LaMotte DC1600 Colorimeter - Manual (Page 67)

pH
COLORIMETRIC METHOD CODE 3700
QUANTITY CONTENTS CODE
60 mL Chlorphenol Red Indicator V-2209-H
60 mL Phenol Red Indicator V-2304-H
60 mL Thymol Blue Indicator V-2213-H
3 Pipets, 0.5 mL, plastic w/caps 0369
The term pH (always written with a lower case p and an upper case H) is correctly defined as the negative logarithm of the
hydrogen ion concentration. More simply, the term pH can be considered to be an “index” of the amount of hydrogen ion
present in a substance, or is a measure of the acidity of the substance. This “index” is important as it can be used to quickly
identify the acid, neutral or alkaline (basic) nature of materials. Acidic substances have a pH less than 7.0, neutral
substances have a pH equal to 7.0 and alkaline substances have a pH greater than 7.0.
Most natural waters have pH values from pH 5.0 to pH 8.5. Acidic, freshly fallen rain water may have a pH value of pH 5.5
to pH 6.0. When it reacts with soils and minerals containing weakly alkaline materials, the hydroxyl ion concentration will
increase and the hydrogen ion concentration will decrease. Then the water may become slightly alkaline with a pH of 8.0
to 8.5. Natural sea water has a pH value of 8.1, and changes from this value indicate that water from an inland source is
entering the body of sea water.
Waters more acidic than pH 5.0 and more alkaline than pH 8.5 to 9.0 should be viewed with suspicion. Mine drainage and
acidic industrial wastes are the principal factors in increasing the acidity of water, and alkaline industrial wastes are the
cause of high pH values.
Because pH measurements can be made so simply, and because they can tell so much about the past and future reactions of
water, they are routinely made in water quality studies. Sudden changes in pH values serve as warning signals that water
quality may be adversely affected through the introduction of contaminants.
APPLICATION: Drinking, surface, and saline waters, swimming pool water; domestic and industrial wastes.
METHOD: The various pH indicators exhibit a specific color change over a narrow pH range. The color
changes are measured colorimetrically.
SAMPLE HANDLING
& PRESERVATION: Sample should be analyzed immediately after collection.
INTERFERENCES: Sample color and turbidity interfere with the colorimetric pH measurement. Color
interfernece may be eliminated by adjusting the instrument to 100%T with a sample blank.
Two drops of 0.1N sodium thiosulfate per 100 mL of sample will eliminate the chlorine
interference.
RANGE & SETTING: pH Indicator pH Range Wavelength Setting
Chlorphenol Red 5.0 – 7.0 4
Phenol Red 6.5 – 8.3 4
Thymol Blue 8.0 – 9.5 5
PROCEDURE
1. Rinse a clean colorimeter tube (0967) with sample water. Fill to 10 mL line with sample.
2. Use “Range & Setting” chart (above) to select wavelength setting on the “Select Wavelength” knob, corresponding
to anticipated pH range.
3. Press “30 Second Read” button and insert tube into colorimeter chamber. Adjust to 100%T with with “Set Blank”
knob. This is 100%T blank.
4. Remove tube from colorimeter. Use the 0.5 mL pipet (0369) to add exactly 0.5 mL of the pH indicator for the chosen
range. Cap and mix.
5. Press “30 Second Read” button and insert tube into colorimeter chamber. Record %T as soon as reading stabilizes.
6. Consult the calibration chart to find the pH value for the indicator selected.
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