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Confused about pH and GH

idris

idris

Member
Joined
3 Jan 2011
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816
Location
Herts
1st point for confusion: There seem to be a lot of references to Alkalinity (as measured by KH test kits) as if it something entirely different to Acidity and pH. From what I remember of A-Level chemistry (albeit 20 years ago), Alkalinity and Acidity are just the opposite ends of the pH scale. 1-6 = Acid, 7 = Neutral, 7-14 = Alkaline.
So surely an Alkalinity Test Kit is just a pH Test Kit?
Is the term Alkalinity being used to mean somehting different? And if not why does it seem to be refered to as something different to pH?

2nd point for confusion: I have very hard water. I'd like to reduce the hardness of the water going into my 250L tank. For various reasons neither boiling, rain water, buying RO water from an LFS, nor using an RO filter are easy options and in the short term I'm really not sure I can justify preasuised CO2. I understand that ion exchage softeners aren't a great idea either. So what options does that leave me?
(Appologies if this is answered elsehwere - I've done some reading but the solutions offered are generally those listed above.)

FWIW There is part of me just thinks to hell with it - I might as well fill the tank up ans see what dies ... but I'd rather not ;)
 
pH is the measure of acidity and alkalinity. 0 is very acidic, 7 is neutral, 14 is very alkaline. For tanks we go from extremes of around ph 5 as very acidic (though I've run tanks lower than this!) to around pH 9 as very alkaline.

Hardness is broken into two components called Carbonate Hardness (KH) and General Hardness (GH). You can also measure Total Hardness or the Total Dissolved Solids (TDS) using a meter.

KH measures the amount of carbonates in the water. Carbonates can be thought of as a buffer to acidity and they help to stabilise the pH. Think of them as the anti-acid tablets you take for indegstion; they will absorb the acidity present in the liquid.

GH is the general hardness and is the measure of the common metals in the water - specifically Calcium and Magnesium. These are the most common salts (with carbonate ions) in tap water so give a fairly accurate measure for most of our purposes.
 
Thanks for that Ed but (don't take his as ingratitude) it doesn't actually address either of the questions I asked.
 
idris said:
Thanks for that Ed but (don't take his as ingratitude) it doesn't actually address either of the questions I asked.
Click to expand...

Good point! Sorry about that!

Basically Alkalikity isn't the best name for a KH test kit and really it should be called a KH kit. A pH test kit measures acidity and alkalinity, yes.

Once you understand what GH, KH, pH and TDS mean then you can see that ion exchange softeners don't really reduce the hardness, they just switch calcium ions for sodium ones that don't register on the GH test kit. If you want to meaningfully reduce your water hardness then you need to reduce the TDS and to do that you need to get some water with lower TDS or use something that reduces that.

Boiling reduces the KH not the GH so won't help much either. And CO2 will not reduce the hardness, it will only lower the pH so won't help at all - it is excellent for helping the plants to grow though.

Most people on here don't soften their water and their plants grow fine so I'd go ahead and see what happens.
 
Thanks again Ed.

So other than RO and rain water are there any real ways to reduce hardness? I had read that hardness wasn't a problem for the plants but I was thinking of softening the water for the fish. (I'm not interested in Cichlids ;))
 
you can use peat to soften your water slightly, the humic acid binds the positively charged calcium and magnesium ions, and exchanges them for positively charged hydrogen ions, this also decreases the ph. this does have the side effect of staining your water yellow, but the tannins and humic acid compounds are good for fish. you need to keep replacing the peat as this process slows down and stops over time.

you also get water softening pillows, they work by switching out calcium and magnesium ions for sodium or potassium, i would stay away from the sodium ones...

if you are really worried about it then invest in a ro unit...
 
Hi all,
The terms all relate to one another, but are expressed in different terms.
pH
Is a bit of a funny measurement. Because acidity/alkalinity is difficult to calculate, or compare (without using titremetric methods) the pH scale was developed which acts as a proxy, by expressing acidity/alkalinity in terms of the equivalent activity of H+ ions (also called "protons", and actually in the form of hydronium ions H3O+)

I'll use Wikipedia for this as it covers the important bit fairly succinctly.
"In a solution pH approximates ... to p[H], the negative logarithm (base 10) of the molar concentration of dissolved hydronium ions (H3O+); a low pH indicates a high concentration of hydronium ions, while a high pH indicates a low concentration. Crudely, this negative of the logarithm matches the number of places behind the decimal point, so for example 0.1 molar hydrochloric acid should be near pH 1 and 0.0001 molar HCl should be near pH 4 (the base 10 logarithms of 0.1 and 0.0001 being -1, and -4, respectively). Pure (de-ionised) water is neutral, and can be considered either a very weak acid or a very weak base (centre of the 0 to 14 pH scale), giving it a pH of 7 (at 25 °C (77 °F)), or 0.0000001 M H+.[1] For an aqueous solution to have a higher pH, a base must be dissolved in it, which binds away many of these rare hydrogen ions. Hydrogen ions in water can be written simply as H+ or as hydronium (H3O+) ions."

Acidity
"An acid is a substance which can act as a proton (H+) donor. Most acids encountered in everyday life are aqueous solutions, or can be dissolved in water, and these two definitions are most relevant. The reason why pH of acids are less than 7 is that the concentration of hydronium ions is greater than 10-7 moles per litre. Since pH is defined as the negative logarithm of the concentration of hydronium ions, acids have pH of less than 7.

Alkalinity or basicity
"A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions (OH-) quantitatively. The Brønsted-Lowry theory defines bases as proton (hydrogen ion H+) acceptors".

So out of all that the important bit is acids are H+ donors and alkalis are H+ acceptors.

This is why "wearsbunnyslippers" is 100% correct when he says "the humic acid (from the peat) binds the positively charged calcium and magnesium ions, and exchanges them for positively charged hydrogen ions ......decreasing the pH. " So this is an acid base reaction. dGH is the measure of the divalent cations (Ca2+ and Mg2+), from the peat reaction you can see that if you have water with a high dGH (conc. of Ca and Mg ions), you need a lot of peat to donate enough H+ ions to accept all the Ca/Mg 2+ ions. This process doesn't greatly effect the TDS, all you have done is replaced one sort of compound with another. Probably this is best shown by an image (NaCl/H2SO4 can be any salt/acid):

Dia_1.jpg


These are general acid base interactions, but we can then look at a specific acid / base pair, the pairing that is probably most relevant to us is the reaction between CO2 and carbonates, this is slightly complicated by the disassociation into bicarbonate and carbonate, but starts with
CO2 dissolving to form carbonic acid (H2CO3):

CO2 + H2O is in equilibrium with H2CO3.

You can drive the equation in either direction by adding either CO2 or a source of carbonates:

CO32- +2 H2O ? HCO3- + H2O + OH- ? H2CO3 +2 OH-
&
H2CO3 +2 H2O ? HCO3- + H3O+ + H2O ? CO32- +2 H3O+

In your very hard carbonate rich water (water with a high dKH "K(c)arbonate Hardness") you have a large supply of carbonate, this is "buffering", the potential to neutralise acids (or technically to accept H+ ions). The dKH is from the carbon dioxide (CO2, but dissolved as H2CO3 in rainwater) reacting with limestone (calcium carbonate (CaCO3)) to form soluble calcium bicarbonate (Ca(HCO3)2) and giving you a large reserve of carbonate buffering in the water.

Neither fish or plants are entirely problematic in very hard water, if you think of the English Chalk Streams they are species rich in both plants and animals. Other than fish from the Rift Valley Lakes, there are plenty of live bearers, CA cichlids, Rainbow fish etc that thrive in hard water. I'm not an expert in these at all, but this looks a reasonable list:
<http://www.wetwebmedia.com/fwsubwebindex/fwhardness.htm>.

cheers Darrel
 
Oh b*****x! I knew someone was going to beat the c**p out of me with some full on chemistry ;)

Unfortunately I'm still not clear whether I've got any options (other than those I've listed as difficult for me) to soften my hard water. From the above it sounds like peat will help (wearsbunnyslippers), but at the same time it won't really do any good (dw1305). :crazy:
 
Unless you are really looking to keep/breed some particular type of fish that needs soft water then I would not worry about it too much. London water is also very hard and we manage to grow nice tanks down here and have good colonies of shrimp breeding ;)
 
@LondonDragon - we used to live in N London and got water from the R Lea which is chalkstream so should be pretty hard, and we certainly got a lot of scale in the kettle.
According to Thames Water the hardness is 289 mgl CaCO3. Where we are now is supposed to be 348, so even harder than London. I'm just looking for an easy way to get it so it's not so hard the fish have to use a hammer to drink their tea.
 
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