ACS Primary Standard purity level is typically what?

• A. 100 ± 0.02%
• B. 99.9%
• C. 98%
• D. 100%

Answer: (A)

ACS Primary Standards are prepared with extremely high purity and accompanied by a certified purity value with a small uncertainty. The typical specification is 100 ± 0.02%, meaning the true purity is expected to be within 99.98% and 100.02%. This tight tolerance accounts for tiny amounts of impurities, moisture, and measurement limitations that occur during weighing and preparation, ensuring the standard can calibrate analytical methods with traceable accuracy. A fixed 100% value would ignore these real uncertainties, while a purity like 99.9% or 98% indicates notably lower purity and would not be suitable for a primary standard.