Water report contains carbonic acid content, but not bicarbonate

[ebbelwoi]

My water report is written in Japanese. There are a few things missing, but I've been doing my best with it for the last few years. I dug it up again today, to make sure my water's alkalinity is what I think it is.

I had requested levels for HCO3 and CaCO3. The guy who put the report together wrote, "While we don't test for those, I'll give you the figure for carbonic acid, which is similar." I just took his word for it, and entered 2 ppm in the HCO3 field of my Brewfather water profile.

Looking at it again, and using a different translator for part of it, my first reaction was, "Wait, that's carbonic acid, and that's a totally different thing! Have I been wrong about my water chemistry this whole time?"

My question is, if my water contains 2 ppm of carbonic acid, is it correct (or close enough) to estimate my bicarbonate at 2 ppm as well? Do they balance each other out, similar to the way anions and cations do? FWIW, the report indicates a pH of 7.6.

And just for giggles, here it is:

 

[VikeMan]

You are correct. H2CO3 not the same thing as HCO3. And at ph 7.6 the two numbers will be very different. Knowing your pH and the H2CO3, we could compute HCO3, but starting with a nice round number of "2," I wouldn't trust it, as any rounding error is going to be amplified quite a bit.

But, If you can supply the mg/L for each of the following common ions, we can pretty closely estimate your total alkalinity as HCO3 and as CaCO3:

Ca
Mg
Na
K
Cl
SO4

 

[ebbelwoi]

Thanks. Here's the best I can do:


Ca
Mg
I was given 31 as total hardness, so I guessed around Ca 8-9 and Mg 2. Hopefully that's not too far off?

Na 8
K unknown
Cl 11
SO4 29

 

[VikeMan]

Ok. K can (often) be ignored, because it's (usually) so small.

Backing into Ca and Mg from a total hardness number can be a crapshoot, but let's call it 9.1 and 2. That, and the other ions, would mean a total alkalinity of about 3.3 as HCO3 or 2.7 as CaCO3. The problem is, that's too low, given an H2CO3 number of 2 at 7.6 pH. It may be that you do have a significant amount of K or (less likely) some uncommon cation, in which case, the total alkalinity number would be higher, to balance. Or it may be that the H2CO3 number is suspect.

But I think it's probably safe to say that you have very soft, low alkalinity water. If I had your water, and had no way to get the measured total alkalinity, I'd probably hedge my bets and call it about 8 as HCO3 (or 7 as CaCO3).

 

[mabrungard]

Carbonic acid is definitely not a value that you can use. But if your hardness value is accurate, it's unlikely that the alkalinity is very high. I believe that most of Japan's geology is volcanic and it's less probable that there would be carbonate aquifers in that part of the world. That means that alkalinity is probably low.

 

[ebbelwoi]

Thanks for the replies, guys! It's good to know that the figures I was working with weren't too far off.

A house about half a mile from here has a K reading of 1.1ppm, so I'm guessing mine isn't too far off from that. That house is on a different water line that's actually a blend of two sources (and it changes over the course of the year). One of those sources is the same as mine.

@VikeMan , the SO4 reading was an estimate that you helped me with in a different thread about a year and a half ago. It's possible that it's off by a few ticks, but I'm happy just to be in the ballpark.

Thanks again.

 

[Silver_Is_Money]

For the presumption that your water contains 1.1 mg/L (ppm) of Potassium and the guess that it also contains 0.1 mg/L Nitrate, my guess is that your Alkalinity (as CaCO3) is about 3.7 mg/L (ppm), and your HCO3- Bicarbonate ion is about 4.5 mg/L (ppm).

If nitrate is instead zero this changes to about 3.8 mg/L Alkalinity and 4.6 mg/L Bicarb.