Water Profile - Alkalinity value difference

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Water Profile - Alkalinity value difference

Postby Bob57702 » Thu Feb 15, 2007 12:20 am

I just got the results from my water analysis. So I put the values into the Water Profile and my Alkalinity from BTP is lower than from the lab. Which is correct?

Ca = 45
Mg = 24
Na = 7
SO4 = 16
Cl = 9
HCO3 = 198
Hardness = 213
Alkalinity = 173 (lab) 162.4 (BTP)

Thanks,
Bob
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Postby just-cj » Thu Feb 15, 2007 6:30 am

I would say that if you got your water analysis from a qualified lab, it would be more accurate than the estimates that BTP gives you.
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My screen shows different hardness, too.

Postby billvelek » Thu Feb 15, 2007 6:52 am

I just entered your list of minerals, and my BTP screen shows a "Hardness" of 211 rather than the 213 that you have listed. Is your figure from the lab, or has BTP somehow generated a different number for you than for me?

Also, when I changed the alkalinity field in my water chemistry screen to match the lab value, the only mineral that it changed in order to compensate was HCO3. I'm no chemist and so I have absolutely no idea, but I want to verify that that is correct. Just want to be sure that if there is a bug here, as per the lab report, that we find out everything that is affected.

Cheers.

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Water analysis

Postby Bob57702 » Thu Feb 15, 2007 11:15 am

All of the values listed with the exception of one Alkalinity are from Ward Labs in Nebraska. I also had the BTP calculated Hardness value of 211. I wish I had another lab report from a different water source to compare the BTP values with. I think this would be very helpful in determining what we're seeing. I guessing it's likely calculation rounding errors but they seem to be a little big. I'd have to see the calculations to be sure. I'm not a chemist either.
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Postby slothrob » Thu Feb 15, 2007 11:49 am

I'm no chemist, I'm a biochemist, which means I know as little as possible about chemistry :lol: , but my suspicion is that what you're seeing is something like assay error and rounding error.

Basically each of the assays performed to determine the individual values have a built in error range, +/- some %. Since BTP takes the individual measurements and calculates the alkalinity, it is subject to the cumulative error from each measurement. The testing lab titrates the alkalinity as an independent value, with it's own error, but not subject on the cumulative errors. Depending on how chance makes the results fall, things can average out to the same result, or tend to skew things in one direction.

I suspect there may also be some difficulty getting a good measure of HCO3 as it is affected by the equilibrium of dissolved gas and might fluctuate a bit.

Either way, I think you might be seeing evidence that your alkalinity probably isn't exactly 162, nor 171, but more like 166+/-3% (which really is good precision, 5% being more common in real world samples, so 162+/- 5.5% could easily be true, though I suspect that error is conservative given a single sample, and is probably closer to 10%.)

Also, other compunds can affect alkalinity, other than the ones mentioned and inluded in BTP's calculations, though usually minor, it might be enough to explain the difference between measured alkalinity and calculated alkalinity.

For these reasons, I think I would take the directly measured alkalinity as the most reliable data point, as CJ suggested, adjust the HCO3 as Bill points out, and remember that the target alkalinity for a given SRM is a range and you should target the middle of that range to account for error.

Sorry to be so wordy.
Last edited by slothrob on Thu Feb 15, 2007 2:07 pm, edited 2 times in total.
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Agreed

Postby Bob57702 » Thu Feb 15, 2007 1:04 pm

A little less wordy :wink:, I agree. Since I'm really only concerned about being in a general pH range I think the numbers are likely to be close enough. Afterall this is homebrewing and not some large scale commercial brewery where I need to squeeze out every possible fraction of efficiency.

Thanks everyone.
Bob
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Re: Water analysis

Postby billvelek » Thu Feb 15, 2007 3:08 pm

Bob wrote:Snip ...I also had the BTP calculated Hardness value of 211. ... snip
Anyway, hardness values of 211 versus 213 seem to be very close to me; would it make any detectable difference?

By the way, since your post I have focused a lot of my attention on water chemistry today, and posted a rather long message in the 'bugs' thread. At the same time -- and I hope Jeff reads this -- I happened upon a thread in rec.crafts.brewing about water chemistry, and one of the posts suggested this as a solution for a brewer with a particular water analysis: "I would treat a ten-gallon batch by adding one-half teaspoon of pickling lime and one tablespoon (2.8tsp) calcium chloride and one-half teaspoon powdered chalk, ...", so if those are legitimate additions, I think it would be useful for BTP database to include "pickling lime" and "powdered chalk", too.

Just a thought.

Cheers.

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Postby slothrob » Thu Feb 15, 2007 4:48 pm

I think Chalk = Calcium Carbonate = Calcite.

Is Pickling Lime = NaOH = Sodium Hydroxide?
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I googled it and ...

Postby billvelek » Thu Feb 15, 2007 5:10 pm

Pickling Lime is calcium hydroxide and Lye is Sodium Hydroxide.

Don't know if that is correct or not, but looks like several websites agree on that.

Once again, I would like to stress how convenient it would be to have both the chemical names and common layman's terms together in the browser; that's because in posts on different forums, one or the other are just as easily used.

Cheers.

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