CaCO3 (Chalk) Incorrect in Basic Water Chemistry Calculator???

[Brewer #144342]

Hi. I've been playing with the Basic Water Chemistry Calculator and want to know if there is an error for CaCO3 additions. It seems the reported Ca++ amount is 1/2 of what it should be. For example, go to the calculator and select 1 liter for water volume. Do not have any default salts from a water source (keep all the those fields blank). Then go down below and enter 1 gram of CaCO3. Since CaCO3 is ~40% Ca by weight (anhydrous), you would expect to have 400 ppm Ca++, but it is only 200 ppm. Also, the sum of Ca++ and CO3 (or HCO3 as it's reported) should be ~1000 ppm, but it's not. All this assumes CaCO3 source is anhydrous. Can someone explain or check this out? If the calculator is incorrect, it needs to be fixed. If it is correct, then what are the assumptions about the CaCO3 (Chalk) composition. Again, if correct, it would have to be ~50% water or some other material - but if it has other salts to an appreciable extent (an impure source), they are not displayed and accounted for. All the other salts give the correct numbers either as anhydrous or hydrated forms based on simple MW calculations.

 

[Kevin The Stout]

It's assumed that only 50% of the calcium carbonate is utilised. There's a note on the water chemistry page to this effect. Note that where hardness is reported as bicarbonate equivalent that's twice the carbonate equivalent.

 

[Brewer #144342]

But if only 50% of the CaCO3 is utilized, then the Carbonate anion (or Bicarbonate with 1 unit higher MW) should be 1/2 of what the calculator reports. It's reporting ppm, right? I will look for the notation you mention. When you say "utilized" do you mean dissolved? Only 1/2 goes in to solution? Or are you talking 1/2 of Ca++ ion or 1/2 of CaCO3.

 

[Brewer #144342]

Sorry for all the posts. Not being a jerk. Just trying to understand. I much appreciate your response and the discussion.

I see the notation about CaCO3 utilization but that's on the Advanced Water Chemistry page. I was using the Basic Water Chemistry page. If the Basic page includes a 50% utilization, even though it's not noted, then I believe that the CO3 (or HCO3 as it's reported) should be 1/2 of what it shows. The MW of HCO3 is just 1 unit higher than CO3 (61 vs 60), so a minor difference. The individual salts are reported as ppm which is mass based. No matter what, if it is reporting ppm, the ratio of CO3:Ca should be 1.5:1. What am I missing here?

 

[Kevin The Stout]

I've never used the Basic Water Chemistry page, but it's probably safe to assume that it's just a subset of the Advanced Water Chemistry page.

In the case you describe 50% utilisation does mean that 50% remains undissolved. The reported ppm of HCO3 is not the actual HCO3 concentration - it is the alkalinity reported as equivalent HCO3 ppm. Since the charge on the HCO3 ion is only half the charge on the CO3 ion, 600ppm of HCO3 alkalinity is equivalent of 300ppm CO3 alkalinity.

 

[Brewer #144342]

OK. Thanks so much for the explanation!