YSI pHotoFlex Series - Manual (Page 11)

pHotoFlex
®
Series Practical part
11 ba76126e01 03/2013
1.6 Minimizing interfering effects
1.6.1 General information
The following factors can affect photometric determination and cause
incorrect measurement results:
 Unsuitable pH value of the sample
 Turbidity
 Interferences due to certain water substances (e.g. complexing
agents often disturb the determination of metals)
 Adverse temperature
 Improper, especially not representative, sampling
 Change of the sample due to too long storing period or unsuitable
storing conditions until measurement
Many tests have package inserts included. Read these package inserts
thoroughly. They inform you of special features to be noted during sam-
pling, preparing the sample and carrying out the test and of possible in-
terferences.
The following chapters provide a detailed description of important influ-
encing factors and practical instructions on remedial actions.
1.6.2 Influence of the pH value
The pH value can affect the course of chemical reactions in a photo-
metric determination. For some tests the pH value of a solution has to
be in a certain range. With these tests, the analysis specification in-
forms you of the necessity to check and if necessary adjust the pH val-
ue.
Adjusting the pH value Observe the following points when adjusting the pH range:
 Measure the pH value with the aid of a pH meter or pH indicator
 Use the acids and lyes specified in the analysis specification
 Add the acid or lye drop by drop and measure the pH value after
each drop added. Thus the volume of the sample does not increase
(is not diluted) too much
 The volume increase by the drops is negligible if the resulting dilu-
tion is less than 2 %. With a greater dilution, the measurement result
should be converted accordingly. Adding up to five drops per 10 ml
of solution is uncritical as a rule of thumb.