Standard Density of Water

To ensure that S eas is determined accurately, we calibrate the equipment or instrument used to obtain the signal. Balances are calibrated using standard weights. When necessary, we can also correct for the buoyancy of air. Volumetric glassware can be calibrated by measuring the mass of water contained or delivered and using the density of water to calculate the true volume. Most instruments have calibration standards suggested by the manufacturer. 

Density and Relative Density. Density is mass per unit volume and in SI is normally expressed as kilograms per cubic meter (density of water = 1000 kg/m or 1 g/cm ). The term specific gravity was formerly the accepted dimensionless value describing the ratio of the density of sohds and Hquids to the density of water at 4°C or for gases to the density of ak at standard conditions. The term specific gravity is being replaced by relative mass density, a more descriptive term. 

The density of a substance is the weight per unit volume of the substance. Density of a substance is also known as its specific gravity. The density of water is 1. It is used as the standard to find out the density of other substances. The density of sea water is 1.025. 

Density is defined as the mass of a unit volume of material at a specified temperature and has the dimensions of grams per cubic centimeter (a close approximation to grams per milliliter). Specific gravity is the ratio of the mass of a volume of the substance to the mass of the same volume of water and is dependent on two temperatures, those at which the masses of the sample and the water are measured. When the water temperature is 4°C (39°F), the specific gravity is equal to the density in the centimeter-gram-second (cgs) system, since the volume of 1 g of water at that temperature is, by definition, 1 ml. Thus the density of water, for example, varies with temperature, and its specific gravity at equal temperatures is always unity. The standard temperatures for a specific gravity in the petroleum industry in North America are 60/60°F (15.6/15.6°C). 

The variations in density of water from different sources, due to the variation in proportions of the isotopes of hydrogen and oxygen, exceed the experimental errors in accurate determination of density. The water taken as the standard substance should be ordinary tap water distilled once if laboratory distilled water is used, either with or without further distillation for purification, there is always a possibility that the istope ratio H D (the density of D2O at 4° is 1 1059) will have been disturbed. To 1 lit. of the once distilled tap water 0 5 g. pure sodium hydroxide and 0 2 g. pure potassium permanganate are added and the liquid is distilled in an all-Pyrex or tin-lined vessel with an arrangement to prevent carry-over of spray, the first and last third of the distillate being rejected. ... 

Specific Gravity of a Gas. Specific gravity is defined as the ratio of the density of a substance to the density of some standard substance. In the case of liquids and solids, water is usually taken as tlie standard reference material and its density taken at 20° C and one atmosphere. In en neering units the density of water is usually taken at 60° F and one atmosphere. To avoid ambiguity the temperature and pressure of the reference substance should always be specified. 

For gases the standard reference material is dry air and its density is taken at the same temperature and pressure for which the density of the gas is given. The distinction between specific gravity and density must he kept clearly in mind. In the metric system, the density of water is essentially equal to one. Consequently, specific gravities and densities have substantially the same numerical values in this system of units. However, this is not generally true. 

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