Weighing Materials

1
Weighing Materials

• Proper use of balances

2
Terminology for weighing

• Range
• the span from lightest to heaviest weight that a
  balance is able to measure
• Capacity
• the heaviest sample that a balance can weigh
• Sensitivity
• the smallest value of weight that will cause a
  change in the response of the balance.

3
Proper weighing procedure

• Make sure the balance is level
• Adjust the balance to zero
• Tare the weighing container or weigh the empty
  container
• Place the sample in a the weighing container and
  read the weight
• Remove the sample
• Clean the balance and surrounding area

4
Proper weighing techniques

• Always use a calibrated weight to verify the
  scale is in proper working order (daily)
• Always use a weigh boat or weigh paper do not
  place materials directly on the pan
• Do not touch the chemicals or material being
  weighed
• Do not return unused chemicals to their storage
  bottle (unless you use a sterile spatula or
  spoon)

5
Factors affecting weight measurement

• Not level
• Drafts
• Static
• Not placing the load in the center of the pan
• Placing an amount on the balance which is higher
  than its capacity
• Temperature explain this to me!

6
Calibration of Balances

• First step is to zero the balance. It should
  read zero every time you press the zero button
• Second calibration point is taken at the upper
  end of the capacity of the balance
• Place a certified weight on the balance and
  verify it reads the correct weight
• Some scales will prompt you to enter the weight
• A third reading can be made using a lighter
  calibrated weight and verifying it reads the
  proper weight

7
Equipment Log Books

• Notebooks or binders used to maintain operating
  procedures, calibration records, verification
  checks
• Example the equipment log book in the back of
  the room
• Incubator temp charts
• Refrigerator temp charts
• Pipet calibration records
• Balance calibration and verification charts

8
Critical Thinking Question

• Suppose you need to prepare a solution that is 15
  mg / mL. The material you are weighing is
  extremely difficult to get exactly 15 mgs.
  Suggest a strategy to get the correct
  concentration even if you cannot weigh 15 mgs.
• Weigh out close to 15 mgs and then determine the
  proper amount of solution to add so that your
  solution is still at 15 mgs/mL

9
Critical Thinking Questions

• How much volume of soln. at a conc. of 35ug/mL
  can be made in each of the following
• Sample weighs 0.003560 g
• 101.7 mL
• Sample weighs 0.050 mg
• 1.43 mL
• Sample weighs 1.0897 g
• 31.134 L