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 = 1,000 ppm (1mg/ml), LabChem # LC12700-7
  • 500 ppm working stock: In a clean tube add 5 mL of 1,000 ppm CaCO3 stock and 5 mL dH2O to make a 500 ppm stock.
  • Prepare the following 9 calibration solutions in clean 10 mL tubes.

  Vol CaCO3 500 ppmVol H2O 
   0.00 5.00
  25 0.25 4.75 
  50 0.50 4.50
  75 0.75 4.25
  100 1.00 4.00
  125 1.25 3.75
 150 1.50 3.50
 175 1.75 3.25
 200 2.00 3.00

Prepare baking soda alkalinity solutions

10 dKH solution = 178 ppm CaCO3 equivalent.

  • Make a 500 dKH solution = 7.5 g in 500 mL distilled or DI water 
  • Dilute 50-fold to make a 10 dKH solution. (2mL in 100 mL volumetric flask)
 dKH ppm CaCO3 equiv.Vol 10 dKHVol H2O 
  0 0.00 0.00 5.00 
  1 17.8 0.50 4.50
  2 35.6 1.00 4.00
  3 53.4 1.50 3.50
  4 71.2 2.00 3.00
  5 89.0 2.50 2.50
  6 106.8 3.00 2.00
  7 124.6 3.50 1.50
  8 142.4 4.00 1.00
  9 160.2 4.50 0.50
 10 178 5.00 0.00


  • 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)

 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

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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