# pH

## Introduction

The pH scale is a standard measurement system used to indicate acidity with lower numbers and basicity with higher numbers. Pure water has a pH of 7, which is what we tend to think of as “neutral”.

## —Log Proton Concentration [pH]

Substance | [H+] concentration | scientific notation | pH |

lemon juice | 0.01 mol/L | 1x10^{–2} | 2 |

black coffee | 0.00001 mol/L | 1x10^{–5} | 5 |

pure H_{2}O | 0.0000001 mol/L | 1x10^{–7} | 7 |

human blood | 0.00000004 mol/L | 1x10^{–7.4} | 7.4 |

sea water | 0.00000001 mol/L | 1x10^{–8} | 8 |

baking soda | 0.000000000316227766 mol/L | 1x10^{–9.5} | 9.5 |

## [H+] from pH

What if you know the pH of a substance, and want to know how that translates to proton concentration?

Just remember:

Knowing this, you can simply plug in your pH as your Y value, allowing X to represent your proton concentration.

Using pure water (pH=7), let’s first take the log positive by shifting the negative to the other side of the equation:

Having done, that, we can simply follow the “if log X = Y, then X = 10^{Y}” logic:

Thus, having run the calculation in reverse, we know that if pure water has a pH of 7, then its proton concentration must be 1x10^{-7} M.

## Henderson-Hasselbalch equation

Another way to calculate pH, which is especially useful for buffer solutions: