Total Alkalinity

[bobfloyd]

Hi.

Total Alkalinity - does this buffer just the addition of acid, or does it also buffer the addition of a base (I'm assuming not - that's "acidity" right?)

I understand that if water has a non-zero total alkalinity, it's pH (and TA) will drop when acid is added. The pH drop will be less so than if there was no (or lower) TA.

But what about if a base is added? Will the pH increase less?

I guess the reason for my question is why bother with TA at all? Wouldn't acid be more effective if there were no TA (assuming that TA does not have any effect on the addition of bases). We need to frequently add acid, so why inhibit it's effect with TA?

Bob.

 

[chem geek]

Welcome to TFP!

It buffers in both directions, but technically the TA number itself measures the buffer capacity against a drop in pH. There is a number for Total Acidity that would measure the buffer capacity against a rise in pH, but that's not in standard pool test kits since the primary buffer system in pools is that of bicarbonate which is measured in the Total Alkalinity test.

So yes, when the TA is higher and a base is added the pH increase will be less.

As for why we have any TA, the reasons are as follows:

• With no TA at all, the pH can bounce up or down rather wildly and to extremes with very small additions of acid or base. This can lead to metal corrosion if the pH got low.
• Plaster pools need to be saturated with calcium carbonate to protect such surafces. The carbonate comes from bicarbonate and both count towards TA.

Now it is true that since TA is a source of rising pH on its own due to carbon dioxide outgassing, it makes sense to lower the TA level if the pH tends to rise in one's pool.

Also, if you need to frequently add acid, then the amount of acid you have to add is not related to the pH buffering. If a base were added to the water with low TA, the pH would rise a lot so would take just as much acid to move that high pH back down as when the TA were higher and a base moved the pH up less. The buffering goes both ways.

If the source of pH rise is carbon dioxide outgassing, then lowering the TA level should reduce the rate of such outgassing and therefore lower the amount of acid that needs to be added.

 

[bobfloyd]

Thanks Chem Geek, appreciate the help.

It is interesting to note that 99% of what I read just says that TA refers to it buffering a drop in pH only.

If this were the case, clearly there would be no point having any TA so as to make the use of acid more effective.

But I understand that it works in both "directions" now, so having a reasonable TA makes sense.

Why is there a need for an acidity measure (not included with our pool test kits)? Winemaking?

Scott.

 

[chem geek]

You'd want to measure Total Acidity if you wanted to know the buffer capacity against a rise in pH, but really these are just crude measures anyway. The Total Alkalinity is usually the measure when the pKa of the buffer system is lower than your usual target, as is the case with bicarbonate, while Total Acidity is usually used when the pKa of your buffer system is above your usual pH target.

See the thread pH Buffer Capacity which shows you how the buffer strength increases for carbonic acid / bicarbonate as the pH gets lower while for borates it gets stronger when the pH gets higher. So technically, Total Acidity might be useful for the borates, but since there are other ways of measuring borates directly that's what is normally done instead. Note that buffer strength is at its greatest when the pH is equal to the pKa for the buffer.

 

[bobfloyd]

Chem Geek, a quick question...

I note on my bottle of Hydrochloric Acid it says "to lower pH by 0.1, add 100mls of acid per 10,000 litres".

Do these instructions on the bottle take into account TA? Or would these instructions assume the water has no buffering capacity and you'd therefore have to add a lot more..?

I presume that TA will gradually "erode" over time, eventually necessitating the need to raise it a little. I'm a new pool owner; is TA something that would need to be adjusted at the start of each season (i.e. annually) typically?

Cheers.

 

[chem geek]

Those instructions are an estimate with an assumption of TA AND starting pH. Does the bottle you are looking at say 15-16% Hydrochloric Acid? Because those instructions would be accurate with a TA of 100 ppm and starting from a pH of 7.3. Otherwise, from a starting pH of 7.8 it would drop to 7.59 so a drop of around 0.2 while if it were full-strength Muriatic Acid (31.45% Hydrochloric Acid), the pH would drop from 7.3 to 7.1 or from 7.8 to 7.4.

The TA will drop if you keep adding acid. 100 ml of full-strength Muriatic Acid in 10,000 liters drops the TA by 5 ppm (half-strength by 2.2 ppm -- it's less than half because the density is 1.08 g/ml instead of 1.16 g/ml). This rule is independent of pH or starting TA.

You can use PoolMath for approximate calculations that are reasonable for normal pH and TA ranges in pools.

What you normally do with TA is get it to a point where the pH will be reasonably stable. So when using net acidic sources of chlorine the TA needs to be higher while when using (approximately) net pH neutral sources of chlorine such as hypochlorite, then the TA should be lower. This is because TA is a source of rising pH. You can look at the Pool School Recommended Levels. If you use chlorinating liquid or bleach, then you may use some acid to keep pH in check but the TA will only go down to the extent that there is carbon dioxide outgassing so that is usually pretty slow. So yes, you set up your TA at the start of the season and normally don't need to adjust it very often, but you should test more frequently at first until you are certain it's not changing a lot in your pool.

 

[bobfloyd]

Thanks for that Chem Geek.

I have a salt water pool. My understanding is that the chlorine will push the pH up over time, so I have to add some acid each week to keep the pH down to the right spot. This also means that each week I am slowly eating away at the TA...

 

[JasonLion]

In nearly all cases you can balance your levels so that the PH does not go up constantly. The only major exception is high TA fill water and extreme amounts of aeration (like a negative edge).

PH typically goes up because of TA being too high combined with aeration. There can be a few other causes, but they are much less common (things like fresh plaster). If you balance your TA correctly for the amount of aeration you can normally stop the PH from going up. PH increases over time almost never have anything to do with chlorine.

 

[bobfloyd]

Ah ok...errr.....my understanding is off somewhere.

So, for a pool being manually dosed with chlorine, which is alkaline, routine (e.g. weekly) additions of acid are normal...?

For a SWG pool though, once the TA is set correctly, routine acid addition is not required because of the chlorine gas dissolving in the water etc (i.e. the chlorine is being made from what is already in the water; we aren't adding an alkaline substance manually)???

Scott.

 

[chem geek]

Nope, that's not right either. Both manually dosing and an SWG both are alkaline when adding chlorine. However, chlorine usage/consumption is acidic and cancels this out. Technical details about this are in this post.

There is a small amount of excess lye in bleach and chlorinating liquid and it is possible for chlorine gas from an SWG to not fully dissolve in the water before outgassing, but generally as Jason noted most pH rise is from carbon dioxide outgassing which comes from the TA (technically, the carbonate alkalinity) in the water. Pools are intentionally over-carbonated and the outgassing of carbon dioxide has the pH rise with no change in TA. SWGs also aerate the water with their hydrogen gas bubbles and this increases the rate of carbon dioxide outgassing.

 

[bobfloyd]

Ah cheers, I follow.

Hopefully my final question, frequent acid addition vs lowering the TA resulting in less frequent acid addition....

It's basically a lot of acid now vs lot of acid spread out over time????

So putting in the bulk acid, albeit carefully, to lower the TA down to say 60, just mitigates the need to put in the same amount of acid more frequently with the doses spread out say weekly?

Or does the bulk approach definitely win overall?

Cheers.

 

[chem geek]

No, that's not right either. Lowering the TA may or may not change the frequency of acid addition. It definitely lowers the total amount of acid over time. Usually it lets you add acid less frequently, but that is more normally the case if you add 50 ppm Borates to the pool. Basically you substitute a pH buffer that causes the pH to rise (i.e. carbonates) with a pH buffer that does not cause the pH to rise (i.e. borates).

The "a lot of acid now vs. lot of acid spread out over time" would be the case when only adding 50 ppm Borates and not lowering the TA. In that case, the borates increase the amount of pH buffering so lessen the frequency of acid addition, but don't change the amount of acid added over time. So to sum up:

Lowering TA -- reduces amount of acid over time; may or may not reduce frequency of acid addition
Adding Borates -- reduces frequency of acid addition and reduces the rate of pH rise

 

[bobfloyd]

How are pools over carbonated? How does the CO2 form?

 

[JasonLion]

The CO2 comes from baking soda (and related pool chemicals). Adding baking soda to water is an old-fashioned (i.e. inefficient) way of making soda water. It is one element of the carbonate buffer system, used to help stabilize the PH.

 

[chem geek]

Ground water is often over-carbonated because it dissolves calcium carbonate from limestone rock and if underground it doesn't get a chance to equilibrate with air. It may also be at higher pH in which case it becomes more over-carbonated when you lower the pH for pool water.

 

[bobfloyd]

Hmmm. That doesn't sound right. I keep a large planted fish tank at home and I deliberately inject CO2 into it to fertilise plants. The only way to add CO2 is to pump it in, I can't just add baking soda etc.

I must be misunderstanding you. Due to the low partial pressure of CO2 it's not going to hang around in the pool very long and once it's gone it's gone, unless replenished temporarily with a large water top up.

So the way I see it, once the pH has risen, then corrected, it should stay there if it were primarily CO2 related.

It sounds more to me like an imbalance between acid and base in that salt cell...

 

[chem geek]

If you added baking soda to your planted fish tank, that would indeed also increase the amount of dissolved carbon dioxide. However, when you add CO₂ by injection that also lowers the pH so more of the CO₂ remains that way instead of more of it becoming bicarbonate ion. Technically speaking, adding baking soda and acid is IDENTICAL to injecting carbon dioxide except that the baking soda and acid also increases the salt level. If you weren't concerned about increasing the salinity of your fish tank, you could add baking soda and acid to increase CO₂ instead of injecting it.

NaHCO₃ + HCl = CO₂ + NaCl
Sodium Bicarbonate + Hydrochloric Acid = Carbon Dioxide + Sodium Chloride
Baking Soda + Acid = Carbon Dioxide + Salt

Now the above is not one-way. There is an equilibrium between carbonate ion, bicarbonate ion, carbonic acid, and carbon dioxide.

CO₂ + H₂O <---> H₂CO₃ <---> HCO₃^- + H⁺ <---> CO₃^2- + H⁺ + H⁺
Carbon Dioxide + Water <---> Carbonic Acid <---> Bicarbonate Ion + Hydrogen Ion <---> Carbonate Ion + Hydrogen Ion

These equilibrium are relatively fast. What is slow is the outgassing of carbon dioxide. You are incorrect that CO₂ won't hang around in the pool very long. It would take significant aeration of the pool water to speed up removal of carbon dioxide from it. It would also require adding acid to keep the pH down since higher pH slows down the outgassing rate because the equilibrium shifts. This chart shows how much excess carbon dioxide there is in the water compared to being in equilibrium with air at various TA and pH levels. You can see that the Pool School article on how to Lower Total Alkalinity is based on these principles of aerating at lower pH to drive out carbon dioxide more quickly.

So with a pool with a TA of 80 ppm (I'll assume no CYA for simplicity), and starting pH of 7.5, the pH would have to rise to 8.3 to be in equilibrium with the air, though in practice the rate of pH rise slows down significantly as one approaches that pH. Only 8.6% of the carbon dioxide in the water would have outgassed. If you were to add acid to then lower the pH back to 7.5, you would lower the TA by 6.3 ppm and still have a lot more carbon dioxide to get rid of from the water. The equilibrium TA (again with no CYA) at pH 7.5 would be 11 ppm.

The SWG cell has the same pH effect as adding hypochlorite chlorine to the pool. It raises the pH but then when that chlorine is used/consumed the pH drops back down. Technical details about this are in this post.

 

[bobfloyd]

Hi Chem Geek.

Hmmm....can you Google "pH KH CO2" chart and take a look for me...

I was under the strong impression that when raising carbonate hardness, i.e. adding baking soda, that the pH will rise in conjunction with the KH thus not changing the CO2 levels....

I understand what your saying about adding both baking soda and acid at the same though, that makes sense, but yeah the salt by product would be a problem due to the sheer amount of CO2 required - plants need heaps of it under good lighting.

I know this is a swimming pool forum so I'll stay on topic.

I thought if you sat a container of water with higher CO2 in it overnight it would degassing the CO2 pretty quickly to reach equilibrium with the air...as far as I was aware this didn't need any agitation....

Maybe my other hobby is screwing up my understanding....

A couple of related questions, I guess I can somewhat hijack my own thread. When the SWG is running, what happens to the hydrogen gas? Does it just dissolve too or bubble out from the returns?

Also, why doesn't the sodium hydroxide by-product of the electrolysis immediately re-combine with the hypochlorous acid? Why don't they immediately re-neutralise each other? Is it because the "nasties" are more "attractive" at the ionic level? I guess so.

Is it true that your pool can be slightly warmer if there is a lot of oxidation taking place (I.e. lots of nasties being nuked)?

Cheers for the help.

Bear with me.

 

[JasonLion]

The CO2 outgassing rate depending on the surface to volume ration. More surface area means more outgassing, more volume means less. Swimming pools have drastically higher surface to volume ratios than fish tanks, so drastically slower outgassing rates.

 

[chem geek]

Jason meant the opposite -- pools have SMALLER surface-to-volume ratios than fish tanks. In general, as the volume of the object gets larger, it's surface-to-volume ratio decreases. For a cube, it's the ratio L²/L³ for a single side exposed to air so a ratio of 1/L so gets smaller as L (length of a side of the cube) gets larger. However, for pools and aquariums, the surface is the top exposed to air so the ratio of top surface area to volume will simply be the inverse of the depth. The average depth of a pool with 6' deep end is around 4.5'. Large 180 gallon aquariums are around 2' deep so have 2.25x the aeration rate while smaller aquariums are 1' to 1.5' deep so have 3x to 4x the aeration rate. A glass of water is around 6" (0.5') so has 8x the aeration rate. And that assumes the circulation patterns are similar, but aquariums typically intentionally aerate the water (to give the fish more dissolved oxygen) so outgas carbon dioxide faster than pools.

You can look at my Pool Equations spreadsheet which calculates everything and has all the pKa. If I start with water at a pH of 7.5, TA of 50 ppm (assume no CYA) and I add baking soda to get the TA to 80 ppm, the concentrations of the carbonates are as follows (units are micromoles/liter except for chemicals added which are per 10,000 gallons). I also show what happens if you then add acid (and a little more bicarbonate) to lower the pH back to 7.5. I then show what happens if you just add acid and no bicarbonate. And finally I show what happens if you add carbon dioxide or if you add baking soda and acid for the same result.

............................ Initial ........... Baking Soda ...... and Acid ......... Acid Only ..... CO₂ OR Baking Soda + Acid
Species ............ TA 50, pH 7.5 ... TA 80, pH 7.68 ... TA 80, pH 7.5 ... TA 48.3, pH 7.3 ... TA 50, pH 7.3

CO₂ ....................... 61.138 ............. 64.287 ............. 97.521 ............. 93.677 ............. 97.029
H₂CO₃ ..................... 0.094 ............... 0.099 ............... 0.150 .............. 0.144 ............... 0.150
HCO₃^- .................. 994.66 ........... 1588.0 .............. 1591.6 .............. 962.86 ............. 996.33
CO₃^2- ...................... 2.0391 ............. 4.9712 ............. 3.2929 ............. 1.2471 ............. 1.2891

Baking Soda .................................... 67.24 oz. .......... 71.19 oz. .................................... 4.12 oz.
Acid .......................................................................... 4.50 fl.oz. ........ 4.70 fl.oz. ......... 4.29 fl.oz.
OR CO₂ ...................................................................................................................... 0.14 lb.

So as you can see, adding baking soda without adding acid only somewhat raises the carbon dioxide because most of the baking soda remains as bicarbonate ion because the pH rises. However, if you also add acid to keep the pH constant (and add some more baking soda to get to the same TA), then the carbon dioxide level rises in addition to bicarbonate ion. If you add less baking soda and more acid, then you can lower the pH with no change in TA which is the same that happens if you add carbon dioxide. In this case, you can see that most of the change is an increase in carbon dioxide, BUT you have lowered the pH of the water. If you just added acid to the water to lower the pH you would have nearly all of this same effect (i.e. as injecting CO₂) and if you add some baking soda you would have an identical effect (but the acid and baking soda increase the salt level).

As for the acid produced by chlorine dissolving then combining with the base produced when hydrogen gas is produced, they do combine to form water. However, there is twice as much base produced as acid. All of these details are in the technical post I linked to earlier. The net result from an SWG is the same as adding a hypochlorite source of chlorine except for the extra salt added in chlorinating liquid and bleach.