Lowering alkalinity raises TDS - What is it?

[chem geek]

Tom,

I'll oversimplify this a bit, but essentially all that the lowering alkalinity procedure does is to replace bicarbonate ion ( (HCO3)- ) with chloride ion (Cl-). Technically, the TDS in terms of numbers of molecules of dissolved solids doesn't change at all, though in terms of the normal ppm (mg/liter) measurement it actually declines because chloride has a lower molecular weight than bicarbonate.

Why did you say that lowering alkalinity raises TDS? Why did you think that was the case? If the TDS is measured using a conductivity meter then it will measure slightly higher conductivity and report higher TDS because of my oversimplification since some of the carbonate is in the form of the neutral carbonic acid H2CO3 and does not get measured in conductivity, but this is a failure of the conductivity test to accurately determine TDS since it does not account for any neutral (uncharged) substances.

Since there will already be sodium ion in the water from the sodium bicarbonate that was initially added, removing the alkalinity will simply result in salt (NaCl) dissolved in your water. In fact, if you start with pure water and add calcium chloride and sodium bicarbonate and a little acid so that you are at a starting point with pool water, then if you remove most of the alkalinity you will end up with the calcium chloride still there plus sodium chloride. This of course ignores any "organic junk" that gets into your pool water and doesn't fully oxidize (hopefully most of it does).

Oh, I just realized that you are probably starting with some well water source that is high in TA, rather than starting with pool water. So the basic principle is the same, but the well water probably has significant amounts of calcium and magnesium in addition to high TA. So when you lower the TA you will produce salt (NaCl) dissolved in the water, but there will also be calcium and magnesium leftover as well and they will be charge balanced with choride ion (usually the calcium and magnesium original come from dissolved calcium carbonate and magnesium carbonate from rock that contains such substances).

Richard

[aquarium]

Why did you say that lowering alkalinity raises TDS? Why did you think that was the case? If the TDS is measured using a conductivity meter then it will measure slightly higher conductivity and report higher TDS because of my oversimplification since some of the carbonate is in the form of the neutral carbonic acid H2CO3 and does not get measured in conductivity, but this is a failure of the conductivity test to accurately determine TDS since it does not account for any neutral (uncharged) substances.

Thanks Richard,

Yes, I'm using an electronic TDS meter which uses a 0.5 conversion ratio from microsiemens. Our water starts out very hard at 430ppm calcium hardness and 230ppm alkalinity. The initial TDS measures at 360ppm (720 microsiemens) and then goes up to 460ppm after lowering the alkalinity using acid/aeration.

There are some aquatic plants that -may- be sensitive to higher TDS, but nearly all of them do better with lower alkalinity. I've linked PoolDoc's thread here to threads in some of the aquarium forums, and one response was about the higher TDS. So I was just curious if it was something that could be mitigated. Sounds like not, but not to worry about it either?

Note to admin: This can be moved to the China Shop if you wish, I just thought it should start here on topic.

EDIT:

Oh, I just realized that you are probably starting with some well water source that is high in TA, rather than starting with pool water. So the basic principle is the same, but the well water probably has significant amounts of calcium and magnesium in addition to high TA. So when you lower the TA you will produce salt (NaCl) dissolved in the water, but there will also be calcium and magnesium leftover as well and they will be charge balanced with choride ion (usually the calcium and magnesium original come from dissolved calcium carbonate and magnesium carbonate from rock that contains such substances).

Yes, we are on a rural water system in Central Texas that draws from wells in limestone aquifers.

[chem geek]

Since the TDS you would add to your aquarium is essentially salt (as calcium chloride and sodium chloride), then you should look at this as simply adding salty water to your aquarium (i.e. adding salinity). Obviously, if your aquarium is already salt-water, then this should not be a problem, but if it is fresh water, then the salinity may not be good for plants and fish and that seems to be the primary issue here.

It is not easy to remove the resulting salt as you would have to do something like reverse osmosis or distillation to do that (like a desalination plant). You could also change the calcium to sodium using an ion exchange resin as found in water softeners if calcium where a problem. Probably the easiest way for you to get purer water for your aquarium is to buy distilled water or at least bottled water which should also be low in salt. I suspect that it might not be that much more expensive given how much it would cost for you to do your own "pressured" reverse osmosis or "heat" distillation.

Richard

[aquarium]

Thanks Richard!

A little bit of salt is okay. I have a reverse osmosis system that I use for evaporation replacement water, but it's looking like this is the easiest way to make larger quantities of aquarium water. My apologies for going off-topic poolwise, but the water parameters issues are similar.