# An introduction to Ionic Strength

### Background

To quantify the effect of inter-ionic interactions, such as molecular attraction and repelling, one has to have a `parameter` describing themagnitude of these forces and how they alter the way certain calculation are done.

 Topic: Equilibriums The pH scale

The ionic strength (I) of a solution is calculated from all theions `i` in the solution and as follow:

I = ½ .∑ [i] . z2

This equation states that ionic strength is calculated as the squared sum of the valence and concentration of all ions in the solution.

The concentration metric used is usually the molar concentration (mol/L), but the most accurate is actually to use the molal concentration (mol/kg).In most cases it is easier to use molarities and in most cases the difference is not that important.

Ionic strength is used to correct equations used in e.g. pH calculations and to get more accurate estimates of concentrations of ions and compounds inequilibriums.

 Topic: pKa & Ka The basics

The relationship between the concentration [H+] and activity of e.g. the hydrogen ion {H+} is:

fH+. [H+] = {H+}

In this equation fH+ is the individual activity of the H+ ion and [H+] the concentration of the hydrogen ionand {H+} the activity of H+.

The calculation of fH+ is relatively straightforward. It is however not entirely clear which formula should be used for the calculationof fH+, ionic strength etc. in solutions that are far from ideal, since there is no consensus about which equations are true underwhich circumstances.

 Topic: Ideal Gas Law Ionic Strength

So if you are working with wastewater, or any biological system, you will have some trouble finding the right equation to use, and arhiing why you’ve pickedany equation for your calculations.

For this example I'll assume that the Güntelberg equation for calculating individual ion activities can be used to describe the relationship between[H+] and {H+}.

The Güntelberg equation states that:

log10 fH+ =-0.51.z2.(√I / (1 + √I))

Let us in this example consider a 0.1 M solution of MgSO4 with a pH of 7, meaning that the activity of the H+ ion or {H+} is10-7 . We start by writing down the concentration of ions in the solution:

 Ion mol/L Mg2+ = 0.1 M SO42+ = 0.1 M H+ = 10-7 M

By using the equation for calculating ionic strength we get that I ≈ 0.40. In these types of calculations the concentration of [H+] initself is often discarded as it only accounts for a fraction of the total ionic strength.

If 0.40 is entered into the Güntelberg equation and the necessary rearrangements are done, fH+ is calculated to approx 0.634 ora fH+ of 10-6.80.

This value is used to calculate [H+] from {H+}.

Still the pH value is 7 as {H+} is 10-7 as measured with a pH-meter. However, if the concentration of some compound in aequilibrium is calculated a value of [H+] of 10-6.80 should be used.

### Disclaimer

Ionization constants or dissociation constants used in equilibrium calculations also depend on ionic strength and should be recalculated as well. See ionicstrength and ionization constants.

NaHCO3 dissolved in water - how is pH calculated? Background about buffersA buffer is a solution that has the ability to keep pH within a certain and narrow range: even if an excessof hydrogen ions (H+) is applied to the solution. In other words:buffers resist changes in pH whenreasonable amount of either H+ or OH- is added to the buffer solution.Th...

Buffers and Equilibriums As shortly explained in the section about how to calculate pH in a solution of dissolved NaHCO3, a buffer has the capability of keeping pH withina certain and narrow range even if an excess of hydrogen ions H+ or hydroxide ions OH- are added.In this particular page I will elaborate a bit on why buffers actually have the ca...

Acids and bases - Broensted-Lowry definition A more fundamental definition of acids and bases than the one provided on the index page was given by the Danish chemists Johannes Broensted.The definitions are:A Broensted acid is a proton doner.A Broensted base is a proton acceptor.The English chemist, Thomas Lowry, proposed exactly the same at roug...

pH meter pH strips BackgroundpH meters have been used since 1936 when they were invented by the Danish company Radiometer.In contrast to many other measuring devices pH meters do not measure concentrations, rather they measure activities. Tobe more specific the pH meter does not measure the concentration of hydrogen ions.pH meters are used to ...