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5.
Reduce the gas supply by partly closing the valve at the base of the burner until the flame is
about 10 cm high.
6.
Adjust the air supply until the inner blue cone is about 3 cm high.
7.
Once you have finished using the flame, immediately turn off the gas supply using the control
valve on the bench top. NEVER leave a Bunsen burner with an open flame unattended or
when it is not being used.
Sometimes the air-gas mixture begins to burn before it reaches the top of the burner. If this
happens, the barrel will become very hot and the flame will become green. The gas supply must
be shut off immediately and the burner allowed to cool before relighting.
As indicated in the above illustration, the hottest part of the flame is directly above the tip of the
inner blue cone.
E.
pH Measurements
The pH meter is an instrument used for measuring the pH of solutions. It consists of a probe
which acts as an electrochemical cell whose potential difference (voltage) depends upon the
hydrogen ion concentration of a solution measured against a standard silver, silver chloride solution
contained within the probe.
This potential difference is measured by a voltmeter whose scale is calibrated in pH units instead
of volts. The meter must be standardized regularly in order to obtain accurate pH readings.
1.
General Notes:
a)
pH meters are very expensive and require special care, therefore, handle them carefully.
Because of the high resistance of the glass membrane, only very low currents can be drawn,
in the order of 10
-13
A. This requires a special voltmeter.
b)
The glass electrodes are very fragile (and expensive) and can be cracked with just a sharp
tap. Therefore, NEVER use the electrode as a stirring rod.
c)
The glass membrane and the wick providing a connection to the reference electrode must
not be allowed to dry out. When the pH meter is not being used, keep the probe in the
storage buffer solution provided.
d)
In very basic solutions ( pH above 12), the glass electrode is unreliable and is gradually
dissolved by the high concentration of hydroxide ions; therefore,
DO NOT allow the electrode to sit in strongly basic solutions. When you have completed a
