pH Values of various acids 5

[micalbrch]

Dear All,

I hope that somebody can help me.

I'm looking for a chart/graph/table which shows the pH values of some inorganic acids vs. the corresponding acid concentration in percentage. I'm looking after it for some weeks now but haven't found anything yet. The acids for which I need such a comparison are:
Nitric Acid
Hydrochloric Acid
Sulphuric Acid
Phophoric Acid

I appreciate any positive feedback. Thanks in advance!

 

[dcasto]

try this

http://www.sensorex.com/support/education/pH_calculator.html

 

[micalbrch]

dcasto: Thanks! It is not what I was looking for but it is at least a step forward and better than anything I found so far.

 

[bchoate]

micalbrch

Let's see what you are after. The sensorex app requires molar (mole solute/liter solution) concentrations. The acids of interest include both strong (sulfuric, hydrochloric, and nitric) and a weak acid (phosphoric)
Assuming that you know the definition of pH:
pH = -log(H3O+) and is constrained to the range of 0 to 14.

pH does not apply to concentrations > 1 molar for strong acids. 1 M gives a pH of zero.

Hydrochloric and nitric acids are monoprotic (one hydrogen). Concentrated HCl is 37% w/w and concentrated nitric acid is 63-70%. The following shows some data for the two acids. It is assumed that the specific gravity of dilute solutions is 1.

molarity %HCl w/w pH
1 3.7 0
0.1 0.37 1
0.05 0.18 1.3
0.001 0.037 2

molarity %HNO3 w/w pH
1 6.3 0
0.1 0.63 1
0.05 0.31 1.3
0.001 0.063 2

Sulfuric acid is diprotic, two hydrogens. The first dissociation is complete but the second dissociation is that of a weak acid and does not impact the pH.

molarity %H2SO4 w/w pH
1 9.8 0
0.1 0.98 1
0.05 0.49 1.3
0.001 0.098 2

Phosphoric acid is a triprotic acid and is a weak acid. The dissociations are not complete and the pH reflects the hydronium ion concentration calculated from the dissociation constants. The following table presents some data for phosphoric acid.

molarity %H3PO43 w/w pH
10 65 0.58
5 39 0.74
2 19 0.94
1 9.8 1.098
0.5 4.9 1.26
0.2 2.0 1.48
0.1 1.0 1.65
0.01 0.1 2.30
0.001 0.01 3.15

Molar concentrations of concentrated acids are Hydrochloric (11.8), nitric (14), sulfuric (7.1, 50%), and phosphoric (14.6)

 

[MortenA]

micalbrch are you a chemical engineer? If not: Be a little carefull here. This is something that chemical engs spend some time on - that is easy to forget (sic).

If you are not a chemical eng. then i would advice you to seek assitance beyound Google and the friendly posters at this site! Once you get to mix weaker acids or add other chemicals it can get a little complicated.

Best regards

Morten

 

[micalbrch]

MortenA: No, I'm a mechanical engineer. I had chemistry at the university but that was 20 years ago. I'll be careful, be assured. I'm just looking for a kind of "feeling". When I read a phosphorid acid concentration of 10 %, I'm not sure whether the pH value is more 1 or more 4 (as an example).
Thanks "bchoate". That is the kind of "feeling" I was looking for.

 

[25362]

Bchoate, can you verify and confirm the pH values for 0.001 N dilutions ?

In general, the figures generally quoted for dilute solutions are only approximations based on the equation: pH = -log[sub]10[/sub][A], where [A] is the molar concentration of the monoprotic acid assumed totally dissociated at 25[sup]o[/sup]C.

The pH is defined as the negative log[sub]10[/sub] of the activity, not the concentration, of the hydronium ion. The activity itself is the product of the ion's concentration multiplied by a correcting factor named coefficient of activity.

For example, the CRC Handbook gives the following activitiy coefficients for HCl and HNO[sub]3[/sub]:

molal activity coefficients @ 25[sup]o[/sup]C
concentration HCl HNO[sub]3[/sub]

0.001 0.965 0.965
0.01 0.905 0.905
0.1 0.797 0.792
1.0 0.811 0.730
5.0 2.380 1.063
10.0 10.4 1.644

Meaning the above equation for lower than 1.0 molar solutions is just an approximation and the actual pH values are a bit greater than those estimated by the equation.

Please note the table refers to molalities, which at very dilute solutions, are about equal to molarities. Also note the 'anomality' of the activity coefficient at higher concentrations.

For weak acids and bases the dissociation is far for complete, thus the pH values need a more complex estimation based on the actual measured dissociation constants as explained by MortenA.

 

[bchoate]

micalbrch
25362 thanks for your comments. The info I posted was a simplified version because I suspected micalbrch was not a chemical engineer and maybe not totally comfortable with info for the posted discussion. It did seem that the Sensorex link did not completely answer his question because it dealt with concentrations in molarity. That is why I posted a simplified generic reply that illustrated trends with concentration in %w/w. Nonetheless a complete discussion of this topic is beyond most readers of these posts and would consume an inordinate amount of space. This post is to take this discussion a little farther to clarify some points and bring a little closure.

First there is a freeware calculator which will do all of the calculations necessary including activity coefficients.
[ww2.iq.usp.br/docente/gutz/curtipot.html]

Second, there are five concentration definitions that are used with solutions. Two, %w/w and %v/v, are self descriptive and need no further discussion.

molality (m) - moles solute dissolved in 1 kg. of solvent. activity coefficients are often described in terms of molality.
molarity (M) - moles solute in one liter of solution.
normality (N) - number of gram equivalents of a solute per liter of solution. Normality is an applicable construct in the discussion of reactions.

Considering pH of aqueous solutions of monoprotic acids -- hydrochloric and nitric acid. An 0.5 molar solution hydrochloric acid has a pH = 0.3 using the simplified definition I posted previously. Incorporating the activity coefficients, the pH is 0.44. Similarly the pH of a 1.0 molar solution is 0 or 0.1, respectively. Either way the pH / molarity relationship is demonstrated. There are no thermodynamically sound methods to measure activity coefficients. Activity coefficients are calculated from theoretical bases such as the Davies equation. The link above will show both pH values and the activity coefficient used.

Sulfuric acid has two protons and dissociates in two steps. The intermediate species does not dissociate to any great extent and the pH is dependent upon the first dissociation step alone. The pH of a 0.5 M sulfuric acid solution is 0.3 or .47 (using activity coefficient = 0.6713). Sulfuric acid is not in the freeware above.

Sulfuric acid becomes oxidative at a concentration greater than 2-3 M. Nitric acid becomes oxidative above 3 M.

Phosphoric is a triprotic weak acid. It's pH is largely represented by the first dissociation stage. An initial estimate of the hydrogen ion concentration is given by he following:
[H+] = SQRT (K(A1) * acid concentration)
An 0.5 M solution has an estimated first step hydrogen ion concentration of 0.059 M. The estimated pH is 1.23 or 1.32. The app from above gives 1.16 and 1.25 using all the dissociations.

 

[25362]

I feel the need to revert to the concept of activitiy saying that, in short: activities are, in fact, 'effective' concentrations.

In a previous post bchoate has stated "pH does not apply to concentrations > 1 molar for strong acids."
IMO this means that the resulting pH values at these concentrations are negative (pH<0). This happens because at higher concentrations, activities can depart a lot from 'analytical' concentrations with activity coefficient values far greater than 1.

For example, at, say, 37% mass (~12 M) HCl has a quoted activity coefficient = 207. Under these conditions the 'effective' concentration of H[sup]+[/sup] is not the 'analytical' 12 M, but ~ 2500 M (12[&times;]207) pointing to a pH [&cong;] -3.4. The current explanation: the formation of water-captured ion hydration shells and the electrostatic 'repulsion' between the ions.

On the other hand, using similar arguments, a 0.1 M HCl (0.37 mass %) solution shows an activity coefficient of ~ 0.8. It doesn't mean that the acid is just 80% ionized. It means that 20% of the [H[sub]3[/sub]O[sup]+[/sup]] and [Cl[sup]-[/sup]] ions have formed complexes in which oppositely-charged species are loosely 'bound' by electrostatic forces.

At ionic concentrations lower than 0.001 M, activity coefficients approach 1.0, and concentrations can generally be used instead of activities.

 

[BRT549]

Thanks for the great link, dcasto. Gave you a star.