edited by SHELDON H. COHEN Washburn University
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An Inexpensive pH Meter Dennls Sievers Central High School Breese, IL 62230
In lieht of the financial pr,d>lemsof many schools and the rising kflation ;ate, it is b&ominK more difficult to purchase basic instrumentation for a chemistry laburamry. A pH meter is certainly one of the most useful devices available;yet, a good unit with temperature control can cost from $200-400and is beyond the range of most schools, especially small ones. The pH meter shown in the figure can he built for under $25 less meter and electrodes. Comhination electrodes are available for under $40, and the meter is a 50-0-50 type. Many schools will have such a meter in the form of a galvonorneter, or if needed, one of suitable accuracy may he purchased for under $25 from Allied Electronics.' By having to purchase all of the items for this pH meter, one will have invested less than half of the cost of commercially available units. Several methods of construction are possible depending on one's background in electronics. These could be terminal strips, perforated board, or printed circuit. Choose the one that is easiest for your situation. The entire meter may he assembled and mounted in a suitable housing by two students in less than two hours. Little experience~n~electronics is needed.. onlv. a little patience. A parts list is given along with the circuit diagram in the figure. Followine construction, the meter must be calihrated initially. This begins by checking the voltage between pins 2 and 3 which should be 0 V. If this voltage is not zero, R7 should be rotated until a VOM shows 0 V. The voltage between pin 6 and ground should he checked also. If a VOM shows a reading other than 9 is adiusted to eive a readinn of zero. The .. ~ .~~ ~0 V. . R ~ ahove stepsshould bedone with theelectrudesand meter, MI, disconnected. This calibration need he done only once unless ~~~-~~ the meter fails to indicate the proper pH of known samples. If recalibration fails to eive the . proper - results, then IC 1should be replaced. With the electrodes and meter connected, the meter is calibrated using standard buffers at pH 4,7,10. R6 is used to adiust the meter to give the values listed in the table. Any vaiiation from these values that cannot he adjusted using R6 may be made by a minor movement of R8.The meter is now ready for use. The instrument is extremely useful and highly accurate. The primary factor in accuracy and reproducibility is the quality of the meter. My pH meter uses a galvonorneter that I have had on hand for several years, and the results are comparable to those of commercially produced models. Pin jacks were used at the instrument housing, and the galvonometer is connected to these with a short length of wire. Thus, the meter may be used for other purposes. The instrument requires little adjustment during use and the batteries will give 3W40 hr of continuous use. Alkaline or rechargeable nickel-cadmium hatteries will give even longer periods of use. A 100-0-100 meter may be substituted for the one indicated to allow for a pH range of 1-14. The parts cited in the figure are available from a local radio parts store or Allied Electronics.' Many parts may he salvaged from old TV sets and radios; therefore, checking first with local repair shops may he helpful. ~~~
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Schematic circuit diagram of me pH Meter. IC1 741 op amp. R6 01FET2N3819 R1 3.3 Megohm resislor 1 % R2 8.2 K resistor 1% R3 15 K resistor 1% R4 2.2 K resistor 1 % R5 4.7 K resistor 1%
IK potentiometer R7 10 K potentiometer R8 2.5 K potentiometer R9 2.5 K potentiometer CI ,001 uf capicator M I 500.50 microamp meter
Standard Settings tor Calibrating pH Meter
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pH
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Meter reading 50 left of o 40 left of 0 30 left of 0 20 leftof 0 10 leR of 0 0 10 right of 0 20 right of 0 30 right of 0 40 right of 0 50 right of 0
MBmOds and details of how yo, can save money oy b~ la ng you own ~ q u i p r w nare t oulinw n his featve f y a have made romethmg which is particularly useful or money saving. please share it with our readers. Sheldon H. Cohen received his B.S. in 1956 from the University of Wisconsin, and his Ph.D. In 1962 horn the University of Kansas. He has been teaching at Washbun University since 1960, wiwe he is presently
Chairman of the Chemistrv Deoartment. Dr. Cohen has been actjve indeveloping a mmoonad advancs lakwat0ry pogam a r i nar o w n "song a self-pacedheshman aboraton,.oroaram faman" vears She has been involved in directins And teachinq inst tneo fame leachmg of scmnce wile mentary schoo s ana has lor many years Dam offerlogan enrichmem laboratory program for mldale rrnwl age ch~ldren.
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Allied Electtonica, 401 E. 8th Street, Fort Worth, TX 16102. Volume 58 Number 3 March 1981
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