LaMotte SMART 2 - Manual (Page 214)

INTERFERENCES: Sample color and turbidity interfere with the colorimetric
pH measurement. Color interference may be removed
bystandardizing the instrument with the original water
sample. Two drops of 0.1N sodium thiosulfate per 100 mL of
sample will eliminate chlorine interference.
INDICATOR,
RANGE,
& TEST NAME: pH Indicator pH Smart2
Test Name
Chlorphenol Red 5.0-6.8 74 pH CPR
Phenol Red 6.6-8.4 75 pH PR
Thymol Blue 8.0-9.6 76 pH TB
pH 2/3 Smart2 TEST PROCEDURES 2.04