Do it yourself Chemistry Hero Kit


How to dry-lab seemingly impossible problems


Figure 1. Equiligraph Template. Region 1 is the equiligraph. Region 2 helps draw the region around each pKa. Regions 3 and 4 are guides for drawing lines with slopes of +/- {1,2,3}.

I am going to show you how make a very simple set of equiligraph tools that will  help you dry-lab most of the problems you will likely encounter, ranging from simple pH problems to problems which seem so complicated that any attempt at analysis seems futile.

After you make this kit and learn to use it, as I will show you, people will think you are some kind of whiz-bang chemistry wizard. Please do not show them your kit, or this page. If you do, then they will begin to think that you are not quite azmazing as you really are!

We are going to make versatile, reusable equiligraphs for several common chemicals, which are used in the laboratory and can be used to represent, or simulate other chemicals you will encounter. 

In the process, you will learn how to make versatile equiligraphs for other, not so common, chemicals which you may be interested in.

We will make charts for:

  1. *Acetic Acid            CH3CO2H

  2. *Ammonia               NH3

  3. *Carbonic Acid        HCO(OH)2

  4. *Phosphoric Acid      H3PO4

  5. *Sulfuric Acid           H2SO4

Start by looking at the Creating an Equiligraph page.

Next get yourself  a pad of graph paper and some viewgraph transparencies.

Start by drawing an equiligraph template, as shown in figure 1.

There are 4 regions marked.

Region 1 is a normal equiligraph.

Region 2 is a guide to help draw the region near each pKa.  The horizontal line aligns with the CTotal line. The gap is 1.4 units wide with 0.7 units on either side of the vertical pKa line. The bottom vertical line begins at a point 0.3 units below the CTotal line and is on the vertical pKa line.

Regions 3 and 4 are guides for drawing lines with slopes of +/- {1,2,3}

Now, make about 10 photocopies of your template, as shown in figure 2, and put the template away in a safe place, as you might want it again.  Everything we make from here on will scale to this master graph.

Figure 2. Make about 10 photocopies of the template, then put your template away in a safe place, as you may need it again.

Now, using the principles described in Creating an Equiligraph , make monoprotic equiligraphs for acetic acid (pKa = 4.76) and ammonia (pKa = 9.24).  You should get graphs similar to those shown in figure 3 and 4.

The CTotal line should be between -3 and -5 on the Y axis.

Figure 3. Equiligraph of Ammonia.

Figure 4. Equiligraph of Acetic Acid.

We now convert these fixed concentration graphs to variable concentration graphs by tracing only the α0 and α1 lines, along with the two vertical lines marking pH = 0 and pH = 14. This is shown in figure 5.

After drawing the vertical lines to the full length of the transparency, reposition it so that the CTotal line is more in the center of the page.

Figure 6 shows an equiligraph for 0.1 (M) ammonia made by aligning the transparency for ammonia and a photocopy graph sheet from figure 2.

Any CTotal concentration can now be represented  by moving the transparency up or down as needed.

Figure 5. Equiligraph of Ammonia on the transparency. Only the pH = 0, 14 vertical lines ( marked 1,2)  and the α0 and α1 lines are copied.

Figure 6. Variable equiligraph, currently showing ammonia with CTotal = 0.1 (M) is made by overlaying the transparency (figure 5) on the photocopy (figure 2).

Acetic acid an Ammonia are both monoprotic. Polyprotic acids, such as carbonic, sulfuric and phosphoric acid are a bit trickier, but the same techniques we’ve already used need only be modified slightly.

The problem is that when the α0 line crosses the second pka, it’s slope decreases by 1. The transition is smooth, and begins 0.7 units before the α0 line crosses the pK2 and the slope is -2 after it crosses the pK2, but it crosses at a point 0.3 units below where the initial α0  line would cross pK2.

This sounds complex, but it is really the same as we’ve done before for monoprotic acids, but the initial slope was 0 and went to -1. Now the initial slope is -1 and goes to -2.

Figure 7 shows a portion of the sulfuric acid  equiligraph, with the transition marked as described above.

Figure 7.  Drawing the lines for polyprotic acids. As a line crosses each pKa, the slope changes by + or -1. The transition begins 0.7 units before the line crosses the next pKa. The new slope begins at a point 0.3 units below where the initial line would have crossed the new pKa.

Figure 8.  Base graph for carbonic acid.

Next, Draw equiligraphs for:

  1. *carbonic acid             pK1 = 6.32         pK2 = 10.32

  2. *sulfuric acid                pK1 = -3.0            pK2 = 1.99

  3. *o-phosphoric acid       pK1 = 2.15           pK2 =7.92       pK3 = 12.32

Your graphs should look like those in figure 8 through 13

Figure 10. base graph for ortho-phosphoric acid. 

Figure 9. Base graph for sulfuric acid.

Figure 11. Transparency for carbonic acid

Figure 12. Transparency for sulfuric acid

Figure 13. Transparency for phosphoric acid

It is time to use your kit.  Let’s start with the finding the total concentration of ammonia species and the partition in a solution that is buffered at pH = 5, with a standing partial pressure of 10 (ppmv) ammonia gas.

Sound difficult?  It really isn’t!

Go to the Soluble Gasses page.

Quick Links;


Tutorials / Interpretation:

Equiligraph Basics

Polyprotic acids and bases

Creating an Equiligraph

DIY Chemistry Hero Kit

Worked examples:

Sourdough Bread

Soluble Gasses

Physics / Theory:


Contact me:


Software / Apps:

Titration:    Apple AppStore

    Equiligraph, buffer capacity and titration curve.


**  Not yet available!

    A calculator showing CaCO3 solubility as a function of atmospheric CO2


****  Not yet available1

    Equiligraph showing solubility as function of pH