Lab Balance and Scales – Techniques and Uncertainty

Weighing Techniques for Lab Balances and Scales

As simple as a Lab Balance Scale is to use there exists multiple techniques for determining the correct weight of a sample. One might ask why does a lab balance need to be sensitive enough to weigh something so light as say a fingerprint for example. After all, a person is not going to be adding or removing a sample granule by granule in order to hit an exact target weight. You would be surprised at how many people do just that – they will try to add a target weight to their lab scale by adding or removing granule by granule trying to get an exact target weight. This is not the proper way to use a lab balance scale.

Say for example a method says to weigh 2 grams of a material and your lab balance scale is readable to 0.0001 grams. You do not need to “pick” at the sample until you get 2.0000 g. In fact you will more than likely not be able to hit the exact weight but many will attempt to, at least to say the second decimal place. This is not the proper technique. Trying to hit an exact target weight will take too much time and possible skew your measurement. If you spend more than a minute picking at your sample trying to hit a target weight you are more than likely to introduce errors associated with the ability of the sample to either absorb some of the components of a sample or it may slowly volatilize off some of the sample.

The best rule of thumb is to hit a target weight of ± 10%. For example, it your target weight is 2 grams, then it is perfectly acceptable to achieve an actual sample weight in the range of 1.8 to 2.2 grams. Weigh your sample accurately and quickly. The more time you take the more error you are likely to introduce.

If a sample is volatile in nature, say for example ammonium bicarbonate, you would be able to watch the loss of weight in real time right from your lab balance scale readout. You should in most cases, if possible, weigh your sample in a closeable vessel. It might be something as simple as a small beaker with a small lid you place on top of it – a lid for example like small watch glass or maybe a small aluminum weigh pan.

Direct Weighing With Lab Balances and Scales

Direct weighing is the easiest and most used technique. A simple summary of a typical procedure is worth noting. Once again, say for example you are to weigh 2 grams of ammonium bicarbonate.

1. Place a small beaker with a lid on the balance.
2. Zero or tare the lab balance scale.
3. Remove the beaker lid and place it under the beaker ( both the beaker and lid you zeroed need to be on the balance or your target weight reading will be off ), quickly add your sample target weight (about 1.8 to 2.2 g) and place the lid back on top of the beaker.
4. When the lab balance scale reading stabilizes record your weight.

It is worth noting that some people have issues with spilling small amounts of sample on the balance pan when delivering their sample to the weighing vessel. This will skew your reading and can produce a significant error. Even it you cannot see the granules or particles you dropped on the pan. If this is a concern it is perfectly acceptable to remove your sample vessel completely off of the lab balance scale when you are adding your sample with a spatula. Just make sure when you remove it for sample addition that you set it down on a clean surface. You don’t want to pick up stray particles of dirt or the like onto the bottom of your sample vessel.

Using Indirect Weighing on Lab Balances and Scales

Indirect weighing, or weighing by difference, can be controversial to some people. This need not be the case. Weighing by difference is every bit as accurate as direct weighing.

Here is an example of weighing by difference. Say for example your sample is a liquid and you draw up 1 mL of it into a glass syringe. You then place the syringe in a vessel ( say for example a beaker again ). You place the beaker with the full syringe in it on the balance pan and then you zero the lab balance scale. The reading now says zero. You then remove the syringe and inject half the volume of the syringe into your instrument or a piece of lab glassware for subsequent treatment. You now place the half empty syringe back in the beaker and then place this back on the lab balance weighing pan. The weight is lighter now and what you get is a negative reading, say for example it now reads – 0.5763 grams ( a negative reading). The amount of your sample delivered is the absolute amount of + 0.5763 grams. So why is it some people have trouble with this?

Even auditors have questioned this technique. They wonder if a lab balance scale is designed to be used in this fashion. Is a negative reading just as accurate as a positive reading they ask. The answer is yes, of course it is. The best way to look at this is to consider the useable range of the balance being used. Say for example the range of a balance is 0 to 200 grams. This does not mean it has a useable range of -200 to 200 grams ( that would actually be a 400 gram range ). A 200 gram range is 200 grams no matter how you use the balance.

For example, say you place a beaker that weighs 100 grams on this balance and then zero it. You now have a sample weight range of 0 – 100 grams. The 100 gram beaker plus a sample weight of 100 grams makes a total of 200 grams, the maximum for your lab scale.

So lets break down this techniques like this:

• You place a 100 gram beaker on the balance and zero it.
• The display reading says 0.0000 g, but internally 100 grams of the useable 200 grams has been used.
• You place a syringe with 1 mL of liquid sample in the beaker on the pan and zero it again ( say for example this filled syringe weighed 3 grams ). The display reads 0.0000 g again but once again internally 103 grams of the useable range has been used up.
• You then inject half the volume of the syringe into whatever you needed it for then return it to the beaker sitting on the balance pan.
• Now the lab balance scale reads for example, – 0.5763 grams, a negative reading. But, internally it weighed three positive readings all within its range of 0 to 200 grams. The first reading was 100 grams, then the second was 103 grams and then the third was actually 102.4237 g ( 103.0000 – 0.5763 = 102.4237 g ).

Just because the reading is a negative value is no reason to suspect an error in using a lab balance scale. Internally all weights were within the useable “positive” range of 0 to 200 grams.

Uncertainty With Your Lab Balances and Scales

Uncertainty in weighing with a lab balance scale can be easily misunderstood. Many think that just because a lab scale is readable down to 0.0001 grams that a measurement of 0.0010 grams will have the same degree of accuracy as a weighing of 100.0000 grams. This is certainly not the case. As with all analytical tools of measurement whether it be a glass burette or a lab balance, the larger the measurement the smaller the uncertainty and conversely the smaller the measurement the larger the uncertainty.

To keep this matter simple consider the weighing of a large sample versus a small sample. If you were to weigh a 100.0000 gram sample on a balance capable of measuring down to 0.0001 grams with an error of ± 0.0001 gram then the actual reading would be in the range of 99.9999 g to 100.0001 g. A 0.0002 g spread. The accuracy is then determined simply like this: 0.0002 g / 100.0000 g × 100 = 0.0002 % accuracy, which is a remarkably accurate measurement.

Now we do the same calculation for a small sample size of 0.0010 g. The accuracy would be 0.0002 g / 0.0010 × 100 = 20%. Yikes, not very accurate at all. So sample size can certainly make a difference in accuracy.

 

 

 

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