Total Alkalinity, CaCO3 as a function of Bicarbonate, HCO3

My most recent Lab Wards report shows my Total Alkalinity, CaCO3 as 89, my Bicarbonate, HCO3 as 97 and my pH as 8.1.

I had read that for pH under 8.3 Total Alkalinity, CaCO3 = 50/61 * Bicarbonate, HCO3. That does not hold true for my numbers. My report also lists Carbonate, CO3 as 6. Does that explain why the math doesn't work out? If so, is there a formula that calculates Total Alkalinty as a function of Bicarbonate plus Carbonate?

My Cations / Anions , me/L are 2.5/2.5 which gives me confidence that my test results are valid.

 

Either the pH is in error or it was miss-keyed when entered in the program. For the reported HCO3 and CO3, the pH would have to be 9.1. If the pH is actually 8.1, then the HCO3 and CO3 would be about 107 and 0.6, respectively.

 

How about this one (all in mg/l - I think that's the same as ppm)

Calcium 1.59
Magnesium < 0.100
Alk as CaCo3 190
Ca++ hardness 3.98
Total hard CaC03 3.97
pH 8.80

Any ideas on how to treat this with 10% phosphoric acid ?

Want to get to a 6.00 pH for my mash water.........5.20 pH for sparge water

Should I treat the water over a period of 1-2 days to check where the pH is - maybe let the water stabilize

Thanks in advance for help

 

That is an approximation and the closer you get to 8.3 the worse it becomes. Nonetheless I have long noted that Ward Labs alkalinity, bicarbonate and carbonate numbers do not jive. For example, I found a Ward Labs report under Brew Science where the Alkalinity is listed as 199, the bicarbonate as 231 and the carbonate as 6. Ordinarily, the alkalinity titration is done to 4.3 (in brewing). If this were the case, the bicarbonate would, for a pH 8 water, be 239 and the carbonate 1 mg/L. If I tweak the titration pH down to 3.77 then the bicarbonate becomes 231 but the carbonate stays right around 1. Ward Labs may be overlooking the fact that distilled water has intrinsic alkalinity of 2.5 ppm as CaCO3 to pH 4.3 (because you must add that much acid to pure water to lower its pH to 4.3) but that doesn't explain the discrepancy.

Furthermore, at pH 8 the ratio of bicarbonate to carbonate is (60/61)*10^(pH - 10.3756) =0.00414 regardless of how you define alkalinity and Ward Labs is coming up with something 6 times this. They can only do this by using a value for pK2 other than the correct one. Even putting in crazy temperatures does not let me come up with a pK2 that would explain this.

I have noted this discrepancy in every Ward Labs report I have checked. I always have been and still am mystified by this.

If you knock off the 2.5 ppm for the alkalinity of the water you get a bit closer to the correct number but not nearly as close as one would hope. I have no idea how they calculate the bicarbonate and carbonate numbers. It's usually done from the alkalinity in which case they are off quite a bit esp. WRT to carbonate.

 

Can I put faith in any of these 3 numbers? Is it more likely the reported carbonate value is suspect?

 

Carbonate would not be at issue for brewing so use either the alkalinity or the bicarbonate number. At the level of accuracy we deal with here it shouldn't matter. The alkalinity is the easiest to measure and they probably do that with an automatic titrator so perhaps that's the best one to use.

 

jmf143:

I suggest you go with your measured test results - that is your actual data. Certainly there can be noncarbonate sources of alkalinity, so the data points to those as being present.

These are not great links, but they do at least provide some reassurance that non carbonate alkalinity is real.

This one hints at the relationship between calculated and actuual total alkalinity:
http://steamofboiler.blogspot.com/2011/08/carbonate-hardness-and-non-carbonate.html

Here is the abstract for the other link:
Total alkalinity (TAlk) has long been used to evaluate the buffering capacity of aquatic
systems. TAlk has also been used, together with measurements of either pH or dissolved
inorganic carbon (DIC), to indirectly estimate the partial pressure of carbon dioxide
(5 pCO2) in inland waters, estuaries, and marine systems. These estimates typically
assume that carbonate and bicarbonate ions comprise nearly all the species contributing
to TAlk; however, other inorganic and organic acids have the potential to contribute
significant non-carbonate alkalinity. To evaluate the potential for error in using TAlk to
estimate pCO2, we measured pH, TAlk, and DIC in samples of river water.
http://www.biogeosciences-discuss.net/8/5159/2011/bgd-8-5159-2011.pdf

What is your water source?

Cheers!

 

There should be no buffering system in potable water other than carbonic/bicarbonate/carbonate (with the exception of the buffering capacity of water itself which is responsible for the 2.5 ppm as CaCO3 referenced in the earlier post). If there is I would suggest not brewing with or drinking that water until the source has been identified.

I note again that there are discrepancies in all the Ward Labs reports I have looked at but there are no discrepancies in water that has been analyzed by either me or other laboratories/agencies.

 

There is a lot of data missing from that report. To come close to balancing, there would have to be a lot of sodium or potassium in the profile. It looks like water softener water. The bicarb shows about 219 ppm and carbonate is about 6 ppm at that pH. I'm doubting that this is a suitable brewing water.

 

My tap is in Wixom, Mi but the water is City of Detroit water, drawn from Lake Huron.

 

Here is the comment from a guy I contacted at Ward. I don't think he read the whole thread, I only sent him part of it in my email.

We analyze carbonate by titrating to end point with phenolphthalein and then titrate to end point of methyl orange. Phenolphthalein gives us carbonate and methyl orange gives us bicarbonate. Total alkalinity is calculated from carbonate and bicarbonate values. If you can estimate carbonate from pH go ahead, but you can have tremendous amounts of ions in solution at the same the pH. I hope this helps.

 

Here is more info: Water is pulled from 4 wells in the city area - Slidell, LA


Sodium 94
Potassium 0.100
Sulfate 11.5
Chloride 16
Hydroxide < 0.50
Carbon Dioxide 164
Free CO2 range < .50 - 1.41


Thanks

 

That scheme works - sort of - and is found in lots of older books on carbonate analysis (including Standard Methods) but is approximate. To use it one assumes that the bicarbonate content, as CaCO3, is 2*P -2*[OH-] where P is the phenolpthalein alkalinity and [OH-] is as CaCO3 and that the bicarbonate content is T - 2*P + [OH-]. In fact one doesn't need the P alkalinity as it can be calculated from the total alkalinity assuming the only buffer in the water is carbo (which, as noted in an earlier post, should be the case in potable water).

Alkalinity is the equivalent amount of acid required to move the pH of the water from whatever pH it comes to you at to the titration endpoint. It is

alk = Ct*{[(f1e-f1s) - (f3e-f3s)] + 1000*[(10^-pHe - 10^-pHs) + 10^(pHs-14) -10^(pHe-14)}

where the alkalinity is in mEq/L and Ct is the sum of the millimoles of carbonic plus mmol of bicarbonate plus mmol of carbonate per liter. f1 is the fraction of carbo that is carbonic and f3 is the fraction that is carbonate. These depend only on the sample (s) and end point (e) pH values. Ct is unknown but once the titration is done and alk is known the equation can be solved for Ct and then the carbonic is simply f1s*Ct, the bicarbonate f2s*Ct = (1-f1s-f3s)*Ct and the carbonate f3s*Ct - all in mEq/L which are converted to mg/L by multiplying by the molecular (carbonic) or equivalent (bicarbonate, carbonate) weights.

This method is obviously more accurate and simpler as P alkalinity does not need to be measured. To give an idea of the order of magnitude of the differences, if a sample is given to us with pH 9 and T alkalinity of 106.9 ppm as CaCO3 (2.138 mEq/L) then the carbonate content is 5.4 mg/L and the bicarbonate content 116.3. Ct for such a sample is 2.0 and the P alkalinity would be 5.2 ppm as CaCO3.

Using the approximation formulas the carbonate calculates as 6.3 mg/L and the bicarbonate as 128.7. These numbers depend on the assumption that the "methyl orange end point" is 4.3. In fact different laboratories use different end points - some a fixed one and some one that varies dependent on the amount of bicarbonate they expect to find (equivalence end point). Standard methods says one can use whatever end point he chooses as long as the report states what it is. This is consistent with the method I have given where one simply plugs pHe into the formula and solves for Ct. Ward Labs does not report end point pH.

I never considered that Ward Labs would use this method. Given that they have ICP machines for cation analysis I rather thought they'd be more modern wrt to carbonate and bicarbonate analysis. But then it's been done this way for so long that perhaps it is expected that it would be done this way.

Brewers don't really care about bicarbonate and carbonate levels. They care about the amount of acid it takes to get pH to 5.2 - 5.4. The alkalinity is a good measure of this.

 

This thread was referenced in another which caused me to come back here and look over these posts. This one was puzzling:

I wondered 'what are these tremendous amounts of ions ?'. And then it clicked. Ward Labs is in business to serve agriculture - not the drinking water industry. Thus their unusual practice of reporting sulfate "as sulfur". While drinking water contains no buffers other than bicarbonate raw water intended for irrigation well may especially if from a polluted source. Phosphate immediately comes to mind.