Chemical Analysis
Chemical Analysis
5940022ChemAnal.fm Page 31
3.2 Checking for Accuracy and Precision
Accuracy defines the closeness of a test result to the true value. Precision defines
the closeness of repeated measurements to each other. Although precise results
suggest accuracy, they can be inaccurate. Both the accuracy and the precision of
test results can be evaluated by using standard additions or standard solutions.
Figure 8 Precision and Accuracy Illustrated
3.2.1 Standard Solutions
A standard solution may be ordered as a prepared reagent or it may be made in
the laboratory. It is a solution of a known composition and concentration. The
accuracy of your instrument may be checked by using a standard solution in
place of the sample water in a procedure.
2
Polyethylene (P) or glass (G), or PTFE Teflon
3
Sample preservation should be performed immediately upon sample collection. For composite chemical samples each portion
should be preserved at the time of collection. When use of an automated sampler makes it impossible to preserve each portion,
then chemical samples may be preserved by maintaining at 4 °C until compositing and sample splitting is completed.
4
When any sample is to be shipped by common carrier or sent through United States Mails, it must comply with the Department of
Transportation Hazardous Material Regulations (49 CFR Part 172). The person offering such material for transportation is
responsible for ensuring such compliance. For the preservation requirements of Table II, the Office of Hazardous Materials,
Materials Transportation Bureau, Department of Transportation has determined that the Hazardous Materials Regulations do
not apply to the following materials: Hydrochloric acid (HCl) in water solutions at concentrations of 0.04% by weight or less
(pH about 1.96 or greater); Nitric acid (HNO
3
) in water solutions at concentrations of 0.15% by weight or less (pH about 1.62 or
greater); Sulfuric acid (H
2
SO
4
) in water solutions at concentrations of 0.35% by weight or less (pH about 1.15 or greater); and
Sodium hydroxide (NaOH) in water solutions at concentrations of 0.080% by weight or less (pH about 12.30 or less).
5
Samples should be analyzed as soon as possible after collection. The times listed are the maximum times that samples may be
held before analysis and still be considered valid. Samples may be held for longer periods only if the permittee, or monitoring
laboratory has data on file to show that the specific types of samples under study are stable for the longer time, and has received
a variance from the Regional Administrator under §136.3(e). Some samples may not be stable for the maximum time period
given in the table. A permittee or monitoring laboratory is obligated to hold the sample for a shorter time if knowledge exists to
show that this is necessary to maintain sample stability. See §136.3(e) for details. The term “analyze immediately” usually means
within 15 minutes or less after sample collection.
6
Should only be used in the presence of residual chlorine.
7
Maximum holding time is 24 hours when sulfide is present. Optionally all samples may be tested with lead acetate paper before
pH adjustments in order to determine if sulfide is present. If sulfide is present, it can be removed by the addition of cadmium
nitrate powder until a negative spot test is obtained. The sample is filtered and then NaOH is added to pH 12.
8
Samples should be filtered immediately on-site before adding preservative for dissolved metals.
9
Numbers refer to parameter numbers in 40 CFR, Part 136.3, Table 1B.
Not accurate,
not precise
Accurate,
not precise
Precise,
not accurate
Accurate and
precise
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