Ph Interference - Hach DR/700 Procedures Manual

Interference also may be caused by a high concentration of the constituent
under analysis. For example, the presence of a larger excess of chlorine
will cause the test to read less than full scale. Dilution of the sample to
5 mg/L will result in a reading higher than full scale. This indicates the
need for more dilution until the instrument reading is "on scale."
When an unusual answer is obtained, a color other than that expected is
formed, or an unusual odor or turbidity is noticed, the result is suspect.
Repeat the test on a sample diluted with deionized water; see Sample
Dilution Techniques. Compare the result (corrected for the dilution)
with the result of the original test. If these two are not identical, the
original result probably is in error and an additional dilution should be
made to check the second test (first dilution). This process is repeated
until the same corrected result is obtained on two successive dilutions.
More complete information about interferences and methods to
overcome them is contained in the General Introduction Section of
APHA Standard Methods. The analyst is urged to obtain this book and
refer to it when problems are encountered.

pH Interference

Many of the procedures in this manual are pH dependent. Hach reagents
contain built-in buffers to adjust the pH of the typical sample to the
correct pH range. However, the reagent buffer capacity may not be
sufficient for some unusual samples. This occurs most often with highly
buffered samples or samples with extreme sample pH. Check for pH
interference in the following manner:
1. From the Sampling and Storage section of your procedure determine
the optimum pH range of the test. This is the pH the preserved sample is
adjusted to just before running the test. For some procedures this
information may not be given. If the pH of your sample is within the
optimum pH range, buffering is not needed.
2. Measure the pH of your analyzed sample with a pH meter.
3. Prepare a reagent blank using deionized water as the sample, add all
reagents called for in the procedure. Timer sequences, etc., may be
ignored. Mix well.
4. Measure the pH of the reagent blank with a pH meter.
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