Alkalinity

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Alkalinity levels of 50 to 170 are the values generally recommended in the literature for Koi ponds.

Alkalinity is the measure of the amounts of carbonate (CO₃⁼) and bicarbonate (HCO₃^-) ions in the pond. Alkalinity is related to the buffering capacity, or pH stability of the pond.

Test kits measure the alkalinity as carbonate hardness or KH often in degrees of German Hardness (^odH). To convert German Hardness to parts per million (ppm) simply multiply by 17. Remember ppm is equal to milligrams per liter (mg/l) and grams per cubic meter.

Baking soda, sodium bicarbonate (NaHCO₃), can be added to increase the alkalinity and buffering capacity of the pond. The Arm & Hammer brand baking soda you can buy at the local supermarket is pure (USP) sodium bicarbonate.

So let's see what happens when we add 1 cup (~300g) of baking soda to a 2,000 gallon pond. The measured KH before the addition was 5^odH or 85 ppm. After the addition the KH increased to 6.5^odH or 110 ppm. In other words, the addition added 25 ppm of carbonate to the pond as measured.

Could we have calculated the expected increase? Remember that ppm has to use the same units, i.e., grams of baking soda to grams of water in the pond. So we added 300 g of baking soda to 7,500,000 g of pond water. Once we have the same units we divide the 300 by 7,500,000 and then multiply the result by 1,000,000 to get the increase in baking soda in ppm. That gives us a result of 40 ppm.

How much of that 40 ppm is carbonate? We need to know the amount of carbonate in baking soda. The molecular weight of sodium bicarbonate is 84. The molecular weight of the carbonate ion is 60. So the fractional weight of carbonate in baking soda is 60/84 = 0.71 (bicarbonate is 61/84 = .73). So let's multiply 40 ppm by 0.71 which gives us 28 ppm of carbonate actually added versus 25 ppm measured, which is certainly within the accuracy limits of the test.

Now that we know how to calculate the alkalinity remember that sodium carbonate and sodium bicarbonate generally increase the pH. You might want to measure the pH of your water with the level of baking soda you want to add in a small test sample. Don't be surprised if the result is in the neighborhood of pH = 8.5. The bad news is 8.5 is pretty close to the upper pH limit for Koi, the good news (?) is it should be pretty stable at the 8.5 level.

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