Revised USP Chapters 41 & 1251

USP - Are you PreparedIt is now out and official: USP published revised General Chapters 41 “Balances” and 1251 “Weighing on an Analytical Balance” in the Second Supplement to USP 36-NF 31. After a six months transition period the new chapters will be official December 1st 2013. All affected companies need to be compliant with their balance SOPs updated on that day at the latest.

The below overview summarizes the most import changes in respect to the revision of the mandatory general “Balances” and the information chapter 1251″ Weighing on an Analytical Balance”


Chapter 41
Current Official 1st Dec 2013, can be applied 3rd June 2013
Scope Defines  ‘the requirement for balances used to weigh analytes for quantitative measures’ Same scope with improved wording for greater clarity
Calibrated Balance Not specified New requirement to use a calibrated balance
Number of Balance Test 1 – repeatability 2- repeatability and accuracy
Test 1 – Repeatability (A different and more stringent rounding rule has been introduced)
Limit of repeatability test ≤ 0.1% ≤ 0.10%
Expansion factor 3 2
No. of replicate weighings 10 10
Acceptance criterion (RP) 3*s/m ≤ 0.1% 2*s/m ≤ 0.10%
Other criterion No If the repeatability obtained is smaller than 0.41 d, where d is the scale interval, replace the standard deviation with 0.41 d
Test 2 – Accuracy
Limit of accuracy test N/A ≤ 0.10%
Test weights N/A Must be between 5% and 100% of the balance capacity


Chapter 1251
Current Official 1st Dec 2013, can be applied 3rd June 2013
Scope Applies to ‘all analytical procedures’ Same
Balance Test
Performance Test Drift check to be performed daily Risk analysis determines the frequency of any test
Built-in weights N/A, only a statement on Checks can be partially replaced using automatic or manually triggered adjustment by means of built-in weight.
Minimum Weight Not specified It is clearly stated that minimum weight applies to the sample weight, not the tare or gross weight
Calculation of Minimum Weight Not specified A new method for calculating minimum weight is given with the standard deviation, s, derived by using a test weight ‘up to a few percent of the balance capacity’: m = 2000*s
Repeatability Test Not specified As repeatability fluctuates over time, when possible, weighing should be performed at larger values than the minimum weight (safety factor).
Gravimetric Dosing Not specified Now described as an alternative to gravimetric-volumetric methods – so Quantos is accepted

METTLER TOLEDO has created a portfolio of materials to help you reach the December 1st, 2013 deadline. To find out how General Chapters 41 & 1251 revisions affect your weighing processes, visit to:

  • Request a free in-house seminar today
  • Watch a video: Three key things you should know about USP 41 & 1251
  • Register for a USP webinar
  • Download a USP white paper
  • Register for eLearning


You’ll learn how the changes (especially clarifications to minimum weight determination and balance test frequency) affect your lab’s protocols, helping you gain audit-proof USP compliance and accurate weighing results.