A pH meter measures the hydrogen-ion activity in water-based solutions.

I have stressed "activity" because the relation shown in the attachment is based on theory and a fix temperature.

I am thinking use pH instead of pOH and the relation between pH and Ka (acid constant) There is a relatinoship btween pH and PKa and the HA and H+ concentrations.

Thank you Roberto Molteni Jose A. Garcia-Regueiro for your answers.

They have made me rethink my problem and now realise what I was asking is how to calculate the amount of strong acid used in a titration of a strong base past the equivalence point to an acid solution.

The amount of acid needed is the moles to take the solution from the old pH to the equivalence point, plus the moles needed to take the solution from the equivalence point to the new pH.

In other words, the amount needed is [H+] + [OH-], not [H+] - [OH-], where [H+] is 10-newpH and [OH-] = 10-(14 - oldpH).

The expression «10-pOH - 10-7 + 10-pH - 10-7» (units omitted) is not dimensionally correct.

For the initially basic analyte solution, as acid titrant is added, we have:

initial (analyte) solution → titrated (considered neutral) solution → final acidic solution

As long as I could understand from the enunciate:

Units of 1st parcel, 10-pOH: mol OH-/dm3 of initial (analyte) solution

Units of 2nd parcel, 10-7: mol OH-/dm3 of titrated (considered neutral) solution

Units of 3rd parcel,10-pH: mol H3O+/dm3 of final acidic solution

Units of 4th parcel,10-7: mol H3O+/dm3 of titrated (considered neutral) solution

Suggested calculation:

Note that the neutral salt ― obtained from the titration of a strong base with a strong acid ― is not expected to significantly contribute for final pH.

Excess volume (Vexcess) of monoprotic strong acid (HA) aq. sol., of nominal molar concentration (formality to be precise), cHA, is added to a neutral salt aq. sol., so that the excess acid ― not consumed by the acid-base reaction ― becomes diluted by the dilution factor f = Vexcess / Vfinal.

The pH of the final acidic solution would be: pH = -log10(f·cHA/(1 M)).

Hence: Vexcess = Vfinal·f = Vfinal·10-pH/{cHA/(1 M)}.

Carlos Araújo Queiroz

Thank you for your reply.

However, my problem is not with a solution of a neutral salt, but rather with the solution of a basic salt i.e. ocean water with a pH of ~8.0. I want to know how much (monoprotic) acid I have to add to lower its pH to 4.5 thus making the solution acidic (not neutral.)

I reckon I first have to make the solution neutral by adding acid to change the pH from 8.0 to 7.0. That is change the pOH from 6.0 to 7.0 and the [OH-] by 10-6 - 10-7.

Then I second have to make the solution acidic by adding acid to change the pH from 7.0 to 4.5 and the [H+] by 10-4.5 - 10-7.

Therefore the total amount of acid that has to be added is:

10-6 - 10-7 + 10-4.5 - 10-7

= 10-6 + 10-4.5 - 2x10-7

= [OH-] + [H+]ep - 2x10-7.

where [H+]ep is the concentration of hydronium ions at the end point and [OH-] is the concentration of hydroxyl ions at the initial point.

At present titration alkalinity/total alkalinity, which is the moles of acid that has to be added to a sample such that it reaches the equivalence point of carbonic acid is stated (in moles per litre) as:

AT = [HCO3-] + 2[CO32-] + [OH-] - [H+] + [other weak acid anions]

e.g. See page 145 of http://www-naweb.iaea.org/napc/ih/documents/global_cycle/vol%20I/cht_i_09.pdf

It should be stated as:

AT = [HCO3-] + 2[CO32-] + [OH-] + [H+]ep + [other weak acid anions] - 2x10-7.

Do you agree?

Dear Alastair:

i) Your last post seems to contradict the query enunciate. Carbonic acid is not a monoprotic acid, and, being this a weak acid ― while a basic salt (not neutral) is involved ― it can not be correctly stated that there are «no chemical buffers present», as said in the enunciate.

ii) That is why I was lead to think that the query dealt with the titration of a strong base with a strong acid, from which a neutral salt results, while there is no buffer system involved.

iii) If carbonic acid is present (or its salts; carbonate or bicarbonate), its two dissociation/acidity constants should generally be taken into account in what deals with pH calculation/prediction; except in specific simplifying conditions (related with pH range), where one constant can be enough.

iv) It seems (your post above) that conjugate bases of other weak acids may be also involved. That being the case; their hydrolytic equilibria should, in principle, be also accounted for.

v) Neither (free) H3O+ or OH- can generally be taken as conserved molar quantities for a mixing solution process accompanied by acid-base equilibria.

vi) Equations involved in a typical pH problem should in principle ultimately translate conserved quantities (such as atomic molar balances or charge balance) and/or equilibrium relationships between the actual (also called 'free') molar concentrations of the relevant chemical species. These concentrations are usually expressed with square brackets (without the subscript T, meaning total ― see § ix, below), as they account for speciation ― unlike nominal molar concentrations, viz. formality.

vii) Total alkalinity (AT) can be defined in terms of total concentrations of selected contributing chemical species (*). Subscript T (with square brackets) may unambiguously identify such concentrations, but is often omitted. These are not, necessarily, the actual ('free') molar concentrations referred to above (§ viii). Instead; they should be understood as measured/measurable concentrations, hence possibly not accounting for speciation. Particularly in oceanographic/hydrographic literature, [H+]T often collectively denotes the concentration of several proton-donor species, besides H3O+. Concentration units used with AT are often other than molarity.

(*) cf. https://en.wikipedia.org/wiki/Alkalinity

The answer to this question is situational. It is based on the amount and pH of either a base or an acid. The level of pH of an acidic substance should cancel out the pH of a basic substance with resultant pH values of an acidic substance remaining in that mixture. The amount of an acidic substance does not necessarily have to be greater than that of a basic substance. But the quantity has to lead to the cancellation out of the basic level of pH with resultant pH values of that acidic substance remaining in that mixture.