Chemistry Reference
In-Depth Information
Procedure: A specific volume of the concentrated sodium hydroxide solution is to
be diluted at a ratio of 1:10 twice: an approximate 1-M solution and a 0.1-M
solution are produced. The gas syringes are to be filled with carbon dioxide to the
100 mL-mark, the tap is to be closed and connected to the rubber stopper. Three test
tubes are to be filled with 5 mL of the stated solutions. One after the other is to be
overlaid with the gas: the gas syringe has to be put on the test tube first loosely, air
has to be pushed out of the tube with 50 mL of gas, 50 mL remain in the syringe.
The test tube is then to be closed gas-tight. The gas volume of all three gas syringes
is to be observed after a couple of minutes.
Observation: All three carbon dioxide volumes decrease. The concentrated sodium
hydroxide solution reacts with the entire amount of gas, a suspension forms which
contains a white precipitate. 1-molar solution reacts with half of the gas volume;
0.1-M solution only with a few milliliter of the gas volume.
E5.14: Concentration and pH-Value of Sodium Hydroxide Solution
Problem: Measuring pH-values of acidic and alkaline solutions is the typical form
of collecting data. According to the definition of pH, pH-values are a direct measure
of the concentration of hydronium or hydroxide ions, respectively. Therefore, these
concentrations can be determined easily by measuring the pH. Another way is the
titration of a specific volume of alkaline solution with an acid standard solution and
recording the changing pH-values to the equivalence point. The result is a table of
data, which can be illustrated with a diagram.
Material: pH-meter and glass electrode, beakers, transfer pipette (20 mL), Erlenmeyer
flask, magnetic stirrer, burette; buffer solution for alkaline range, diluted sodium
hydroxide solutions from E5.13, hydrochloric acid-standard solution 0.1 mol/L.
Procedure: The pH-meter is to be calibrated with the help of two buffer solutions of
pH-values in the alkaline range (see instructions for the pH-meter). The pH-values
of the two alkaline solutions are to be measured by immersing the glass electrode
and then waiting until the pH value stays constant.
With the help of a transfer pipette, 20 mL of the very diluted sodium hydroxide
solution are to be poured into the Erlenmeyer flask and stirred on the magnetic
stirrer. The glass electrode of the pH-meter is to be dipped into the sodium
hydroxide solution to meter the pH-value. After that, it is to be titrated with
hydrochloric acid in 1 mL-steps and pH-values are to be measured until they exceed
the neutral range. These values are to be recorded in a table and transferred to
a diagram; the diagram is to be analyzed in terms of the consumption of hydro-
chloric acid. The amount of reacting hydrochloric acid to the equivalence point is
to be used for a calculation of the concentration of the sodium hydroxide solution.
Observation: At the first step the measured values for both solutions are to be
displayed: the pH-values are about 14 and 13. The consumption of hydrochloric
acid standard solution is to be observed at the second step and the concentration is
to be calculated exactly.
E5.15: Dilution Series for Illustrating pH-Values
Problem: Students might not be able to classify and correlate the pH-value of E5.14
with the concentration of OH
(aq) ions in the solution. To explain this connection,
