Darrel B. Hoff State College of Iowa Cedar Falls
Overhead Proiection with the pH Meter
can he visualized readily by the menihers of the class soon after the titration is completed. Common-ion Gecl: A solution of ammonium hydroxide is placed urider the electrodes aud the expected pH can be predicted from its molarity. The meter is turned on and the pH noted. When solid aninionium chloride is added to the solution, the pH will hc observed to drop. Ru,fer action: The addition of a single drop of hydrochloric acid to distilled water will cause a sharp drop in the pH. The addition of a single drop of hydrochloric acid to a second sample of water containing a small amount of sodium acetate causes very little effect. Further additions of the acid are necessary to achieve a drop in pH con~parableto the first saniple. This can be extended by the use of indicators, aud the differences between buffered and rion-buffered aspirin can he shown. The aut,hor noted no adverse effects on the pH meter, in spite of the rather high temperatures that it was suhjected to under the projector. The combination has been used approxi~natelyten hours to date without any complication. It should he noted that the exhaust from t,he cooling fan of the projector tended to warn1 the solution under the elect,rodes. I t is recomnieuded that a heat shield be devised to hold the solution at a coristant temperature.
Detnonstrations centering on p ~ acid, base reactions, common-ion effects, buffers, and hydrolysis can he performed quantitatively to large groups by using an overhead projected image from a pH meter. These experiments employ a Beseler Vu Lyte I1 proj e c t , ~along ~ ' with a Coleman Metrion I1 pH meter. By removing t,he st,age on the opaque projector (requiring the removal of three screws and two springs) and hy removing the electrode jack from the pI-I met,er and n~ount,ingit. on a ring stand, the meter will easily fit under t,he heat filter of the projector. With ordinary window shades, t,he image of the meter face can be enlarged to 4 X 5 ft without loss of clarity. Rot only can t,he meter dial he seen, but the controls of t,he met,er cau be easily manipulated by t,he demonstrator and sceri by the students. Once the basic operation of the pH meter has becn demonstrated, the con~hinationcan serve to dernonstrate the following: Acid-base tilraiions: After completing routine titrations using indicalors, a group of st,udents can be familiarized with t,he operation of the meter, and then can be allowed lo demonst,rate its use in a tkation. If a transparency projector2 is placed in front and a little to the side of the opaque projector, one of the st,udeots can make a plot of t,he data as another performs the titration. A certain amount of prior planning has to be done so t,hat the scale selected for the approximate volumes and pH range will fit on the graph to be projected. In this fashion, the end-point det,ermination
Figure
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Figure 2. 4 X 5 RI.
Assembly with meter in place.
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' Other projectors, e.g. the A 0 Delineascope, ran be used. The same is true of a varietv of pH meters. > A Beseler Portmcribe works well hecause of its compact sine. Charles Beseler, Co., East Orange, New Jersey.
Journol o f Chemicol Educofion
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Projected i m a g e of the m e t e r on the screen ivpproximotely
