Hardness, PH, and Co2

[urville]

okay just verifying info please.
much appreciated.
i read an article about this and it's my understanding that, co2 can be dangerous only when it lowers the ph of harder alkaline water because no buffering is taking place. but since i have soft water already at gh-6 i'm actually at risk because i dont have the calcium buffering. so if i lower my ph by adding co2 i'm buffered already in that acid zone. gh-6 and kh-5

where if i used peat to lower my ph i will bring my gh down too and i'm already soft. it would be disastorous....
right? :? :?

 

[jsoong]

Er, I think you got it backwards. Harder water has more buffering capacity (higher KH), & so should have less problems with pH swings. Therefore, it is generally recommended that you do not inject CO2 unless your KH is adaquate (at least 4+).
So, CO2 is dangerous in soft water, as it induces wide pH swings, but is an excellent way of lowering pH in harder water.

Also, water is buffered by carbonates (or actually bicarbonates), not Calcium. This is measured by KH, not GH. Although, usually, hard water with high GH will have a correspondingly high KH; this is not always true.

A useful website about all this is:
http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm

and also the KRIB FAQ:
http://fins.actwin.com/mirror/begin.html

 

[urville]

rellay?
it was my understanding that gh is the measure of hardness or calcium and magnesium salts in the water and that
kh the measure of carbonate hardness or amount of sodium bicarbonate aka baking soda also related to co2 and the waters ability to absorb it
also that buffering happens both in the alkaline or 7.1 to 10.0 range of ph
AND the acid or 1.0 to 6.9 ph range 7.0 being neutral so the water is either buffered alkaline or buffered acid for instance Kent PH Minus has Buffered acid hydrogen sulfates...

this is why it seemed to me to fit right in that lowering ph with say biphosphates would lower the ph but not buffer it acid leaving it prone to swings, or say having alkaline buffered water but lowering the ph by co2 if the co2 quits it will just run back up to a high ph because the water wasnt buffered for acidity... thats why i thought people used peat to soften water and acidify because it will buffer that water to the acid spectrum both for biotope tanks like discus, AND so that loss of co2 for any reason wouldnt so heavily affect the ph...rather than just lowering the ph like co2. adding acids but not buffering to the acid side.

chemists anyone?

 

[urville]

okay so i read some more and the only thing i can definitly see i got wrong was that kh is related to alkalinity, your right. higher kh means higher ph.
so hers my problem i have a ph of 7.4 and a kh of 5 i use DIY Co2. i just took off the power filter and media i was using from another tank to seed bacteria, once my mini cycle from the tank change over to a lerger tank was over, i took it off.

the question is at one bublle every 1 to 2 seconds can i acheive 15ppm co2 in this 30 gal. and how quickly will the ph change at that rate i could not find a difinitive answer to this so i'm calling on experience of others here.... and still is my ascertation of buffering correct or not?
oh i use the water intake of a powerhead to diffuse the bubbles and puch them around what i noticed is bubbles are dissolving but not all of them... this worries me, has anyone used this method and had any experience on that?

 

[BrianNY]

The chart jsoong linked above is excellent. You can see urville, that at your current pH and kh, you've got very little CO2 in your water. You'll need to increase the bubble count keeping a close eye on the ph. Yep, it's a balancing act. IME the pH will drop slowly. It wouldn't be a violent swing with gradual adjustments.

HTH

 

[urville]

am i right about the ph or an imbecile?

 

[jsoong]

am i right about the ph or an imbecile?

No one is an imbecile when he/she asks questions!

You have some of the concepts, but not quite right. Here's some basic chemistry <sorry, to understand the stuff, you need to get technical>

CO2 + H2O -> H+ + HCO3-

CO2 added to water makes hydrogen ion (acid) and bicarbonates.

This is a reversible reaction (ie goes forward & backwards). Not all the CO2 added will turn into acid. The exact amount is determined by the Ka (association constant) & defined by the Henderson-Hasselbach equation.

If you just look at the equation, I think you can intuitively see that the more HCO3- you have in the system, the less CO2 will turn into H+ & HCO3- , you can think of it as the HCO3- pushing the reaction backwards.

Thus, the more HCO3 you have (HCO3 = KH), the less likely CO2 is to react to form H+, giving you less H+ & less of a pH drop.

Similar thing happens when you add other acids - the HCO3 mops up the H+, reducing your pH change. This is what is meant by buffering. Incidentially, HCO3 also blunts any pH rise ... buffering works both ways.

The bicarbonate system is the major buffer in water, but is not the only one. Any acid (actually, the associated base) can act as a buffer. Each buffering system has an intrincic balancing point, where the pH will be if the water is saturated with the stuff. HCO3 happens to drive pH towards high 8's. Phosphates will tend to drive the pH to 6's (that's the stuff in pH down).

In the usual aquarium, there is far more bicarbonates than any other buffers. That is the reason pH down & the like will not work well in altering pH. You have to add a ton of the stuff to overwhelm the native buffer (HCO3), anything short of that will cause pH swings.

Therefore, the only safe way to change pH is: add HCO3 (or CO3 eg limestone, coral, which will be converted to bicarb) to raise pH, or remove HCO3 (by peat) to lower pH.

CO2 will drop pH, but not by too much in system with any amount of HCO3. Usually, people add CO2 for the plants, the lowering of the pH is a side effect (which may be desirable for certain fish). If you have to do a BIG pH change with CO2 alone, you will need to add so much CO2 that the CO2 level will become toxic to the fish ..... so in that case, you use peat, reverse osmosis, or other method.