 Third, since we need to measure rates experimentally, we'd better have some experimental ways of doing it. We want to know how fast a reactant is being used up, or a product is being produced, so we need some way of detecting amounts of these chemicals at different points in the reaction. Here are some common ways of doing it. If one of your reactants or products is a gas, you have a couple of ways of doing it. You could measure the volume of the gas used or produced. You could measure a change in pressure. Or you could measure a change in mass, either as the gas escapes if it's a product, or as the gas is incorporated into the reaction mixture if it's a reactant. If you have a solution with a reactant or product that can be titrated, then you can do titrometric analysis, where you remove small samples from your reaction mixture at regular times and titrate it to find out how much there is. To do this accurately, you need a way of quenching or stopping the reaction in the sample that you remove, so that your little sample doesn't keep on reacting while you're trying to titrate it. A back titration is a good way to do this. If you have a reactant or product that's coloured, or absorbs ultraviolet light, then you can track its presence using spectrometry. A spectrometer shines a beam of light through your sample and detects how much of that light is absorbed. The absorbance of the sample is directly proportional to the concentration of your chemical, so that can be a very convenient way of detecting concentration. If you have an aqueous reactant or product that's ionic, then you can track its presence using conductivity. As the quantity of ions increases or decreases in your solution, you'll register a change in conductivity, and this can also be used to calculate concentrations. And finally, there's also pH. If acidic or basic species are involved in the reaction, then any change in their concentration will lead to a change in pH, which can be tracked with a pH meter.