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I was doing a lab at school yesterday which involved transferring a precise amount of solid reagent into a beaker.

The way my professor told me to do it was to tap the reagent container so that the solid would fall out into the preweighed beaker and then weigh the beaker. However, I always thought that the correct procedure was to weigh a piece of paper, then place the solid onto the paper, reweigh the paper to obtain the correct mass reading, and then use the paper to pour the solid into the beaker.

What are the advantages of using the teacher's way over the way I proposed?

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There is no correct or universal weighing protocol per se. The key question is what is the desired level of accuracy? Your teacher's approach is quite cautious but this is not the universal way, as what you did is also common. He/she is apparently avoiding a spatula as well. I personally dislike paper and plastic weighing boats. If you ever had a chance to weigh dry salts or organic crystals on a dry day, these materials charge like crazy and literally all particles fly apart the moment you touch the plastic boat with gloves.

Pharma companies, where a high level of accuracy is required and legal formalities are important, chemists often use weighing funnels. They are not popular in academia (pricey!). In the picture, you see a latest type of balance which avoids static issues. On the top you see a weighing funnel. This one is plastic coated with anti-static material but glass is more common. After weighing, the funnel containing the weighed substance is placed on the mouth of a volumetric flask and washed very carefully. You can't do this with paper.

enter image description here https://www.laboratorynetwork.com/doc/new-smartprep-weighing-funnel-weigh-in-samples-more-accurately-0001

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I am listing some advantages for either method. Depending on the specific task at hand, one or the other (or using a weighing boat or an aluminum weighing dish or yet another methods) might be optimal.

Advantages of weighing paper

It has a lower mass than a beaker, so you can use it in an analytical balance. You can fold the weighing paper and use it as a chute to get the solid into a small vessel. You can put the weighing paper back on the balance to see if any solid is still on it.

Advantages of a beaker

If your next step is to add water to the solid in a beaker, you skip a step. More solid fits into a beaker. It is easier to transport the solid in the beaker. If you are weighing in something that is very dusty (like growth medium) you lose less if you can transfer the powder directly into a beaker.

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Many times the decision has to do with what type of balance is available.

Advantages of using a beaker:

  • Paper cost money.
  • Paper may adsorb or react with the reagent which has the potential to throw off the measurement.
  • If the reagent is hazardous, then using paper increases the amount of hazardous waste generated.

Advantages of paper:

  • Paper is much lighter than a beaker. This is useful if an analytical balance without sufficient range and/or resolution is available.

For example. Say a beaker weighs 250g and the reagent weighs 3g.

If the range of the available balance is 300g and the resolution is 0.01g then the beaker method will be the better method.

If the range of the available balance is 5g with a 0.01g resolution the paper method will be better because the beaker will overload the balance.

In these two cases, both methods have the same accuracy.

If a 5g balance with 0.001g resolution is available, then the paper method might be more accurate assuming the paper is not causing issues.

The answer to your question comes down to what type of balance is available.

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  • $\begingroup$ Use a 10ml beaker instead of one liter for a 3g sample? $\endgroup$ – Karl Apr 14 '19 at 19:34
  • $\begingroup$ @karl You are making assumptions about the sample density. It is entirely possible 3g sample might not fit in a 10ml beaker. $\endgroup$ – Agriculturist Jun 25 '20 at 14:34
  • $\begingroup$ And what kind of sample did you have in mind for the 1L / 3g combination? (1 L standard lab beaker weighs ~150g, so more like 2L, actually) $\endgroup$ – Karl Jun 25 '20 at 17:06
  • $\begingroup$ The combustion of very light organic matter in the heavier than standard beakers used for combustion experiments exaggerates the need to consider the type of balance. If the balance can handle the beaker then the professor's method is best. If the balance cannot handle the beaker then the teachers method is best. A professor is more likely to have the costlier balance with the range and accuracy to handle a beaker than a high school teacher so the reality is both methods were likely proper, but appropriate for the available balance. $\endgroup$ – Agriculturist Jun 25 '20 at 18:55
  • $\begingroup$ Using an unnecessarily oversized beaker to weigh in a substance is Bad Laboratory Practice, period. $\endgroup$ – Karl Jun 26 '20 at 6:21

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