YSI 9300 Photometer - Manual (Page 88)

Notes
1 The color range of the phenol red test is yellow, through orange, to red. The
formation of an intense purple coloration shows that the indicator has been
affected by high chlorine or other disinfectant residuals. In such cases the
result should be disregarded.
2 Phenol red does not show any further color change at pH values below 6.8 or
above 8.4. Note therefore that when such values are recorded this could
indicate that the sample has a much lower or much higher pH value.
3 Ionic strength, temperature and other water factors may have an effect on
pH readings. This test has been calibrated for conditions most likely to be
encountered in a typical swimming pool at 30°C.
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