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The pH scale is a logarithmic scale, which means each step is a factor of ten away from the one next to it. pH is the negative log of the concentration of the hydronium ion. At a pH of 7 - H ions = 1 x 10E-7 and OH ions = 1 x 10E-7 for a total of 1 x 10E-14. Example: a pH of 6 is 10 times more acidic than a pH of 7. A pH of 9 is ten times more basic that a pH of 8. When you hear “drop the pH a full log" it means to add co2 until the pH drops from your baseline, (we’ll say 7.8 until you test it) to a full log lower (to 6.8 if your baseline was 7.8).
To find your baseline draw a cup of water from the tap and let it sit out for 24 hours, then test the pH. This is your baseline or starting point.
When you add co2 to water it forms carbonic acid and lowers the pH of the water. Testing ph tells you how much co2 you’re injecting. At a rough guestimate, dropping the pH a full log will give you roughly 25-30 ppm of co2. Dropping the pH .8 log will give around 20-25 ppm of co2. A drop checker will help you more accurately measure how much co2 is in solution.
There is a chart on the web by Chuck Gadd that compares KH, pH and co2. If you know your KH and your pH you can calculate what your co2 saturation is. This chart assumes that your KH, your water’s buffering capacity, is made up of only carbonate. If your water contains other buffers it throws the chart off and renders it effectively useless.
You can make a KH solution with distilled water and baking soda that will help you in making your drop checker. 1 degree of KH is equal to 17.6 ppm of carbonate, or 17.6 mg/l. A solution with a KH of 4 is commonly used to make drop checkers.
Since DIY co2 systems really have no way to adjust the co2 going into the tank, the co2 is seldom steady or where you want it.
Mark