by "Grog" (Alan W. Grogono), Professor Emeritus, Tulane University Department of Anesthesiology

# pH Playgound (understanding pH)

### Definition of pH.

Caution!  If words like pH and logarithm intimidate you, skip this paragraph and go straight to the Playground below. The pH is the negative logarithm of the [H+] measured in moles per liter. A pH of 7.4 is equivalent to 40 nanoMols/liter [H+] (hydrogen ion concentration).

pH confuses many of us because we are not entirely comfortable with Logarithm. Moreover, as the acidity increases, the pH decreases. The Playground below is designed to take mystery out of pH.

### Try the pH Playground

Buttons: Click on any of the Blue Multiply or Divide Buttons. For example, the number 2 buttons double or halve the [H+] concentration and the number 10 buttons have a tenfold effect.

Watch the changes on the pH, the [H+] and the Units. In particular look at how cumbersome it is to express dilute [H+] concentrations in mol/L.

The Clinical Range is between about 80 and 20 nanomol/L (pH 7.1 to pH 7.7).

pH[H+]pH[H+]
-0.1 +0.1
-0.3 +0.3
-1.0 +1.0
-3.0 +3.0

# Change in pH and [H+]

pH =
[H+]=
or

Accuracy: This Playground uses approximate values to make it easy to visualize the concentration of Hydrogen Ions in a solution.

### Conquer pH Yourself.

 Number Log pH Concentration 1,000,000 6 -6 1 megaMol/L 1,000 3 -3 1 kiloMol/L 1 0 0 1 Mol/L 0.001 -3 3 1milliMol/L 0.000,001 -6 6 1 micMol/L 0,000,000,001 -9 9 1 nanoMol/L 0,000,000,01 -8 8 10 nanoMol/L 0,000,000,1 -7 7 100 nanoMol/L

Steps of 1000. First step down in thousand fold jumps (See Table on Left). Of course it is impossible to make such concentrated solutions as a megaMol/L, but most of us can remember the logarithm of a thousand and a million.

Starting Numbers: Next, write a list of pH values starting at 8.0 and finishing at 6.8 (See Table below). Below these numbers, write in the values for pH 8.0 and 7.0 from the first table. These numbers are starting points.

 8 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.2 7.1 7 6.9 6.8 10 12.5 16 20 25 32 40 50 64 80 100 128 160

Now use: x 2 = + 0.3  i.e., a log jump of 0.3 corresponds almost exactly to doubling. Halve 100 (50, 25, 12.5) and double 10 (20, 40, 80, 160) to obtain the numbers in the table. When you get to 160, pH 6.8 is a tenfold jump away from pH 7.8 (16 followed by 32, 64, 128). Notice that the final number you insert (128 for pH = 6.9) also demonstrates the slight error of this method. Compare it to the value for pH = 7.9 (12.5). The correct values are 126 and 12.6 - not errors which would be critical in medicine.