Illustrated Guide to Home Chemistry Experiments

Chapter 3: Equipping a Home Chemistry Laboratory 31

TABLE 3-3: Recommended supplemental laboratory glassware also have a minimum capacity, below which their readings
may be inaccurate.

Reasonably high capacity is important, because you will often

tare and weigh by difference. For example, before you begin

a procedure, you may weigh the beaker in which the reaction

will take place. Rather than weighing reagents directly, you

place the beaker on the balance and use the tare function to

“zero out” the balance, so that the balance shows a weight

of zero with the beaker on it. You then add the reagents to

the beaker. You can read the weight of the first reagent you

add directly on the display. You can determine the weight

of the second reagent you add by subtracting the original

weight from the new weight, and so on. When the reaction

is complete, you can filter the precipitate and determine the

weight of the product.

Accuracy and precision

Accuracy and precision (also called repeatability) are

separate but related characteristics. Accuracy specifies how

closely a balance indicates the true weight of a sample. For

example, if you weigh a calibration weight with a known weight

of 100.0000 g, an accurate balance indicates that weight to

within its resolution (e.g., an accurate centigram balance reads

100.00 g, and an accurate milligram balance reads 100.000 g).

Precision or repeatability specifies how closely repeated

weighings of the same sample agree. For example, if you weigh

the calibration weight nine times, a precise but inaccurate

balance may report weights of 100.83 g seven times, 100.82 g

once, and 100.84 g once. Although the indicated weights are far

from the true weight, which means the balance is inaccurate,

the separate weighings agree closely, which means the balance

is precise. Conversely, an accurate but imprecise balance may

report weights of 99.96 g, 99.97 g, 99.98 g, 99.99 g, 100.00 g,

100.01 g, 100.02 g, 100.03 g, and 100.04 g. No two weighings

gave the same result, which means the balance is imprecise,

but none of the results differed much from the actual weight,

which means the balance is accurate.

Repeatability is more important than accuracy. An inaccurate

balance can be calibrated by using a known weight and

adjusting the display accordingly. An imprecise balance cannot

be adjusted to be more precise.

Linearity

Linearity specifies the accuracy and precision of a balance

across its range of capacities from minimum to maximum. For

example, a centigram balance with a 200 g capacity should

have similar accuracy and precision whether you are weighing

a 1 g sample or a 195 g sample. The best way to test the

linearity of a balance is to weigh multiple items individually and

then together. For example, you might weigh three labeled bolts

SRISE oU SCIENCE

The major factor that relegates even the best

chemistry sets to the realm of scientific toys rather

than serious tools for doing real science is their lack of a

decent balance. There’s only so far you can go in learning

chemistry if your measurement methods consist of

using a scoop of this or three drops of that. Chemistry

in general and lab work in particular is founded on

accurate measurements, which makes a good balance

indispensable.

and get results of 34.18 g for bolt A, 44.77 g for bolt B, and 61.29

g for bolt C. When you weigh bolts A and B together, a balance

with good linearity will read 78.95 g (34.18 g + 44.77 g = 78.95

g). When you weigh bolts A, B, and C together, a balance with

good linearity will read 140.24 g. Other than the least-expensive

models, most decent electronic balances have good linearity

through most of their range of capacities.

Of course, price is often a major consideration for a home lab,

but I suggest you don’t skimp too much. If you’re on a tight

budget, choose a $60 model like the My-Weigh Dura 50 Digital

Scale (0.01 g/50 g). If you can afford more, choose the $100

My-Weigh iBalance 201 (0.01 g/200 g) shown in Figure 3-19,

which is what I used for the lab sessions in this book. If you need

a milligram scale, the least expensive model I can recommend is

the $170 My-Weigh GemPro 250 (0.001 g/50 g), although I think

the $220 Acculab VIC-123 Milligram Scale (0.001 g/120 g) or the

$270 Acculab VIC-303 (0.001 g/300 g) is a much better choice.

Most laboratory supply places carry balances suitable for home

labs. The best source we know of in terms of selection and price is

Precision Weighing Balances (www.balances.com).

FIGURE 3-19: My-Weigh iBalance 201 digital balance