CLINICAL LAB SCIENEC

CHAPTER 9: LABORATORY MATHEMATICS 221
meter probes that are placed into the solution being analyzed must be cleaned
with distilled or deionized water between uses but are not stored in water. Electrodes
are calibrated by the use of known values of pH solutions or standards,
with both acid and alkaline solutions used in the adjustments of the instrument.
Specific Gravity
Specific gravity refers to the concentration of a solution. This calculation may
also be thought of as the density of a solution and is based on the comparison
of pure water to a solution with dissolved substances in it. Density is
described as the ratio of the concentration (the amount of solutes dissolved
in the solvent) of a material at a certain temperature. Occasionally, it is necessary
to determine the specific gravity and to adjust a reagent to a specific
level when performing certain procedures, such as a flotation test for parasitic
ova. Urine specimens in the laboratory are commonly tested for specific gravity.
This is estimated by the use of a test that is not specifically a test of the
density but compares well with an actual specific gravity. This test measures
the presence of ionic concentration of the urine specimen by color changes
on a chemically-treated dipstick. This method is acceptable for determining
that the pH range is normal. A urinometer, a modified cylinder
with a flotation device, gives an accurate specific gravity when
the conditions of temperature and cleanliness of the device are
acceptable (Figure 9-7); it compares urine with pure water. Most
urinometers are designed for optimum performance at ambient
or room temperature (RT) of 26–27°C, but some are gauged for
20°C. An example of the calculation of the specific gravity (density)
of a given solution would be as follows:
Weight of solution for analysis/Weight of pure water (1.000) = Specific gravity
Another method for determining specific gravity is the use of a
refractometer, where dissolved solids bend light waves; it compares
a solution with air (Figures 9-8 and 9-9).
1.000
1.010
1.020
1.030
1.040
1.050
1.060
1.070
Stem
Meniscus
(read at bottom)
Solution
Urinometer
Cylinder
Differences between Values by Three Types of Specific Gravity
Determinations
Except in rare circumstances, a specific gravity measured by any of
the three methods above is acceptable because, in most cases, differences
between the three methods are negligible, although each
relies on a different measurement principle.
• Urinalysis dipstick—Measures dissolved ions
• Urinometer—Compares solution with pure water
• Refractometer—Measures specimen against air
Mercury
weight
FIGURE 9-7 A urinometer is sometimes used
to determine the specific gravity of a urine
specimen.
Source: Delmar/Cengage Learning.
Copyright 2010 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.