Hi all,
You do need to buy or make standard buffering solutions for your pH meter, usually pH4 and pH7:
For pH7 buffer "Add 29.1 ml of 0.1 molar NaOH to 50 ml 0.1 molar potassium dihydrogen phosphate."
or "dissolve 1.20g of sodium dihydrogen phosphate and 0.885g of disodium hydrogen phosphate in 1 litre distilled water." For pH4 "Add 0.1 ml of 0.1 molar NaOH to 50 ml of 0.1 molar potassium hydrogen phthalate." So probably easiest to buy the solutions made up.
In de-ionized water pH doesn't really have any meaning as pure H2O has no buffering, any addition of acid or alkali will cause the pH to drop or rise to extreme values. In this case it is the buffer that counts, via James' C Planted tank "6.0g of Sodium Bicarbonate and add to 5 litres of deionised water to make a 40dKH solution. Mix 10ml of the 40dKH solution with 90ml of deionised water to give you 1 litre of 4dKH reference solution." Sodium bicarbonate is a base, so that this solution would be alkaline, you also add the pH indicator "Bromothymol blue", which is yellow below pH6, blue above pH 7.6.
http://en.wikipedia.org/wiki/Bromothymol_blue
The drop checkers solutions pH range around pH7 happens because you have "buffered solution", in this case the acid and "conjugated base" are carbonic acid and bicarbonate, and the reactions are:
NaHCO3 = Na(+1) + HCO3(-1) dissolved in water
HCO3(-1) + H2O = H2CO3 + OH(-1) dissolved in water
H2CO3 = H2O + CO2 dissolved in water
Both of these reactions are at equilibrium and reversible. If you add components to one side of the reaction, you drive it in the other direction, so if you add CO2 to an aquarium the pH will fall and the pH indicator will change colour. This is also why hard water resists pH changes much better than soft water, and we re-mineralise R.O water to 4dKH to achieve a much larger reserve of buffering (or carbonate hardness - KH).
cheers Darrel