A ninhydrin reagent for use in a method for analyzing nitrogen-containing compounds, in particular visualizing nitrogen-containing compounds by a color forming reaction, is provided. The ninhydrin reagent contains ninhydrin; an aqueous buffer; and a temperature-dependent reducing agent, which agent is inactive in the reduction of ninhydrin at a first temperature and active in reducing ninhydrin to hydrindantin at a second temperature, wherein the second temperature is higher than the first temperature. The reagent is particularly useful in the analysis of amino acids.