Quote:
Originally Posted by tedr108
-- Add 6 grams of bicarbonate of soda to one liter of distilled water. This gives you one liter of water with a KH of 200 dKH. Now take 10 ml of that water and mix with 490 ml of distilled water (a dilution of 1 in 50) and you get 500 ml of 4 dKH water.
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The problem with trying to add 6 grams of sodium bicarbonate is that the amount of bicarbonates that gives you depends on how much water has been absorbed by the sodium bicarbonate. And, you can't easily dry out the sodium bicarbonate without also converting part of it to sodium carbonate, which has a different ratio of carbonate to sodium than does sodium bicarbonate. A better method is the second one you mentioned. Since the indicated ppm of CO2 is directly proportional to the KH, any error in KH is the same error in ppm of CO2. But, a similar error in pH makes a huge error in ppm of CO2, because the ppm is inversely proportional to 10 raised to the pH power. This is why the drop checker, when green, only tells you that the ppm of CO2 is from about 25 to 40 ppm, and even that accuracy is possible only if you can discriminate shades of green accurately.