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Construction of a Photoelectric Calorimeter and Application to Students' Experiments (Part 2) Tsutomu Matsuo Kobe Technical College Maikodai TarumiXu Kobe 655, Japan
Akihiko Muromatsu, Minako Mori, and Kazuko Katayama ~ o b university e Facully of Education Nada-ku Tsurukabuto Kobe 657, Japan
Recently, students have been apt to purchase everything and anything they want. Very few students take the time to make anything. However, it is essential for people to make things. Chemical education should provide students with the skills necessary to make measuring instruments. In the process of making them, students can come to understand clearly the principles of measurement. Also through such experiences,students will learn some points of improvement and their creativity will be developed. In fact, many chemists have made various measuring instruments by themselves. Consequently measuring instruments are routinely being invented and improved. Therefore, the authors have tried to use the measuring instruments assembled by students (1, 2). If the students assemble the instrument, they become attached to the instrument and their interest appreciably increases. In our previous paper (3),we reported on a photoelectric colorimeter (PEC) and the student's experiments with it. In this paper, the construction of a version of the PEC and its new uses are presented. Construction of a Portable PEC
A portable PEC was constructed by simplification of the stationary-type PEC (3). It is compact because the power supplies are exchanged for dry cells, and the amplifier is omitted. Therefore, it can be used at a place having no ac
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Journal of Chemical Education
Figure 1. Exterlw view of lhe portable PEC
line, and it can also be employed for field work. As a digital voltage meter is huilt into it, an external meter is not needed. The portable PEC is shown in Figure 1.The numher of the arts is reduced. and so it can be assembled easilv and economically. In Fieure 1.1and 2 are switches for the Dower suo~liesand digital Goltage meter, respectively. 3 is the meter display (0.1 mV-15 V, SOAR 3020). The digital meter serves to enhance the analytical sensitivity. 4 is a cover for shutting out the environmental light. After the cell is inserted, the cover is closed. 5 is the light source of the optical system. 6 is a cell. 7
Ku"1
I
X lo-"Ol/'l
FiQure 4. Calibration graph for Cue+(ammonia memod).
Figure 3. Circuns for Uw p o w supply and h LIS. (1) Voltage stabillring ckcult for me bulb. (2) Circuit for me meesurement of the light intensky.
is a knob for the adjustment of the light intensity sensor (LIS, Siemens, TFA 1001W). The detailed structure of the optical system is shown in Figure 2. Number 1is a bulb bracket. (A flashlight bulb with a built-in lens (2.2 V, 220 mA) is used.) A diaphragm in the round plastic plate (hole radii 1-3 mm) and a colored cellophane filter are set up in it. The filter can be replaced accordine to the color of the solution. 2 is a dastic cell for the spectrop~otometer.3 isacellstand. I t is designed to hold the cell in lace because the variation of the cell position affects the absorbance. 4 is the entrance for the trksmitted light. An LIS is set up, and a diaphragm attached to the LIS shades the stray light. The circuit of the power supply for the bulb is shown in Figure 3(1). The dc voltage (1.5 V X 4) is applied to the variable voltage stabilizing circuit and 2.2 V dc is acquired. If the dry cell is directly connected to the bulb, the light intensity gradually decreases, even in the course of the measurement, due to the drop in bulb voltage. The power supply must only be turned on while measuring absorbance in order to keep from using up the cell. The circuit for measuring the light intensity is shown in Figure 3(2). The LIS consists of a photodiode and amplifier, it can linearly convert the light intensity to the voltage. Thus, the light intensity can be measured by the output voltage of LIS (1mV = 1Lx). The parts for assembling the PEC were mostly purchased at Ninomiya (4-11-4, Nipponbashi Naniwa-ku, Osaka, Japan). They are also supplied by the electronic parts shops of big cities in Japan. The LIS was obtained from Kyouritaudenshi (5-7-19, Nipponbashi Naniwa-ku, Osaka, Japan). The total expense was about $25, apart from the digital meter. ($1 = 135 yen.) As with the principle of the measurement described in the
0
5
10 Elapsed
15 time /
20
25
min
Figure 5. Varlatlon of tha sbso&nce wim passage of time (Cu-Ag+ system). Copper plate: 120 X 10 X 0.5 mm; soitition: 0.3 mol1L AgNOs, 50 mL; filter: red.
previous paper (3),the absorbance (4) (A) is also given as follows: A = log (EdE)
(1)
where Eo denotes the output voltage for pure water and E the value for the test solution. Experiments Uslng the Assembled Portable PEC The calibration graph for Cu2+was obtained to confirm the characteristics of the PEC. All reagents were of analytical-reagent grade. For increasing the sensitivity, an excess of NH3 (aq) was added to Cu2+solutions. A yellow cellophane filter was used. As the graph shows in Figure 4, a straight calibration graph (2 X 10-3-8 X 10-3 m o l h ) is obtained. Hence, the assembled portable PEC can be used to determine the concentration of the colored solutions. Reaction between Metal and Metal Ion
When a metal is immersed in a solution containing metal ion that has a smaller ionization tendency than the metal, a chemical reaction will occur. Such chemical reactions were studied with the PEC. Fifty milliliters of 0.3 mol/L AgN03 solution was placed in a 100-mL beaker and stirred with a magnetic stirrer. A copper plate (20 X 10 X 0.5 mm) was immersed, and the elapsed time was measured. The stirrer Volume EE
Number 10
October 1989
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Table 1.
la,
Analysb ol Copper In Brass* F€C
Absorbance Concemratlon (molA) WeigM of Cu
(a
Elapsed time /
min
F l p e 6. Variation 01 he absorbance wim paasage 01 time (Ln-Cu2' system). Ztncplam(1)60X 10X0.5mm.(2) 10X 120X 0.5mm:sot~Ion:O.l5mollL cuso,. so mL: finer: red.
spsctropholometer
0.0783 5.52 X 0.0877
0.3346 5.56 X to-' 0.0883
The following experiment gauged the precipitation of copper. A zinc plate (60 X 10 X 0.5 mm) was immersed in the 0.15 molL CuSOa solution (50 mL), and its absorbance was measured every 2 min. Another zinc plate (120 X 10 X 0.5 mm) of different surface area was also examined. Fieure 6 indicates the results of the measurement. The absoihance gradually decreases, and it reaches zero at 24 min (curve 1) and 18 *in (curve 2). At these points, Cu2+is considered be almost fully deposited. The process of the stripping or precipitation of Cu can he realistically observed by the ahsorbance. Thus the PEC can be used to measure very minute concentration changes. Quantitative Analysis of Copper in Brass
Figure 7. Callbratton gaph of Cu2+ for hanalysis of Cu in brass. (1) PEC (filter: red). (2) spsaropholomster (596 nm).
Since brass is beautiful, it is widely used in homes and the workdace. However, manv students do not know the comnositiod of brass. TW; difflrent measuring instruments, the PEC and a spectrophotometer (Shimazu. U 3200) were used to examine the cobper content. A brass sample (0.1277 g) was placed in 3 mL of 8 molL HN03 and heated. After the sample was dissolved, the solution was evaporated and filtered. The filtrate was diluted to the mark of the volumetric flask (100 mL) with water. Ten milliliters of 6 N NH3(aq) was added to 10 mL of the solution and diluted with water to 25 mL. In order to achieve the same condition as the practical analysis, three samples of pure copper (120-160 mg) were dissolved, and the calibration graph was obtained (Fig. 7). The concentration was determined by this calibration graph, and the copper content was calculated (Table 1). The copper contents in brass obtained with the PEC and spectrophotometer are 68.7% and 69.2%, respectively. Analysis of Iron in Spinach and Tea Leaves
It is said that spinach is rich in iron. The authors decided was shut off 30 s before the measurement. The supernatant to measure the iron content of spinach (5) and tea leaves. was transferred to the cell after a precipitate of silver had The standard solution of Fe3+ (0.0105 molL) was obtained settled. Then the absorbance of the solution was measured, by dissolving iron ammonium sulfate in acidic solution and the measurement was repeated every 2 min for about 25 (H2S04). For the color development, aKSCN solution (0.202 min. molL) was used. A sample (10 g) was placed in a ceramic The correlation between the elapsed time and the absorcrucible and burned for 1h. The ash was transferred in an bance is shown in Fieure 5. The absorhance is linearly inevaporating dish, dissolved in 10 mL of 8 N HCI and evapocreased at first, but it-levels off after 18 min. As the reaEtion rated in a water bath. Then concentrated HCI was added, proceeds. the surface of the comer plate is covered with and the mixture was filtered. The filtrate was diluted with deposited silver, and a chemical e;uilibrium is entahlished. water to the mark of volumetric flask (50 mL). Five milliliThen the reaction between Cu" and Ag was quantitativeters of the solution was transferred to a beaker, and conceuly examined. The total moles of stripped copper were detertrated HN03 (2 mL) was added and heated. After the solumined from the absorhance of the solution ( A = 0.707) by tion was cooled, a KSCN solution (5 mL) was added and usine the other calihration eraoh for Cu" iCu2- = 0-0.225 diluted with water to the mark of the volumetric flask (25 molk, A = 0-1.01); the c&c&ration of cu2+was determL). Then the absorbance was measured to determine the mined to be 0.156 mol/L, and this confirms that 7.80 X concentration. The calibration graph for Fe3+ (Fig. 8) was mol of C U ~was + stripped. On the other hand the deposited obtained by adding an excess of KSCN solution to Fe3+ silver was removed by scrubbing with a wooden rod. Then it solution. The results of the measurement are listed in Table was filtered through a siutered glass disk and dried and weighed (1.72 a). This precipitate corres~ondsto 1.59 X 10W2 moiof silver. f i e mol& ratio of ~ ~ 2 + : ~ g ~ a r t i cin i ~the a t i n ~Determination of the Acid Dissociation Constant of reaction is expressed as follows, Cu2+:Aa = 7.80 X 10-3 BromthymolBlue mol:1.59 X 10--~mol=1.00:2.03 12. hisf fact verifies that The indicator, BTB (BromThyrnol Blue) (6) is familiar to the following reaction formula is correct. the student. Since the color transition region is attributed to its acid dissociation constant (K.), the characteristic of the 850
Journal of Chemical Education
Table 2.
Analysls ol Iron in !3plnach and J m l n e Tea Leaves S~inach
Absorbance Concenbatim (molR) Weight of sample (g) Content of Fe (mg/lM) g) Liieraturevalues (mgIlO0 g)(@
5.10X lo-= 2.63 X 10-I 7.811 4.70 3.7-8.0
Jasmine Tea leaves 8.77 x 4.91 X 1 0 F 5.167 13.3 not available
BTB can be understood if its K. value is known. Hence the K. value of BTB was determined. Two buffer solutions were used for the variation of pH (CH3COOH-CH3COONa,pH < 5.5; NH3-NH4C1, pH > 6). The pH value was measured with a pH meter (TOA, HMSES). The absorbance was measured after adding 2 drops of BTB (0.1%) to each buffer solution (5 mL). Since the total concentration of BTB must be the same in each solution, BTB was dropped with a sharp glass tube. The correlation between pH and the absorbance is shown in Figure 9. The absorbance increases with the increasing pH and an S-shaped curve is obtained. The dissociation of BTB is expressed as follows: HIn * HC+ In'
Figure 8. Calibration gaph of Feat for jasmine tea leaves, filter: yellow.
me analysis of Fe. (1) Spinach, (2)
(6)
where HIn is BTB. Then pK, is given by [HInl pK. = pH + log -
[In-I As shown in Figure 9 A,,, and A,i, are the maximum and minimum absorbance, respectively, and A is the absorbance at the arbitrary pH. If the following equations are valid,
A = %(Am, + A,,)
(10)
the equation pK. = pH can be derived. Accordingly when eq 10 is valid, the pH value corresponds to pK, value of HIn, that is, the average pH of A,, and A,i, absorbance is equal to pK.. Hence from the a w e in Figure 9 pK, = 6.80 is obtained. This DK- value is near the literature value (7.30). The deviation 6ouid result from the wide wavelength range of the cellophane filter. Since the color transition region of the acid-base indicator is near the pK. f 1value, BTB is an indicator that has a color transition region near neutral. Though the determination of pK. is usually performed with a spectrophotometer, it is found that pK, value of the acidbase indicator can be measured with the assembled PEC.
ments can he es~eciallvsienificant in lieht of the current flood of many highly devefuped instrumks. The author; believe that instruments which doevervthine for the student are not the best teaching tools.
Conclusion
Acknowledgment
The portable PEC is very compact because it has no ac power supply nor any amplifier. The circnit is very simple and easily assembled. However, its performance characteristics are comparable to a stationary-type PEC. It can be applicable u; the students' exper&& and can obtain many fruitful results. The student can assemble it and use it to recognize the principles of measurement. Through measurement with such hand-made instruments the student will become familiar with chemistw and will be motivated to learn. All the experiments described here can be incorporated into the curricula of senior hieh school or undermaduate university programs. The hand-made measuring instru-
The authors would like thank to Satomi Murakami and Keiitsu Saito, Faculty of Education Kobe University, for helpful suggestions with this work.
~ l g v 9. e Cwrelanon between pH and the abswbance (BTB); finer: yellow.
1. Matru0.T. Kogoku Kyouiku 1984,32.430. 2. Matsu0.T. Kogaku Kyouiku 1984.82.522. 3. Matsuo,T: Muromatsu, A,: Kstsyama, K.; Mori, M.J. Chem. Educ. 1389.66.329. 4. Ewing, G. W.Inslrumenfol Method8 ofckemirol Anoiysis; MeGraa~Hill:N m York,
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66
Number 10
October 1989
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