3B Scientific Hofmann's Voltameter, Small - Manual (Page 2)

3B Scientific GmbH • Rudorffweg 8 • 21031 Hamburg • Germany • www.3bscientific.com
Subject to technical amendment
© Copyright 2008 3B Scientific GmbH
2. Description
Hofmann's voltameter is used for the electrolysis of
water (converting electrical energy into chemical
energy), quantitative determination of the resulting
gases and confirmation of Faraday’s laws.
The apparatus consists of three vertical glass tubes
connected to each other at the bottom. Taps at the
top ends of the outside tubes are closed whilst the
inner cylinder is open at the top to allow the addition
of water via a reservoir. Gold sheet electrodes are
fitted to the lower ends of the outside tubes and
connected to a low voltage DC power supply unit. The
proportion of hydrogen and oxygen produced by
electrolysis from the water can be read from the
graduations on the side tubes.
By opening the taps at the top of the tubes, gases can
be collected for analysis. Carbon electrodes (U58011)
are also available for analysis of solutions where gold
is unsuitable.
3. Technical data
Dimensions: approx. 580 x 150 mm²
Stand base, A-shaped: 115 mm leg length
Operating voltage: 4 - 12 V DC
4. Additionally required equipment
1 DC Power Supply 0 - 20 V, 0 - 5 A (230 V, 50/60 Hz)
U33020-230
or
1 DC Power Supply 0 - 20 V, 0 - 5 A (115 V, 50/60 Hz)
U33020-115
1 Mechanical Stopwatch, 30 min U40800
1 Digital Pocket Thermometer U11853
and
1 K-Type NiCr-Ni Immersion Sensor
U11854
1 Barometer U10260
Distilled water
Sulphuric acid (c = 1 mol/l approx.)
5. Example experiments
5.1 Investigation of the conductivity and composition
of water
• Pour distilled water into the water reservoir with
both stopcocks open until both tubes are full.
Then close the stopcocks.
• Turn on the power supply and observe the
electrodes.
There is no perceptible reaction at the electrodes.
• Turn the power supply off again.
• Add a few drops of dilute sulphuric acid.
• After waiting for about 5 minutes, switch on the
power supply again.
Gas bubbles should rise from both electrodes.
• When the gas collection tube at the negative pole
(cathode) is half filled with gas, turn off the power
supply.
• Release the gases through the stopcocks and
collect them in upturned test tubes.
• Demonstrate the presence of hydrogen by the pop
test and the presence of oxygen using a glowing
splint.
5.2 Determining the Faraday constant
• Pour distilled water into the water reservoir with
both stopcocks open until both tubes are full.
Then close the stopcocks.
• Add a few drops of dilute sulphuric acid.
• Turn on the power supply and set the current so
that approximately 1 A flows. Check to see that
gas is being emitted into both tubes.
• Turn the power supply off again, open the
stopcocks and release the gas.
• Close the stopcocks. Turn on the power supply
and take the time with a stopwatch.
• When the glass collection tube at the negative
pole (cathode) is nearly full, turn off the power
supply and the stopwatch together and record the
time.
• Determine the volume of the hydrogen.
• Measure the air pressure and water temperature
in the reservoir.
For a known current I (A), time t (s), air pressure p
(Nm
–2 ), temperature T (K), volume of gas VH
2
(m 3 ) and
universal gas constant R (8.3 J mol –1 K –1 ) the Faraday
constant F is given by:
mol / C
V p
T R t I
F
5
10
2
≈
⋅ ⋅
⋅ ⋅ ⋅
=
.