During the summer of 1963, the author had the privilege of attending an Advanced Placement Chemistry Institute in which the participants built some simple instruments useful in chemistry and studied their usual applications. The A. C. Conductivity Bridge built by the participants seemed to offer some advantages in presenting one of the demonstrations accompanying an experiment used in the Chem-Study course at Roosevelt High School in Des Moines, Iowa where the following activity was conducted

Morrison, Rex T.

University of Northern Iowa

Iowa Science Teachers Journal Volume 3 Number 2 Article 5 1965 A Quantitative Study of the Conductivity of Solutions Rex T. Morrison West Essex Regional High School Follow this and additional works at: https://scholarworks.uni.edu/istj  Part of the Science and Mathematics Education Commons Let us know how access to this document benefits you Copyright © Copyright 1965 by the Iowa Academy of Science Recommended Citation Morrison, Rex T. (1965) "A Quantitative Study of the Conductivity of Solutions," Iowa Science Teachers Journal: Vol. 3 : No. 2 , Article 5. Available at: https://scholarworks.uni.edu/istj/vol3/iss2/5 This Article is brought to you for free and open access by the Iowa Academy of Science at UNI ScholarWorks. It has been accepted for inclusion in Iowa Science Teachers Journal by an authorized editor of UNI ScholarWorks. For more information, please contact scholarworks@uni.edu. DECEMBER, 1965 29 A Quantitative Study OF THE Conductivity of Solutions During the summer of 1963, the author had the privilege of attending an Advanced Placement Chemistrv Institute in which the partici p a n t s built some simple instruments useful in chemistry and studied their usual applications. Th e A. C. Conductivity Bridge built by the participants seem-ed to offer some advantages in pre-Morrison senting one of the demonstrations accompanying an ex-periment used in the Chem-Study course at Roosevelt High School in Des Moines, Iowa where the follow-ing activity was conducted. Experiment 17, found in Chemistry -An Experimental Science Labora" tory Manual published by W. H . Free-man and Co., is the experiment re-ferred to above. In this experiment, it is important to determine which acids and bases used were weak or strong electrolytes. Some foundation for this judgment is provided in an Rex Morrison received a B.S. degree in General Science from Parsons College, a B.S.P. degree in Pharmacy from Drake University, a M.S. degree in Science Education from Drake University in 1955, and has attended several N.S.F. Institutes in Chemistry since that time. He taught Chemistry at Roosevelt High School in Des Moines, Iowa for eight years before moving to West Essex Regional High School in North Caldwell, New Jersey where he is currently a teacher of Chem-Study Chem-istry and Chairman of the Science Dept. REX T. MORRISON earlier demonstration in which the relationship of the presence of ions in a solution and its concomitant con• ductance of electricity was demon-strated in a semi-quantitative manner by use of an apparatus which could show conductance by the lighting of a two-watt neon glow bulb, and/ or a ten-watt bulb, and/ or a forty-watt bulb, or non-conductance by the light-ing of none of the bulbs. This appara-tus was suggested for the determina-tion of conductance of the acid and basic solutions used in Experiment 17, and for the study of the change in conductance during the titration of a Ba (OH) 2 solution with H2S04. It was decided that the simple electrical theory of the A. C. Con-ductivity Bridge could be used as a The conductivity bridge, using alter• nating current, is housed in the box at the left. The ring stand supports the 2 plati-mun electrodes and the burettes for titra-tion. A magnetic stirrer may also be used, as shown in the photograph. 30 means of teaching some of the electri-cal units used in Ohm's Law relation-ships, and also as a background for later discussion of such instruments as the mass spectrograph, the teach-ing of Faraday's Laws of electrochem-Using earphones to detect when the conductance of the solution is optimum, a student takes a numerical reading-directly from the dial of the conductivity bridge. "'-OF 300 TITRATION OF 00050 M N..oli (50.<M} WI TH 0 .050M HCL oJO HCL soo 15.00 7.00 ll. 1000 IOWA SCIENCE TEACHERS' JOURNAL istry, and related material which fol-lows closely on the presentation of acid-base theory in the Chem-Study course. It can also be used to show how more quantitative data on con-ductance can be obtained, thus build-ing on the previous experience of the students with the simpler conduct-ance apparatus. Therefore 0.0050M solutions of each acid and base used in Experiment 17 and the accompany-ing demonstration were prepared and their conductance was measured with the bridge. The bridge circuit, Fig. 4, and a reference to further details on its construction , Table 3, are available with this report. The simple theory of the bridge accompanies the circuit diagram. The calibration of the var-iable resistance was discussed and the use of the calibration curve to permit relative conductance calculations was explained. The dial reading of the var-iable resistance minus an intercept of RELATIVE CONDUCTANCE IN ~ alo- 3 LOO Z.00 300 4.00 aoc DECEMBER, 1965 3.3 times the resistance per dial unit from the slope of the calibration curve gave the resistance of the cell. This cell has two small platinized Pt elec-trodes which must be kept under dis-tilled water when not in use. The in-verse of this resistance is the relative conductance. Use of a standard 0.010M KCl, solution makes possible the cal-culation of the cell constant for the cell if equivalent conductanees of so-lutions are desired by calculation from the specific conductance of a solution, k . The relationship used in this calculation is !\ = (1 ,000/ N) k, in which !\ is the equivalent conduct-ance, N is the number of equivalents per liter, and k is the ~pecific conduct-ance. For comparative purposes, such as in the conductimetric titration per-formed, the relative conductances were used on the graphs. In each class, the conductances of the solutions were measured to find which conducted well, and which to FIG. 2 TITRATION OF 50.00.. Of 00045M BA (OHJ.i Willi Q.050 M HzS04 31 a much lesser extent, i.e., which were strong and which were weak electro-lytes. Then, one of the three titrations was demonstrated to each class to familiarize them with the techniques involved. The data from all three were given to the students, and they were asked to write the equations for the reactions involved in the titra-tions. They were also directed to graph the relative conductance calcu-lated from the given dial readings for each addition of titrant against the volume of titrant added. They were asked to explain in a brief paragraph why the graphs had the shape they did in view of the measured conduct-ances of the solutions involved. The titrations performed were those of a strong base, 0.0050M NaOH, with a strong acid, 0.050M HCl, in which a soluble salt was formed. A second ti-tration was that of a strong base, 0.0045M Ba (OH) 2, with a strong acid, 0.050M H2SO4, in which an insoluble RELATIVE CONDUCTANC£ IN OHMS°4 x 10·3 10 8 ll 10.00 32 salt was formed. The final titration demonstrated was that of a strong base, 0.0050M NaOH, with a weak acid, 0.050M CH~COOH. A sample of data actually taken from these demon-strations is given in Table 2. Table 1 gives similar data for the conduct-ances of the solutions used. The graphs of the data from Table 2 such as the students prepared are shown in Fig. 1, 2, and 3. A magnetic stirrer was used to achieve uniform mixing during the titration, although any me-thod of stirring which does not dis-turb the electrodes would be satis-factory. In conclusion, it was felt that though this procedure required more time than the simpler demonstration it replaced, the understanding of the ionic nature of solutions and the effect FIG. 3 TITRATION OF 50.0ML OF 0.0050M NAOH WITH 0050M CH3 COOH ML IOWA SCIENCE TEACHERS' JOURNAL of this nature on their conductance was much more thoroughly under-stood by the students. Their apprecia-tion of the role of instrumentation in science to obtain the quantitative data upon which the theories of solutions are based was also increased. The necessity to understand some simple electrical telationships and units used in electrical measurements increased the student's motivation for gaining a background which was of great as-sistance in developing later topics in which an understanding of these ideas was highly desirable. Better under-standing of these topics seems to make the extra time requirement a worth-while investment, although it also em-phasizes the need for extra laboratory periods for scientific investigation in modern science courses. - 1 -3 RELATIVE CONDUCTANCE IN OHMS x 10 000 1.00 2.00 3.00 400 500 6.00 00 1.00 2.00 3.00 OF 9. 10.00 DECEMBER, 1965 TABLE 1 Conductance of Various 0.00S0M Solutions at 24°C '\,, ... j • 0 8:,, = .50:, 8 QI j i 8 = = bl) .. 8 ... = =I !l 0 -i ;,, ., .£ = loC :c :c ::lo:, ~ !l ii) .!I 1 = ,::, :s- ~~ ! :S..:,S ::I ~! s~ ~8; (/J (/J '1:1 ., 1. HCl P1 16.0 6.73 0.238 1.60 2. HNO3 P1 15.5 7.00 0.238 1.66 ~- H~O4 P1 11.0 1.11 0-238 2.64 4. CH3COOH P2 25.0 3.94 0.238 9.39 X 10·2 5. NaOH P1 24-5 4.04 0.238 9.72 X lO'·l 6. KOH P1 24.5 4,04 0.238 9.72 x 10-l 7. Ba (OH)2 .. P1 15.0 7.31 0.238 1.74 • (The k, specific conductance, of 0.0lOM KCl at 25°C is 1-413 x 10·3 ohms·1cm·l and at 24 °C is 1.386 x 10-3 ohms·1cm·l. The k for d istilled water at 25 °C is 5.8 x 10-8 ohms-lcm•l . Conductance of 0.0lOM KCI at 24 °C based on a dial reading of 18.0 at P1 was 5-82 x 10·3 ohms·1 on this cell which gave a eel !constant of 0.238 from k - (1/ A )L In which 1 is the length of the conductivity cell, A is the area between the electrodes, and L is the reciprocal of the measured resistance of the cell.) •• (The Ba(OH)2 solution was only 0.0045M.) P1 roR o To 1,ooon.~ = l..!!.!l R1 IOK/\ flG. 4 A.C. CONDUCTANC.E BRDGE l'zroR O TO IOI\ ~= J2!!!' 'R1 IOlcl\ "3 fOR OTO 1001\ ~=~ 'R1 l•I\ J . CHEM. ED. B 244 (MAY 19e0) WHEN POINTS A AND B ARE AT THE SAME POTENTIAl, THERE IS NO SIGNAL THROUGH THE EARPHONES. VCA='1"1 Vee =lzR• VCA: Vea 11 R1 = lzRv V=IR ~R1 lzRv 11Rz-1zRc VAD= 11 Rz V!I)= lzRc V~"sD 33 IOWA SCIENCE TEACHERS' JOURNAL TABLE 2 Titration Data Heading 1 - 0.0050M NaOH vs 0.050M HCI Col. 1 - ml of acid added Heading 2 - 0.0045M Ba(OH)2 vs 0.050M H2SO4 Col. 2 - dial reading Heading 3 - 0.0050M NaOH vs 0.050M CH3COOH Col. 3 - Relative Conduc-tance in ohms·l x 10·3 All readings at P1 except •, which are P 2. Heading 1 Heading 2 Heading S Col. 1 . Col. 2 Col. S Col. 1 Col. 2 Col. S Col. 1 Col. 2 Col. s o.oo 25.0 3.95 o.oo 16.0 6.75 0.00 23.0 4.35 0.50 27.0 3.61 0.50 18.0 5-82 0.50 24-0 4-13 1.00 29.0 3.33 1.00 21.0 4.84 1.00 25.0 3.95 1.50 31.0 3.10 1.50 24.0 4-14 1.50 27.0 3-61 2.00 33.0 2.88 2.00 30.0 3.21 2.00 29.0 3.33 2.50 35.0 2.70 2.50 40.0 2.34 2.50 31.0 3.09 3.00 37.5 2.50 3.00 59.0 1.54 3.00 33.0 2.88 3.50 40.0 2.33 3.50 16.5* 0.65 3.50 36.0 2-62 4.00 43.0 2.16 4.00 38.0• 0.25 4.00 40.0 2.33 4.50 45.0 2.05 4.50 59.0 1.54 4.50 44.0 2.10 5.00 45.0 2.05 5.00 35.0 2.70 5.00 49.0 1.87 5.50 38.0 2.47 5.50 25.0 3-95 5.50 49.0 1-87 6.00 30.0 3.21 6.00 20.0 5.13 6.00 49.5 1.85 6.50 25.0 3.95 6.50 17.5 6.03 6.50 49.5 1-85 7.00 22.5 4.45 7.00 16.0 6.75 7.00 50.0 1.83 7.50 20.0 5.12 7.50 14.5 7.64 7.50 50.0 1-83 8.00 18.0 5.82 8.00 13.5 8.39 8.00 50.0 1.83 8.50 17.0 6.25 8.50 12.5 9.32 8.50 50.0 1-83 9.00 16.0 6.75 9.00 12.0 9.84 9.00 50.5 1.81 9.50 15.0 7.31 9.50 11.0 11.1 9.50 50.5 1-81 10.00 14.5 7.63 10.00 10.5 11.9 10.00 50.5 1.81 TABLE 3 Parts List - A. C. Conductance Bridge Quantity Article Cost 1 10 kohm potentiometer- Type J -AB __________ 2.75 1 2 pole, 6 position non-shorting switch, Malory 3226J 1.06 2 Transistors, Raytheon CK 722 2-60 2 Capacitors-2 microfarad 1.16 1 Capacitors--0.2 microfarad -30 1 Capacitor--0.02 microfarad . .28 1 Capacitor--0.005 microfarad .28 2 10 kohm deposited carbon resistor ( + 1 % , ½ w) .83 2 1 kohm deposited carbon resistor (+7%, ½ w) .83 1 1 kohm resistor< + 10%, ½ watt) - .19 1 3.3 kohm resistor <+ 10%, ½ watt) .19 1 18 kohm resistor c+ 10%, ½ watt) .. 19 1 4 kohm headphones (Lafayette F -374) 1.35 1 DPDT switch .60 2 SPST switches .64 1 Dial plate for 10 kohm potentiometer ······- .45 2 Platinum foil electrodes and wire 2.29 2 6 V batteries 2.14 1 1 ½ V battery-type NE .11 1 Tra nsformer (Merit Coil and Transf. Corp.-C2973) 1.35 10 Bana na plugs and knobs 2.07 Miscellaneous: mounting panel and base, hardware, shipping costs _________ _____ 3.35 25.11 

A Quantitative Study of the Conductivity of Solutions

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A Quantitative Study of the Conductivity of Solutions

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