Apera Instruments pH 4.00 Calibration Buffer Solution, 16 oz. Bottle. Apera pH calibration buffers are produced in an ISO 9001:2015 facility with ISO 17025:2017 accreditation using high-purity reagents, deionized water and certified analytical balances in a temperature controlled environment. Reported values are accurate to +/- 0.01 pH at 25°C and are traceable to NIST Standard Reference Materials (SRMs). MSDS documents are downloadable. Certificate of analysis can be achieved by request. All pH buffers have a 2-year expiration from manufacture for an unopened bottle.