CLINICAL LAB SCIENEC

220
ESSENTIALS OF CLINICAL LABORATORY SCIENCE
of units or ions that can combine with or replace 1 mole of hydrogen ions (H + ).
The SI does not recommend normality as a measurement unit in reporting test
results. In previous years, when normality was used routinely, electrolytes such
as sodium, potassium, and chloride were reported as milliequivalents per liter
(mEq/L). The proper SI methodology for reporting these electrolytes is now millimoles
per liter (mmol/L).
pH-Adjusted Solutions
Although it is rare that a laboratory worker finds it necessary to prepare a solution
of a specific pH, sometimes an acid or alkaline solution may be required.
Some requirements for preserving a sample or preparing a volume of solution
with a certain pH may be necessary, and it is well that a laboratory worker be
familiar with this procedure.
The pH of a solution is based on the hydrogen ion (H + ) concentration, which
indicates the acidity or alkalinity. For basic estimates, pH (sometimes called litmus)
paper may be used. A color chart is usually provided with litmus paper for
this purpose. This is sufficient for measuring the pH of a urine specimen, as is
found on the pad on a urine chemistry dipstick, but is not sensitive enough to
provide accuracy for laboratory reagents in many cases.
Principle of Adjusting pH
A neutral pH is 7.0, which is the pH of pure water. The hydrogen ion concentration
and the hydroxyl (OH – ) ion content are equal. Values above a 7.0 pH
are alkaline. Human blood is slightly alkaline (7.35–7.45), which is a critical
value needed for the body’s metabolic processes to continue. Values below 7.0 are
termed acidic. A number of substances are used to adjust the pH of a solution.
Fruit juices, vinegar, and hydrochloric acid (also
called hydrogen chloride and HCl) will serve to
acidify solutions. Baking soda, sodium hydroxide
(NaOH), and potassium hydroxide (KOH)
will make solutions more alkaline. HCl is rich
in hydrogen ions, and sodium or potassium are
rich in hydroxyl ions, so they are most commonly
used to adjust the pH of solutions.
FIGURE 9-6 pH meter along with pH indicator strips used for basic
screening tests for pH.
Source: Delmar/Cengage Learning.
pH Meter
The pH meter (Figure 9-6) reflects the hydrogen
ion content by comparing it to a reference electrode,
in which the electrical potential across a
membrane in the electrode is measured. These
instruments are extremely sensitive and accurate,
providing the value for the pH on a dial or screen,
measured to the hundredths (i.e., 7.82) and on
some instruments even to the thousandths. pH
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