Illustrated Guide to Home Chemistry Experiments

Chapter 3: Equipping a Home Chemistry Laboratory 35

FIGURE 3-25: A pipette pump

FIGURE 3-26: An inexpensive pH meter

dIoN’T dp

Beginners often dip the indicator paper into the solution

to be tested, which may contaminate the solution. The

proper method is to dip a clean stirring rod into the

solution and then use it to transfer one drop of the solution

to the indicator paper.

ranges (e.g., pH 7.0 to pH 10.0), with typical accuracy of
±0.2 to ±0.3 pH.

An electronic pH meter can determine pH very accurately.
Expensive laboratory-grade pH meters are incredibly accurate,
but are overkill for a home lab. The Milwaukee Instruments
pH600 meter shown in Figure 3-26 sells for $30, fits in your
pocket, runs for 700 hours on one battery, and covers the entire
pH 0 to pH 14 range with resolution of 0.1 pH and accuracy
of 0.2 pH. Unless you’re on a very tight budget, we strongly
recommend that you acquire an inexpensive pH meter.

TERMOMETERSH
It’s often important to know the temperature of a solution, so
any chem lab needs at least one thermometer. A traditional tube
thermometer is a glass tube with a narrow bore and a bulb on one
end, filled with a fluid that expands and contracts with changes
in the temperature. In the past, most tube thermometers used
mercury as the indicating fluid, but environmental concerns have
caused a wholesale shift away from mercury to dyed alcohol or
similar fluids.

Alcohol-filled thermometers, also called spirit thermometers,
are as accurate as mercury thermometers, but are limited to
measuring a smaller range of temperatures. A typical wide-range
spirit thermometer measures temperatures from –20°C to
+150°C, whereas a wide-range mercury thermometer might
have an upper limit of +250°C. (Spirit thermometers with a wider
range are available, but are longer, typically 400 mm or more
rather than the standard 300 mm.)

Standard-length thermometers with very wide ranges have lower
resolution. For example, a thermometer with a very wide range
might have 2°C gradations, while a similar thermometer with a
narrower range might have 1°C gradations. For general lab work,
a thermometer with a range of about –20°C to +150°C and 1°C
resolution is suitable. For several of the experiments in this book,
a second thermometer with a narrower range (say, –20°C to
+50°C) allows more accurate measurements.

There are two general classes of tube thermometers, called
partial-immersion thermometers and full-immersion
thermometers. A partial-immersion thermometer, which is
the more common type, is designed to measure temperatures
accurately when a specific length of the tube (usually 3" or
75 mm) is submerged in the liquid being measured. “Full-
immersion” is a misnomer, because it doesn’t mean that you
literally submerge the entire thermometer. A full-immersion
thermometer reads accurately when the tube is submerged in the
liquid to the level of the indicator fluid. (For example, if the full-
immersion thermometer reads 50°C, the thermometer should be
submerged until the level of the liquid being tested is at the 50°C
mark on the thermometer.)

A digital thermometer is also an excellent choice. A wide variety

of models is available, starting at around $10. The least expensive

models are direct substitutes for glass thermometers. They have

about the same range and accuracy as glass thermometers, but

are less fragile. Somewhat more expensive models boast wider

ranges and better accuracy, often to within 0.1°C. The accuracy of

digital thermometers often varies according to the temperature

being measured. For example, one $50 model we looked at had a

range of –50°C to 280°C. The accuracy of that thermometer was

0.1°C in the range –10°C to 150°C and 1°C outside that range.

S SOOPSC , PATULAS, AnD SPOOnS

Scoops, spatulas, and spoons are used to transfer dry chemicals,

typically from a reagent bottle to a weighing boat or weighing

paper. Good laboratory practice requires that to prevent

contamination, you never return unused chemicals to the original

bottle. In theory, that’s correct, but chemicals are expensive

and the economic realities of a home chem lab are such that

I routinely violate this rule in my own lab.