On paper, using distilled water as a pH 7.0 makes sense because the ionization constant is 1 x 10-14 and the concentration of hydrogen ions is 1 x 10-7 molar at 77 °F (25 °C). Converting this to pH by poking 1 x 10-7 into the old calculator and hitting the log key results in -7, and multiplying this by -1 gives us 7; that should be the pH of pure water. I am impressed you remembered that from high school chemistry class. The problem is that water is rarely pure because it is the universal solvent and dissolves all sorts of things, including gases. And even if you were able to keep pure water pure in an environment without CO2, the ionic strength of pure water is too weak for a pH probe to properly function. A pinch of salt could be used to solve that problem.
Although the concentration of carbon dioxide in the atmosphere is only 400 ppm, that’s plenty to affect the pH of water because carbon dioxide sets up a powerful buffer system in water. This same buffering system is present in blood. In the case of water, the carbonate buffer system raises pure water pH up to about 8.2. The takeaway is that you need to buy two buffers to calibrate your pH meter. When you go shopping for pH buffers, you will discover that there are several options, with the most common being pH 4.01, 7.00, and 10.01. Brewers should calibrate their meters using pH 4.01 and 7.00 buffers because brewing biochemistry occurs in the acidic world.
You might be wondering why these pH standards are not affected by carbon dioxide in the atmosphere like water. The answer is because they are buffers themselves. Just in case you were not paying attention during this chapter in chemistry class, buffers are solutions containing a conjugate acid-base pair that are able to resist, or buffer, pH changes. Buffering capacity is directly related to the concentration of these compounds and is limited by how much acid or base can be added before the pH changes. This is why buffers need to be periodically replaced and why they should be stored in closed containers that prevent evaporation. One final point: Don’t ever store your pH meter’s probe in distilled water!