Post boil total alkalinity?

Hi guys, I have another question about water.I was wondering if there is a way to know your alkalinity after boiling the water to remove some bicarbonate.I want to use "bru n water" but my water would have been boiled so my total alkalinity would not be the same as indicated on my water report before the boil.My total alkalinity is 160mg/l and I have a ph of 8.1 on my water report before any boiling.

thanks again.

Jamie.

There are two limits that govern the minimum alkalinity your water might attain through long boiling, a minimum calcium content and a minimum bicarbonate content. Either factor could control the end result. It depends on the starting water quality. Boiling drives off CO2 and causes the dissolved calcium and bicarbonate to precipitate out of solution as chalk (calcium carbonate). The relationship between calcium and bicarbonate is presented in the following equation:

Final Ca content = Starting Ca content - [(starting HCO3 - final HCO3)/3.05]

You obviously have the starting Ca and HCO3 content if you have your water report. The final HCO3 content is a variable that is dependent upon the effectiveness of your boil and it can also be dependent on the starting amount of calcium in the water. For typical boils, you can get HCO3 down to around 60 to 80 ppm. If you have a really effective boil, you might be able to get HCO3 down as low as 40 ppm.

But all that final HCO3 content is dependent upon the amount of calcium in the starting water. That is because if there isn't enough calcium, there won't be enough available to react with the HCO3 to form chalk. There is a absolute lower limit for calcium content in water due to solubility limits. That lower limit for Ca is around 12 ppm. You can't drive the Ca content lower than that with boiling.

If you play with that equation above, you will find that if the starting Ca content isn't high enough that the equation will say that the Ca could be reduced to less than 12 ppm if you assume a low enough final HCO3 content. Since it can't occur, the real answer is that you won't actually be able to reduce the HCO3 content as low as those rules of thumb (60 to 80 ppm HCO3) won't apply.

Sorry, its kind of a convoluted explanation.

It is not easy to tell exactly what is going to happen when hard, carbonaceous water is boiled. One can solve a set of 5 simultaneous equations and come to the conclusion that the pH of the system and hence the distribution of and amounts of bicarbonate, carbonate and carbonic and the amount of calcium in solution depend solely on the partial pressure of carbon dioxide in the atmosphere. At 0.03% CO2 the pH will be about 8.3 and alkalinity and calcium each at about 1 mEq/L. But this is at equilibrium and when one boils a reasonable amount of water and then lets it cool it will take days before equilibrium is reached. Nevertheless it is a general rule of thumb that one can decarbonate to about 1 mEq/L alkalinity as long as there is enough calcium to precipitate with the carbonate. Thus if you have an alkalinity of 3.2 mEq/L (160 ppm as CaCO3) you can expect to get the alkalinity down to about 1 mEq/L (50 ppm as CaCO3) as long as the calcium hardness is originally 3.2 mEq/L or greater. Assuming that it is exactly 160 then you will wind up, after boiling, with water of alkalinity and hardness each around 50 ppm as CaCO3. If you plan to supplement calcium or to add calcium in order to add chloride or sulfate the time to do it is before boiling. Doing this you can decarbonate to a bit lower than 1 mEq/L.

It is really best to check hardness and alkalinity before and after boiling or lime treatment. Wherever precipitation is involved chemistry gets tricky. For planning use 1 mEq/L.

Wow you guys really know your stuff!!! Thanks for the info it explained a lot. Seems like just diluting with di or RO would be less involved and it's also factored in the bru n water software
not to mention my calcium is only 43 so I wouldn't be able to remove much carbonate anyhow.

Thanks.


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