HI70004P is a quality pH 4.01 calibration buffer produced in accordance with ISO 3696/BS3978 standards using high purity salts, deionized water, certified weight-Checked balances, and Class A glassware in a temperature controlled environment monitored with certified thermometers. Reported values are accurate to +/- 0.01 pH @ 25°C and are traceable to NIST Standard Reference Materials (SRMs).