We report a systematic evaluation of the use of refluxing nitric acid as a purification treatment for electric arc-produced single-walled carbon nanotubes (SWNTs), by using a combination of thermogravimetric analysis (TGA) and near-infrared (NIR) spectroscopy. Nitric acid is the standard reagent for purification of SWNTs and has traditionally constituted the first step in many different purification schemes. It has been suggested that nitric acid removes the transition metal catalyst that is used in the production of the SWNTs together with amorphous carbon. Under all conditions, we find that nitric acid destroys SWNTs to produce amorphous carbon while reducing the amount of transition metal catalyst remaining in the sample. Thus, nitric acid is suitable for removing the catalyst from SWNT samples, but only at the expense of a significant destruction of the SWNTs.