b. Interference: Table 4500-F–:I lists common interferences. Because these are neither linear
in effect nor algebraically additive, mathematical compensation is impossible. Whenever any one
substance is present in sufficient quantity to produce an error of 0.1 mg/ L or whenever the total
interfering effect is in doubt, distill the sample. Also distill colored or turbid samples. In some
instances, sample dilution or adding appropriate amounts of interfering substances to the
standards may be used to compensate for the interference effect. If alkalinity is the only
significant interference, neutralize it with either hydrochloric or nitric acid. Chlorine interferes
and provision for its removal is made.
Volumetric measurement of sample and reagent is extremely important to analytical
accuracy. Use samples and standards at the same temperature or at least within 2°C. Maintain
constant temperature throughout the color development period. Prepare different calibration
curves for different temperature ranges.