Imagine my horror then last Sunday when I finally set up the entire system with solution - and added the aeration stone - when within hours the pH climbed from an initial 5.88 to 6.69. Huh?! I added more citric acid to bring the pH back down to 5.6. Again, within hours it had climbed back to 6.8. Left for 24 hours plus, it ends up at around 7.13 - and changing to a different reservoir makes absolutely no difference to this effect.

I then ran an overnight experiment. I made up fresh solution and divided it into four bottles. One bottle stayed in the kitchen, the three others went into the greenhouse - one also closed, one open and a third with an airstone bubbling in it. Next morning, the only bottle to show a pH change was the aerated one - up by several tenths of a unit. I've repeated the above experiment using both tap water, and Brita filtered water too (sadly). Aeration increase my pH by a lot - and citric acid doesn't help for very long.

My water is hard - around 100mg/l total hardness I think, and is probably the main issue, it has a pH out the tap of around 7.9. I came across greystoke's post below (sorry, don't know how to quote properly), and wondered whether anyone else has had success with his technique, i.e. pre-bubbling, then adjusting with vinegar, before adding any nutrients?

Really, any advice would be greatly appreciated!

**************************
* Posted by greystoke Mediterranean (My Page) on
Fri, Mar 14, 08 at 2:39

This may be useful although perhaps a little off-topic.

Here – in Africa – rain is at a premium, and many things are not readily available, so . . . we DIY a lot.

If tapwater is too hard and/or the pH too high we manage to deal with that with simple means.
Hardness is a result of the calcium and magnesium content of the water. It can be measured using a hardness tester bought at an aquarium shop. The test provides two measures:
GH (for General Hardness) is a measure for the total of calcium and magnesium content given in mg calcium oxide (CaO) per 100mL of water.
KH (for Karbonate Hardness) is a measure for the calcium and magnesium bicarbonate content. Also given in mg calcium oxide (CaO) per 100mL of water.
Naturally, the KH is part of the GH, and is always smaller, unless there’s a foreign bicarbonate in solution.
Bicarbonates are strong pH buffers which resist lowering of the pH.
The best way is to store the water under an aerator for a few days, and bring the pH down using a measured quantity of 5% white spirit vinegar. This will turn the bicarbonate ions into water and CO2 gas which will be driven off by the aerator. What’s left is calcium and magnesium acetate.
Quantity: 0.4mL vinegar per Litre of water, per degree KH hardness.
*****************************

Certainly, I've only been aerating my tap water for around four hours now, and the pH has already increased from an initial 7.98 to 8.39:

************************

H2O + CO2 « » H+ + HCO3-

Carbon dioxide, hydrogen sulfide, and ammonia are soluble in water under certain conditions to the extent of 1,700, 3,900, and 531,000 ppm, respectively. Rarely are these concentrations encountered except in certain process condensates. In a normal atmosphere, the partial pressure of each of these gases is practically zero. Consequently, the establishment of a state of equilibrium between water and air by means of aeration results in saturation of the water with nitrogen and oxygen and nearly complete removal of other gases.

As the equations above show, ionization of the gases in water is a reversible reaction. The common ion effect may be used to obtain almost complete removal of these gases by aeration. If the concentration of one of the ions on the right side of the equation is increased, the reaction is driven to the left, forming the gas. In the case of carbon dioxide and hydrogen sulfide, hydrogen ion concentration may be increased by the addition of an acid. Bicarbonate and carbonate ions in the water will form carbon dioxide, which can be removed by aeration.

In a similar manner, an increase in hydroxyl ion concentration through the addition of caustic soda aids in the removal of ammonia.

Gas removal by aeration is achieved as the level of gas in the water approaches equilibrium with the level of the gas in the surrounding atmosphere. The process is improved by an increase in temperature, aeration time, the volume of air in contact with the water, and the surface area of water exposed to the air. As previously indicated, pH is an important consideration. The efficiency of aeration is greater where the concentration of the gas to be removed is high in the water and low in the atmosphere.