Micro Essential Lab 1211U13EA P-12M Hydrion Insta-Chek Surface pH Pencils, 0-13 pH

These pens work great for testing the pH of concrete after surface prep. They mark easily and are very comparable to results from using pH strips. The nice thing about these is that you have the mark on the wall so that you can photograph it for your reports. One thing to note though is sometimes you have to take photos on your phone and then blow them up to see what the real pH of the concrete is. If the surface is very rough, you will leave the lighter colored pen material on the surface where there isn't enough water and concrete to change the pH/color of the material while the parts that are wet and deeper in the surface really are the right color.