3B Scientific Electrochemistry Kit - Manual (Page 27)

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Experiment 11 - measuring pH Teacher's instructions
Chemicals Hazard
symbols
R phrases S phrases Equipment
Buffer solution pH 4 or 7 --- --- Meter
Hydrochloric acid 36/37/38 26 Electrodes:
Combined pH probe
Sodium hydroxide 34 26-36/37/39-45
Sodium chloride solution --- ---
Sodium acetate solution --- ---
Warning: Be careful when handling acids and alkalis! Wear protective glasses!
The Electrochemistry case contains a combined pH probe electrode for measuring pH. The meter displays
highly accurate measurements with a single point calibration. Since the meter is battery powered, it can also
be used to make pH measurements outside the classroom, e.g. in lakes and rivers.
Experiment procedure:
1. Take the combined pH probe out of the case and plug it into the pH connection on the underside of the meter.
2. Change the range switch on the front of the meter from voltage measurement to pH measurement.
3. Now take the pH probe out of its storage flask and rinse it with distilled water.
4. Immerse the pH probe in the buffer solution, wait for a moment and then adjust the zero point until it shows the
correct value for the buffer solution. Make no more adjustments to the rotary knob thereafter.
5. Immerse the probe in the prepared solutions one after the other and read off the corresponding pH values. After
each measurement, thoroughly clean the probe. Take care when doing this not to damage the probe's glass head.
Observation and evaluation:
In order to determine the degree of acidity or alkalinity of fluids, one determines their pH value. The pH value can either be determined
with the help of indicators, which change their color depending on whether they are in an acid or alkaline solution, or with a combined
pH probe, which displays the pH highly accurately on the digital display of a meter. The pH value measured with the probe is much
more accurate as it can also measure fractional values. The pH value is dependent on the concentration of H
3
O - ions in a solution.
The scale runs from pH 0 to pH 14.
pH 0 – 6 = acid
pH 7 = neutral
pH 8 -14 = alkali
Experimental result:
A variety of pH values are found depending on the concentrations of the solutions under investigation Therefore no specific results for
the pH can be given. Use the table to record the pH values measured by the students for the various tested solutions.
Tested solution Measured pH value
Hydrochloric acid
Sodium hydroxide
Sodium chloride solution
Sodium acetate solution