Alkalinity (KH, carbonate hardness)

Measurement of bases (carbonates) in the water. Typically measured by titrations 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 

LaMotte alkalinity test

LaMotte total alkalinity indicator, 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.

Prepare CaCO3 alkalinity solutions

CaCO3 dissociates to Ca and CO32-

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.

  Vol CaCO3 250 ppmVol H2O 
   0.0 5.00
  25 0.5 4.5 
  50 1.0 4.0
  75 1.5 3.5
  100 2.0 3.0
  125 2.5 2.5
 150 3.0 2.0
 175 3.5 1.5
 200 4.0 1.0

Prepare baking soda alkalinity solutions

Alternative to the CaCO3 standard.
10 dKH solution = 178 ppm CaCO3 equivalent.
  • Make a 500 dKH solution = 7.5 g in 500 mL distilled or DI water 
  • Dilute 59.4-fold to make a 8.42 dKH solution = 150 ppm equiv. (1.68 mL in 100 mL volumetric flask)
dKH ppm CaCO3 equiv.Vol 150 CaCO3 equiv.Vol H2O 
  0.00 0.00 5.00 
  15 0.50 4.50

 30 1.00 4.00
  45 1.50 3.50

 60 2.00 3.00
   75 2.50 2.50
  90 3.00 2.00
   105 3.50 1.50
   120 4.00 1.00
   135 4.50 0.50

 150 5.00 0.00


Absorbance Spectrum



Method

  • Add 5 drops of the LaMotte test solution to each tube, cap and invert several times to mix
  • Measure absorbance with the red LED or a custom 615 nm LED board





Notes

Calcium carbonate - Carbonate ion CO32- 
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 CO32- (molecular weight = 100.09, 60% as CO32-)
  • 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-)


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