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Weighing Reagents Protocol

Intro to the Lab Bench

Introduction

For many experiments, you’ll need to make buffers, media, or other solutions. A key part of this task is making sure you’re weighing all reagents precisely to create the solution you expect. Understanding how to obtain the correct amounts of materials for an experiment or reagents for a stock mixture is an essential laboratory technique, as imprecise measurements can affect the outcome of your experiment. In this protocol, you will learn the basics of how to weigh reagents. Wear the appropriate personal protective equipment (PPE) prior to handling the materials.

Equipment

  • Weighing boats or weighing paper
  • Balance
  • Scoopula or spatula
  • Container (beaker, tube, flask, etc.)

Reagents

  • Material that will be weighed out

Procedure

LB powder sits on a weighboat on a scale. The display reads "26.1 g."
The balance in the photo above has a capacity of 200 g, as indicated by the number above the "Zero" button.
  1. Before weighing out your reagents, determine the amount that you need to weigh.
  2. Gather the reagent you will be weighing out, a weighing boat or weighing paper to contain the reagent while on the balance, and an appropriately sized container to put the reagent in. For some reagents, you might need to weigh it out in the fume hood to prevent exposure. Read the reagent’s material safety data sheet (MSDS) to determine if this is best.
  3. Check that the balance has the correct capacity for the material that you are weighing by looking for a weight range on the scale. Make sure that the weight of the material you’re weighing out is within this range. If you need less than a gram of material, use an analytical scale. This will provide more accuracy as these types of scales display more decimal places.
    Pro-Tip
    If you’re weighing out an amount larger than the capacity of your balance or weighing boat, you can weigh out the reagents in batches. For example, if you need 300 g of sucrose, you can weigh out 100 g three times, placing the sucrose in your container after each time.
  4. Place the weighing boat or weighing paper onto the balance. These are used to contain the material you’re weighing and prevent cross-contamination with other substances. They also help you transfer the material to your tube or container.
    Pro-Tip
    If you’re using weighing paper, fold it in half first to create a crease and then place it unfolded onto the balance. This will help you remove it from the scale later on as otherwise, the paper could stick to the surface.
  5. Tare the weighing boat or paper. You need to do this because you don’t want to include the weight of the weighing boat or paper into your measurement. Hit the tare button (or zero button) to set the balance to zero.
    Pro-Tip
    If you’re weighing a minute amount of reagent that will be resuspended in a small volume (e.g. 0.02 g to resuspend in 1 mL), you may place a microcentrifuge tube on the balance and then tare it. Then add the material directly to the tube to weigh it.
  6. Make sure that your scoopula or spatula is clean, and sterile (if necessary), to prevent any potential contamination.
  7. Scoop out the material you are weighing onto the weighing boat or paper. If you weighed out too much material, don’t put any extra back into the original container. Dispose of the excess material into an appropriate waste container if the material is biohazardous. Check your lab’s material safety data sheets (MSDS) to understand how to properly dispose of reagents that you are working with.
    Pro-Tip
    When you weigh out a reagent, you may leave trace amounts of it on the balance. Therefore, it’s best to clean the balance before adding your next weighing boat for the next reagent.
  8. Once you’ve weighed out the correct amount of your reagent, transfer it into your container.
  9. Make sure that any spills produced during the protocol are cleaned up. Discard the weighing boat or weighing paper in the trash if the material weighed is not biohazardous. If it is biohazardous, discard the weighing boat or paper in the biohazard solid waste container.

Conclusion

Appropriately weighing reagents is important to ensure the accurate composition of your solutions and buffers. Now that you have your reagents weighed out, you can begin making your solutions!