Check the listing for details. HEPES Buffer Solution, 1M, pH 7.6, 100mL. Condition: New. Listed at 43.00 USD. Unlike the low pKa of bicarbonate buffer, HEPES has a pKa of 7.66 making it a much stronger buffer than the bicarbonate buffer in the pH range of 7.2-7.6. HEPES is toxic when used over 100 mM concentration.