Alkalinity - colorimetric test

Reagents

• LaMotte total alkalinity indicator, Part # 7038-G (Amazon $12.94). Add 5 drops to a tube with 5 mL of water sample. Cap and mix and measure absorbance at 615 nm.

• Baking soda or CaCO3 standard (Hach)

• Distilled water

Measurement of bases (carbonates) in the water. Typically measured by titration with a pH indicating dye.

• Bromocresol green, pH 3.8 - 5.4

• Methyl red, pH 4.4-6.2

Mix of these two dyes (e.g. Hach test kits) give a sharp transition around pH 5.1 from orange to blue-green.

Also measured colorimetrically using an indicator eg. LaMotte one used here.

Prepare CaCO₃ alkalinity solutions

CaCO₃ dissociates to Ca and CO₃^2-

Calcium carbonate standard = 10,000 ppm as CaCO3. Hach # 218710

• 250 ppm working stock: In a clean 50 mL tube, add 0.5 mL of 10,000 ppm CaCO3 stock and 19.5 mL dH2O to make a 250 ppm stock.

• Prepare the following 9 calibration solutions in clean 10 mL tubes.

Prepare baking soda alkalinity solutions

Alternative to the above CaCO3 standard. 10 dKH solution = 178 ppm CaCO₃ equivalent.

• Weigh out 12.63 g of baking soda. Add to 500 mL of distilled water. Mix until dissolved.

• Dilute 100-fold to yield a 8.42 dKH (150 ppm CaCO3 equivalent) working stock.

• Dilute the working stock as outlined in the Table below

Method

• To each 5 mL sample, add 5 drops of alkalinity solution. Cap and mix by inverting tube several times. An image of all of the calibration solutions after the reagents are added is shown above.

• Transfer approx. 1mL of each calibration solution to a cuvette.

• In the colorimeter plotting program, select the custom 610 nm LED and ppm as units. Calibrate the colorimeter with the 0 ppm alkalinity solution and measure absorbance of all of the solutions in duplicate.

Sample calibration data

• To view/use: Download the file and open in the colorimeter plotting software as described here.

Notes

Calcium carbonate - Carbonate ion CO₃^2-

Sodium bicarbonate (baking soda) - Bicarbonate ion, CO₃^-

From Wikipedia: Carbonate and bicarbonate anions contribute to alkalinity due to their basic nature, hence their ability to neutralize acid. Mathematically, the carbonate anion concentration is counted twice due to its ability to neutralize two protons, while bicarbonate is counted once as it can neutralize one proton.

• One dKH as CaCO₃ = 17.9 ppm (mg/L) CaCO3 and contains 10.716 ppm as CO₃^2- (molecular weight = 100.09, 60% as CO₃^2-)

• One dKH as NaHCO3 = 30.0 ppm (mg/L) NaHCO3 and contains 21.42 ppm as CO₃- (molecular weight = 84.00, 71.4% as CO₃^-)