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**To**:**Aquatic-Plants at actwin_com****Subject**:**Re: KH-pH-CO2 charts****From**:**toado <toado at ihug_co.nz>**- Date: Wed, 16 Apr 1997 23:56:05 +1200
- References: <199704151939.PAA08386 at acme_actwin.com>

This subject has been thrashed recently, but I'd like to add a few comments. As others have pointed out, you unfortunately can't assume a given rate of CO2 addition will give the same CO2 level under different conditions. The CO2 dissolution rate varies with the pH, surface tension etc. After a while you achieve saturation, but what this concentration will be depends on a number of equilibrium reactions, chiefly the bicarbonate system. Thats where pH and carbonate hardness come in. In some situations, like when a non-bicarbonate buffer is present, the actual carbonate hardness can't be easily determined. If we could measure it accurately however, the chart _should_ still be accurate. You can largely account for the contributions of say phosphate buffers to alkalinty and so calculate the 'true' carbonate hardness. To be honest it simply isn't worth the effort :). The chart makes extensive use of equilibrium constants for the bicarbonate system and with them come a few assumptions. The most obvious is that they only (strictly) apply under equilibrium conditions. Few things ever are truly in equilibrium, this allows life to exist, but gives process engineers nightmares. We with our aquaria can assume the next best thing- a 'steady state' so this wont affect the chart noticably. The equations assume an 'activity coefficient' of one. I'm afraid chemists have found many cases where 'constants' based on concentrations aren't strictly constant. Rather than dismiss a concept as usefull as equilibrium constants all the 'concentration' terms get replaced with ones for 'activity'. This gives us a new fiddle factor to make things work :). Mostly the terms differ with highly charged ions like aluminium, or in solutions of high ionic strength- in these situations simple formulae exist relating activity to concentration. There are other exceptions but only in relatively esoteric situations like non-aqueous solutions. In aquaria all the concentrations are dilute, so I doubt this assumption has any effect. The equations assume there are no other equilibria _involving_ CO2 apart from the carbonic acid/bicarbonate/carbonate system. I cant easily see how say phosphate buffers could involve themselves here. My peat extracts do appear to involve themselves with CO2 equilibria- Ive a few guesses but to be honest I'm not exactly sure how :(. Whatever the chart gives you about the best estimate of CO2 levels you can easily obtain. More importantly I reckon it gives you a feel for what affects the CO2 levels- for aquarists thats really the bottom line. Toado. BTW- Dupla add a root growth (hormone?) tablet to the (extortionist) laterite kits they market here. Anyone know what the active ingredient is?.

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