1M HEPES, pH 7.3, is supplied by HiMedia. It is available in the form of a colourless solution. HEPES is a zwitterionic organic buffering agent. It is stable, water soluble and does not react with divalent cations. It has a pKa of 7.5 at 37C and offers an optimal buffering range of pH 6.8 - 8.2. Since it maintains the physiological pH despite changes in the carbon dioxide concentrations which take place due to cellular metabolism, it is widely used in cell culture media.