AN standards are manufactured, tested and certified in accordance with ASTM D664/IP 177 (by potentiometric titration) or ASTM D974 (by color-indicator titration). Verify system functionality, assure accuracy of your laboratory instrumentation and save time with our AN reference standards. Acid number=the quantity of base, expressed as milligrams of potassium hydroxide per gram of sample required to titrate a sample to a well defined inflection point as specified in ASTM D664.