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General Category => All Grain Brewing => Topic started by: pehlman on August 31, 2011, 03:45:04 AM

Title: Another water chemistry question.... Sorry
Post by: pehlman on August 31, 2011, 03:45:04 AM
Was just reading through John Palmer's book in the chapter about water chemistry and I noticed something. He says "if the total hardness exceeds the total alkalinity, then nearly all the alkalinity can be removed down to the 50ppm limit."

After looking at my latest water report, my Hardness as CaCO3 is 246mg/L and my Alkalinity is 112mg/L

However, my Alkalinity doesn't say "as CaCO3" after it like the hardness does. Can I still assume it's pretty close?
Title: Re: Another water chemistry question.... Sorry
Post by: nateo on August 31, 2011, 05:05:30 PM
I believe alkalinity is measured as mg/L of CaCO3 by convention, unless it says otherwise. Converting that to ppm is a different issue, but in most cases mg/L is close enough to ppm to not really matter.
Title: Re: Another water chemistry question.... Sorry
Post by: pehlman on September 02, 2011, 08:18:08 AM
Yeah, I knew that mg/L was pretty close to PPM... it was just the CaCO3 part that really confused me. Cause Im trying to enter my water stats into the Bru'n Water software and I wanted to make sure I wasn't going to get readings that were way off!

Title: Re: Another water chemistry question.... Sorry
Post by: thcipriani on September 03, 2011, 04:14:56 PM
mg/L = ppm


Total alkalinity in your water is probably expressed as CaCO3 - it's like the lowest common denominator. It's easier to express the sum total of all carbonate and bicarbonate as an equivalent weight of CaCO3. Concentration as CaCO3 is just a very convenient means to express concentration since it has an equivalent weight of 50g.

To express the concentration of a substance as an equivalent amount of another substance you first find the mg/L (or ppm - they are equivalent measurements) concentration of the substance for which you are attempting to express as an equivalent weight of another substance.

Then you must find the equivalent weight of that substance, in the above case the substances CO3 and HCO3. The equivalent weight of a substance can be derived by dividing a compound's molar mass by the number of positive or negative electrical charges that result from the dissolution of that compound (or, more conveniently, just Google search, "Equivalent weight of x" and you can usually find it). In the case of CO3 the gram equivalent is 30 (roughly) and in the case of HCO3 the gram equivalent is 60(roughly).

After finding the equivalent weight of the first substance you must find the equivalent weight of the substance in which you'd like to express the concentration of the first substance - in the case of CaCO3 the gram equivalent is 50.

After you find these numbers the math for expressing equivalent weight as CaCO3 is fairly easy.

Concentration CO3 in mg/L (= to ppm) * 30 (which is CO3 equivalent weight)/50 (CaCO3's equivalent weight) = CO3 as CaCO3
Concentration HCO3 in mg/L (= to ppm) * 60 (which is HCO3 equivalent weight)/50 (CaCO3's equivalent weight) = HCO3 as CaCO3

CO3 as CaCO3 + HCO3 as CaCO3 = Alkalinity as CaCO3

That's the long-winded way of saying, "+1 nateo is right".

Also, this is the best thread subject title for a water question ever.
Title: Re: Another water chemistry question.... Sorry
Post by: nateo on September 03, 2011, 05:54:35 PM
mg/L = ppm

When working with an aqueous solution, this is basically correct, but not exactly equivalent, if you want to get to the nitty gritty of it. Mg/L is a weight/vol measurement, while ppm (mg/kg) is a weight/weight measurement. 1L of H2O weighs 1kg at a specific temperature and pressure, so only at that specific temperature and pressure does mg/L = exactly mg/kg or ppm.

By assuming mg/L = ppm we're neglecting any amount of water the solute is displacing, thus changing the volume part of the ratio. In a very dilute aqueous solution at room temperature like this, you can basically ignore any volume the CaCO3 displaces. For what Pehlman is doing, mg/L ~ ppm, but for a chemistry class that wouldn't necessarily be the case, depending on how many significant digits you're measuring out to, and if it's not a dilute, aqueous solution, it's definitely not the case.

Here's what you'd use to figure it out exactly: ((molecular weight)/22400)*ppm=mg/l
Title: Re: Another water chemistry question.... Sorry
Post by: thcipriani on September 03, 2011, 10:44:10 PM
nateo - that's actually a point that I've never really thought about at all and it has some interesting consequences. One thing that springs to mind is that in order to get the oft-quoted 50ppm Ca++ into beer you'd need to vary the amount based on gravity; however, is it possible or practical for homebrewers to even think about that, or is it acceptable to say the 1kg 1.070 beer is close enough to 1,000,000 mg that it shouldn't matter?

After re-reading this thread - is this discussion too esoteric to be relevant? I'm probably not fun at dinner parties.
Title: Re: Another water chemistry question.... Sorry
Post by: nateo on September 03, 2011, 11:26:52 PM
The differences are small, like 1-2ppm, so in this particular application, they're basically irrelevant. Even if you're off by as much as 5ppm, if you're targeting 50ppm of Ca, I doubt being 10% off would make that big of a deal. I would do the math on it, but math isn't my strong suit. And who knows, maybe the oft-quoted 50ppm of Ca takes into account gravity of the wort.

It's a pretty esoteric question for homebrewers, but it's very relevant to chemists. Being off by a few ppm could be a very big deal to a chemical engineer. I mostly pointed it out because a lot of people say ppm = mg/L, so that's what I believed to be true all the time, in every circumstance, so it was extra-confusing when I realized they weren't always exactly equivalent.

I'm all for taking shortcuts and using approximations, but I think it's good to know when and where you're taking shortcuts. Good science is all about significant digits, and for brewers, I'd say they stop at the decimal point with regards to brewing salts.