Hi all,
it is low conductivity that makes pH measurements challenging, not low alkalinity (although alkalinity can contribute to conductivity). A reasonable
guide to measuring pH in low conductivity samples suggests simply adding salt to a sample of the water being tested.
That works, it is <"
add a neutral salt"> (usually NaCl) method. This is because pH meters are a <"
special type of conductivity meter"> and the salt addition raises the amount of ions in solution (Na+ and Cl-), but these cancel out in terms of pH, which is why it is a "
neutral salt" <"
Aqueous Solutions of Salts">.
You can get around the issues of pH meters to some degree with a ISFET meter with REFET reference electrode, but they are expensive bits of kit - <"
Reccomended PH Probe">.
This is the best video I've found (for conventional pH meters).
measuring pH in a low alkalinity, that is, low amounts of hydrogen ions is exceedingly difficult with our common means of measurement (ie not lab grade equipment).. not even sure it actually make much sense to talk about pH in waters extremely low or absent of HCO3.
This is back to the <"
log base 10"> and <"
ratio nature of pH">. This is also why
@_Maq_'s Acid Neutralising Capacity (ANC) is a useful concept, it takes us out of ratios and
into amounts, and
I'm interested in amounts - <"
Some handy facts about water">.
To use an analogy, if you have a <"
two pan balance"> scale, with equal amounts of acids (proton donors) and bases (proton acceptors) it is always at pH7. It doesn't matter whether there is one molecule in both pans, or ten, or a hundred or a thousand etc. As long as the pans have equal amounts in each? The pH is always pH7, and that is the problem with pH interpretation, it tells us about ratios, not amounts*.
*Edit: ......... Just when you think you understand something the truth comes and bites you in the bum. I'll have to bin the balance pan analogy, I'd like to thank
@Andy Pierce for his clarification. I should also say
@Andy Pierce <"
is a real scientist">, rather than some-one <"
who just talks a good game">.
...... Although you can derive the ratio of [H+] and [OH-] if you know one of those values and the equilibrium constant of water, the main point is that pH is a measurement of [H+] concentration (full stop) so it is exactly true that in fact pH does tell you about amounts - it is the measure of the amount of protons per unit volume of solution. That is why the statement "pH interpretation, it tells us about ratios, not amounts" is a misinterpretation of pH.
.......the above statement is incorrect. pH is (by definition) the negative log10 concentration of protons (hydronium). There is no consideration as to the number or concentration of proton acceptors, other than indirectly through influence on the concentration of protons. If the number of protons changes by 10x, the pH changes by 1 unit..........
This is the ANC bit.
In fact, alkalinity or acid neutralizing capacity (ANC) refers to the amount of strong acid needed to change pH from current value to a different (lower) value.
ANC4.5 is used regularly in water processing plants’ reports, and it denotes ANC to reach pH value of 4.5 – which is a point where no bicarbonate ions (HCO3-) remain in water. ANC is not often used in the UK, but is a very useful concept.
We can find out about amounts if we titrate our water against an acid of known strength (if the water is basic) or base (if the water is acidic) and that will tell us out about amounts, and that
is what a drop checker does <"CO2 MEASUREMENT USING A DROP CHECKER">. In a drop checker we have a base of known strength (the 4dKH solution) with a narrow range pH indicator (bromothymol blue) and we then titrate our acid (the H+ ions from the carbonic acid (H2CO3)) against the solution in the drop checker and <"
use the colour change of the pH indicator"> (and our standard curve (the <"
dKH ~ CO2 ~ pH chart"> from
@Andy Pierce )) to define how much CO2 we've added.
Is it specifically hydrogen ions that need to be present, or any ions in general?
A bit of both. Around the neutral pH (pH 7) values are always going to be less stable, you need relatively small additions of acids or bases to lower or raise pH. At pH6 you have a ratio of 10 : 1 H+ : OH- and pH5 100 : 1 etc.
In some ways pH meters are useful for confirming what you already know (........
Amazing, this orange juice is acidic and this sea water is alkaline), but when you start diluting the orange juice and seawater with DI water and mixing them together, the pH (and the solution) becomes a lot more opaque.
cheers Darrel
