may be graduated at I, 5 or 10 cm ‘ intervals according to size. Readings on all these instruments are always taken at the level of the bottom of the meniscus or curved surface of the liquid. Mercury is an exception, as its meniscus curves DE inwards. Care should be taken to place the eye correctly so as to avoid
parallax errors (Fig. 1.11). When taking readings, the pipette and Burnett must be upright and the cylinder and flask must stand on a horizontal bench, otherwise errors may arise from tilting.