Standardized against NIST standards, premixed, and ready to use. Pouch contains 4.01 pH water used as a calibration solution. "Singles" are convenient for high-precision calibration on-the-go.All Oakton pH buffer solutions are freshness dated, and standardized against NIST-certified references to ensure quality. ±0.01 pH at 25°C.SpecificationsIdeal for field calibration.NIST traceable materials.±1% accuracy at 25°C.