weighing and measuring Flashcards
how to use a prescription balance
Drugs go on left, weights go on right
prescription balance is also known as
double-pan Torsion balance
– a Class A balance according to the USP
– a Class III balance according to NIST (National Institute for Standards and Technology)
prescription balances must meet the reqs of
NIST, formerly known as NBS (National Bureau of Standards)
how must you use a rx balance
- Must always use weighing papers or weighing boats
- Weights should not be left on the balance when the balance is not in use
- Must be locked during weighing
operations
Top-Loading Balance
single-pan balance with internal weights and digital display features
top loading balance sensitivity req
1mg (avoid errors of 5% or more so don’t weigh anything less than 20mg)
Percentage of Error Formula
Least Weighable Quantity
minimum quantity that can be weighed with the desired degree of accuracy
Two factors determine the least weighable quantity on a given balance
- The sensitivity requirement of the balance being used
– The potential error that may be allowed to incur in the weighing process
Least Weighable Quantity Equation
Aliquot Method of Weighing
Calculate the small quantity of a substance that can be weighed on a prescription balance
(SR x 100%)/LWQ = PPE
Calculate a multiple of the desired quantity that can be weighed with the required precision
Determine the amount of aliquot to be weighed in order to obtain the desired quantity of the substance
Calculate the weight of aliquot mixture to be prepared
Calculate the amount of inert diluent to be used
DRx/WAliquot = LWQ/WTotal
Dilute the amount of substance weighed with the inert diluent
Weigh the amount of aliquot in order to obtain the desired quantity of substance
Measuring Devices
graduates, medicine dropper, graduated prescription bottle
types of graduates
cylindrical or conical
types of medicine droppers
non graduated or graduated
