Chemistry Reference
In-Depth Information
concentrated sodium chloride solution and the graphite electrode is to be fixed.
A direct voltage of about 10 V is to be applied.
Observation: A yellow-green gas with a characteristic smell forms at the plus pole:
chlorine. After a couple of minutes the indicator solution turns burgundy at the
minus pole, starting from the mercury: sodium hydroxide solution is produced. At
the same time gas bubbles rise through the solution on the left side: hydrogen.
Please note: After decanting the aqueous solution, water is to be added to the sodium
amalgam and one has to wait until the formation of hydrogen stops. Then the aqueous
solution can be decanted and the mercury can be returned to the storage bottle.
E5.17: Combustion Analysis by Liebig
Problem: Sodium hydroxide solution and potassium hydroxide solution play an
important role in the historical combustion analysis by Liebig. He successfully used
the “potash apparatus,” which he developed himself. This apparatus completely
absorbs the produced carbon dioxide from the gas stream. A similar apparatus can
be demonstrated schematically for the comprehension of this analysis method (see
Fig.
5.7
): a combustion tube attached to an oxygen bottle on one side and to two U-
tubes on the other side, one filled with calcium chloride and weighed for absorbing
water vapor, the other filled with potassium hydroxide and weighed for absorbing
carbon dioxide. By weighing a specific amount of organic substance and the
U-tubes afterwards one is able to estimate the composition of the substance.
In another version of E.17, however, only carbon dioxide is being collected
quantitatively in a gas syringe, while the mass of condensed water is not being
determined. It can be calculated that 1 C
4
H
10
molecule forms 4 CO
2
molecules:
after the reaction of butane with copper oxide the volume of carbon dioxide will be
four times larger than the volume of butane before.
Material: 2 gas syringes, combustion tube with copper oxide (wire type) enclosed
with quartz wool, connecting rubber tubes; butane gas (F
+
), lime water (Xi).
Procedure: One gas syringe is to be filled with 20 mL of butane gas. The apparatus
is to be assembled and fixed (see figure). Copper oxide is to be heated strongly with
the hot flame. The butane gas is to be pushed or pulled slowly back and forth
through the combustion tube - until the volume remains constant. The volume of
the produced gas is to be measured. The gas is to be passed from the syringe into
a small amount of lime water.
copper oxide
quartz wool
butane
Observation: The copper oxide turns into bright red copper. Eighty milliliter of a
colorless gas is formed. As expected, the lime water test shows that the produced
gas is carbon dioxide.
