Why is carbonate special?

I'm just starting to learn about water and I'm struggling with alkalinity. Wikipedia says that most of the bases in solution come from bicarbonate and carbonate, due to environmental contact with carbonate rocks and atmospheric CO2. It also says that there are other bases including borate, hydroxide, phosphate, silicate, nitrate, dissolved ammonia, and more.

Meanwhile, Total Alkalinity ("TA") seems to be the most widely used measure, and it is defined to be "the amount of acid required to lower the pH of the sample to the point where all of the bicarbonate [HCO3-] and carbonate [CO3--] could be converted to carbonic acid [H2CO3]."

So here is what I don't understand about the above:

1. Why would scientists call this number (the amount of acid needed to convert bicarbonate and carbonate) "Total Alkalinity", when in fact it is not really the total, given that there may be other bases in solution?

2. Why does TA imply a specific endpoint pH (eg pH=4.2 for seawater or a range of pHs for freshwater)? Aren't there a ton of other contributors to pH that could make the point at which carbonate is balanced much lower or higher?

3. Is alkalinity interesting to brewers for any reason other than predicting the correct amount of acid needed to get a mash with a certain pH? For example, say I have two sources of water, one with much higher alkalinity than the other, and I use them each to brew a beer with an identical grain bill. Say I use the right amount of acid in each such that each one results in a mash with a pH of exactly 5.4, will I be able to distinguish the effect on the beer contributed by the difference in alkalinity?

All good questions.

wuertele said:

I'm just starting to learn about water and I'm struggling with alkalinity. Wikipedia says that most of the bases in solution come from bicarbonate and carbonate, due to environmental contact with carbonate rocks and atmospheric CO2. It also says that there are other bases including borate, hydroxide, phosphate, silicate, nitrate, dissolved ammonia, and more.

Meanwhile, Total Alkalinity ("TA") seems to be the most widely used measure, and it is defined to be "the amount of acid required to lower the pH of the sample to the point where all of the bicarbonate [HCO3-] and carbonate [CO3--] could be converted to carbonic acid [H2CO3]."

So here is what I don't understand about the above:

1. Why would scientists call this number (the amount of acid needed to convert bicarbonate and carbonate) "Total Alkalinity", when in fact it is not really the total, given that there may be other bases in solution?

Click to expand...

Total alkalinity is actually defined as the number of milliequivalents of acid required to move 1 liter of the water from whatever pH it comes to the analyst at to an endpoint pH whose choice is at the analysts discretion though he is required to put the end point pH in his report. I mention this because I have never seen an end point pH specified in a report.

In brewing we tend to use 4.3. Why? Just because Manfred Moll said that's the right number in the first edition of Handbook of Brewing perhaps. Hanna sells an automatic titrator that goes to 4.5 and that is not adjustable (if it were I might buy one). Some analysts use variable end points that result in equivalence of hydrogen ion and bicarbonate. I'll stick with 4.3.

To do the test you put 100 mL sample in a flask with a stirbar, lower in a pH electrode and titrate 0.1N acid (sulfuric is good). 0.1N and 0.1L means the number of cc used is the number of mEq per liter of sample. Don't get confused by the "as Calcium Carbonate" units frequently used (though you certainly are justified in some confusion on that point - I wish they'd standardize on mEq/L). Alkalinity as CaCO3 is just 50 times the alkalinity in mEq/L. There's a reason for doing this but I'll leave that for another time.

If someone snuck into your lab and dosed you sample with lye or amonia or lime you would obviously pick that up in a titration. The assumption made in measuring alkalinity for brewing water is that the water is potable and that it doesn't contain mine runoff or anything of the sort i.e. that carbonate is the sole source of alkalinity unless the pH is higher than about 9 in which case hydroxyl is significant but drinking water isn't supposed to have pH that high. When measuring the alkalinity of waste water those other bases are definitely acting but that's OK as the desire is to come up with an estimate of how much acid needs to be added to get a particular pH.

An interesting source of alkalinity that is usually overlooked is the water itself. First, it contains hydroxyl ions which must be neutralized and then in setting pure water to pH 4.2 you must come up with 10^-4.2 mEq/L acid. These two effects (mostly the second) give distilled water an alkalinity of about 2.5 ppm as CaCO3 (end point 4.3).

wuertele said:

2. Why does TA imply a specific endpoint pH (eg pH=4.2 for seawater or a range of pHs for freshwater)? Aren't there a ton of other contributors to pH that could make the point at which carbonate is balanced much lower or higher?

Click to expand...

No, not really. In potable water bicarbonate is definitely going to be dominant. The pKa's and pKb's of things like ammonia,silica and nitrates are such they don't contribute to alkalinity (unless the pH is higher than normal). Phosphate will contribute but phosphate tends to be low in natural waters because of the presence of calcium. Calcium phosphate isn't very soluble at all.

The reason water chemists can allow arbitrary end point specification is because it doesn't really matter what end point you use. If you give me an alkalinity number and an endpoint I'll use the assumption that it's all bicarb to compute the alkalinity at any other end point. If bicarb isn't the main contributor to alkalinity by a large margin you shouldn't be brewing with this water (or drinking it).

wuertele said:

3. Is alkalinity interesting to brewers for any reason other than predicting the correct amount of acid needed to get a mash with a certain pH? For example, say I have two sources of water, one with much higher alkalinity than the other, and I use them each to brew a beer with an identical grain bill. Say I use the right amount of acid in each such that each one results in a mash with a pH of exactly 5.4, will I be able to distinguish the effect on the beer contributed by the difference in alkalinity?

Click to expand...

We generally just say "Alkalinity = bad" and leave it at that. The main reason alkalinity is bad is that it holds mash pH high. But there is a second effect. At high pH not as much bicarbonate converts to CO2 as at lower so more bicarbonate stays in the beer. Bicarbonate doesn't taste that good. When you use acid to get rid of bicarbonate the reaction is

H2SO4 + 2HCO3- --->2CO2 + 2H2O + SO4--

Thus every equivalent of bicarb you got rid of has been replaced, in this case, by 1 equivalent of sulfate. If you are trying to make Boh Pils with alkaline water and get rid of the alkalinity with sulfuric acid you will have solved one problem (alkalinity) only to have created another (high sulfate which is disastrous in Boh. Pils). In other cases, this is a perfectly fine strategy. It depends on the beer and the original water.