A summary, though not comprehensive, of the common laboratory measurements that can be performed to supplement information obtained by another analytical procedure is provided in this section. Many of the methods can be used in the field or in process control apparatus as well
as in the laboratory.
Some physical measurements that do not require instrumentation other than an accurate balance can be useful in selected circumstances. Density, specific gravity, viscosity, and pH measurements are among the more useful measurements in this category.
This property is defined as the ratio of mass to volume of a substance. Generally the mass is measured in grams and the volume in millilitres or cubic centimetres. Density measurements of liquids are straightforward and sometimes can aid in identifying pure substances or mixtures that contain two or three known components; they are most useful in assays of simple mixtures whose components differ significantly in their individual densities. Densities can be used, for example, as an aid in the quantitative analysis of aqueous sugar solutions. Liquid densities usually are measured by using a calibrated glass vessel called a pycnometer, which typically has a volume of about 10 millilitres. The vessel is weighed by using an analytical balance with an accuracy of at least 0.0001 gram and is subsequently filled to the calibration mark with the liquid. After the filled vessel has been weighed, the mass of the liquid is determined by subtracting the mass of the empty vessel. The density is calculated by dividing the mass of the liquid by the volume of the pycnometer.
Specific gravity is a related quantity that is defined as the ratio of the density of the analyte to the density of water at a specified temperature. The procedure used to measure specific gravity is similar to that used to measure density, although it does not require accurate knowledge of the volume of the vessel that contains the liquid. After the weight of the vessel when empty has been obtained, the vessel is filled to the calibration mark with distilled water at a specified temperature (often 4,
20, or 25 °C [39, 68 , or 77° F, respectively]) and weighed. From the difference between the weights, the mass of the water is determined. The vessel is emptied and then filled with the analyte and reweighed. The mass of the analyte is determined as during density measurements (i.e., by subtracting the mass of the empty vessel), and the ratio of the analyte mass to the water mass is calculated. The resultant ratio is the specific gravity of the analyte. It is not necessary to know accurately the volume of the container, because it and the volume of the analyte cancel one another while the ratio of the densities is obtained. Density and specific gravity measurements rarely provide sufficient information to qualitatively identify a pure analyte. They can be used as supporting evidence,
however, when an assay is performed by another procedure.
Measurements of this kind also provide limited analytical information. Viscosity is a measure of the resistance of a substance to change of shape. Often it is defined as the resistance to flow of a fluid. It is measured in units of poises (dyne-seconds per square centimetre) or a subdivision of poises. For liquids viscosity is measured in a calibrated glass vessel known as a viscometer, of which there are various types. After inversion, the upper glass bulb is filled to the lower calibration mark by applying suction with a rubber bulb and drawing the liquid analyte into the apparatus. The device is stoppered at the end near the lower bulb, inverted to its upright position, and placed in a constant-temperature
bath. After temperature equilibrium has been established, the stopper is removed. The time required for the volume of liquid between the two marks to drain from the bulb is measured. The time elapsed is used in conjunction with a table supplied by the manufacturer of the bulb to determine the viscosity. The tube at the lower end of the upper bulb has a fixed length and radius that is used along with the pressure differential between the upper and lower ends of the apparatus to measure the
viscosity. Viscosity measurements are common in industries that produce oils or other relatively slow-flowing liquids. They often are employed in oil refineries to determine the viscosities of refined oils.
The pH of a solution is the negative logarithm (base 10) of the activity (the product of the molar concentration and the activity coefficient) of the hydrogen ions (H+) in the solution. In solutions of low ionic strength, pH can be defined as the negative logarithm of the molar concentration of the
hydrogen ions because activity and concentration are nearly identical in these solutions. One method for determining pH is by use of a chemical acid-base indicator, which consists of a dye that is either a weak acid or a weak base. The dye has one colour in its acidic form and a second colour in its basic form. Because different dyes change from the acidic to the basic form at different pH values, it is possible to use a series of dyes to determine the pH of a solution. A small portion of the dye or dye
mixture is added to the analyte, or a portion of the analyte is added to the dye mixture (often on a piece of paper that is permeated with the indicator). By comparing the colour of the indicator or indicator mixture that is in contact with the sample to the colours of the dyes in their acidic and basic forms, it is possible to determine the pH of the solution. Although this method is rapid and inexpensive, it rarely is used to determine pH with an accuracy greater than about 0.5 pH units. More
accurate measurements are performed instrumentally as described below (see Instrumental methods: Electroanalysis: Potentiometry).