Edward N. Wise University
of Arizona Tucson
A Modular Approach to Chemical Instrumentation
Modern complcx instruments are becoming commoli tools of t,he professional chemist, and the problem of cducating future chemists to use these tools intelligently is heginning to receive serious attention. An at,t,empt to simplify the problem by regarding inst,ruments as "black boxes," whose internal operations are of no concern to the chemist using them, has resulted in costly errors and wasted effort. Instruments are used without proper regard for their operational charact,eristics and limitations. A common complaint. of industrial supervisors and research directors is that the recent graduate usually believes that almost any problem can be solved quickly and easily by the purchase and application of some inst,rument, (usually an expensive one) and that problems present,ed by limitations of the principle employed by the instrument become apparent t,o him only after he has had experience with its application. Thc problem of properly preparing future chemists to make the best use of contemporary instruments, and of inslrumrnts yet to he developed, may be more soundly approached by helping the st.udent to acquire a sufficicnt knowledge of the principles of chemical instrumentation and their application, so that he has an int.uitive sense in the use of an instrument. Instrumental principles are fairly easy to present in lectures and in texts, but, familiarity with the application of these principlcs is not so easy for the student to ohtain. The modern instrument,^ which apply the principles are designed for precise, trouble-free operation, which usually mcans that t,hey are enclosed, and somet,imes sealed, t,o prevent the entrance of dust or light. The morc complex the instruments, the less likely will they be open to inspect,ion that will reveal their operation. To assist the student to acquire familiarity with the application of inst,rumental principles, a modular approach is taken. The student is provided with relat,ively simple instrument building blocks, or modules, each of which may be studied in detail to observe thc application of a principle of chemiral inst,rument,at,ion, and which may subsequently he interronnccted to form operating instruments. The student progresses from the relatively simple to the very complex by the interconnect,ion of appropriate modulrs. The principle considerations which have influenced the design of the modules are (1) simplicity of construction (including t,he use of commercially available hardware), (2) accessibility of all parts to inspection. (3) interchangeability of all modules having similar
This work has hrrn nopportrd hy a grant from thc Nat,ional Science Foundation.
functions, (4) low cost, and ( 5 ) nlgg~dncssto withstand student. handling, inspect,ion, and operation. Each module is sufficiently simple to permit its construction on a limited budget. The set of basic modules is listed in Table 1. Additional modules for more complex automatic and recording instrumentation are being tested, and will he described in a later publication. The modules included in this paper have been used by st,udents participating in the development program for two years and in formal laboratory instruction for one year. Detailed photographs of the modules have heen taken, and dimensions have been drawn on the photographs so that they may be duplicated hy student,^ or instructors. Publication of a construction manual, including the dimensioned photographs, is now heing undertaken. Figure 1 is a dimensioned photograph of a light source (Rlodule 3) with its side open. When in use, a metal side-plat,e held by four spring clips closrs the side. Other views of this module have been prepared xith the remaining dimensions necessary to facilitate its
Figure
1.
Figure 2.
Module 3 (light rourcel.
Modules 3.6, and 15.
Volume 40, Number
2, February 1963
/ 73
Table 1 .
Instrumentation Modules
Photovoltaio cell housing-with shnt,ter Ma, 1 ma, 10 nru ranger, overload-pra2. JIicn,xmmeter-100 terted 3. Light source-produces fooused heam 4. Light source supply transformer 5. Constant voltage light source supply tmnafnrnmer 6 . Test t,ube holder-with filter hulder 7. Spaker holder-with filter holder and interehnngeahle lids ( o m with a light-shirld fur photometric titrations) 8. Beam splitter-prism or secnisilvered mirror 9 . Test t,ube holder, right angle-with filter holden 10. Beaker holder, right angle--with filter holders and i n k ehnngmhle lids as in module 7 inonochmmiltor-with adjustahlr mtranre and exit 11. (>r;~t,ing dits and wavelength selector adjustahlc entrance and exit 12. Prism monochromator-with 1.
-
~
~
- .
- - ~
out,pot f 1.5 V.D.C. at 1500 oh& (or + I ma) 18. (hrrent smplifier-input 0.1, 1, 10, and IOU Ma, 1 ma, output I volt at 1000 ohms (or 1 ma) Rcgulntcd power supply-out,put of +IS0 V.D.C., +I05 1 T.Ij.C.. - 150 V.D.C. ;st, 20 mn ( d l reerdntedi, and 6.3 V.A.C. unrrguistrd 20. Vnrixhlr powcr supply-output of 0 to 250 V.1j.C. a t 125 ma and 6.3 V.A.C. 21. Photomult,iplirr tub^. housing-with shuttrr 22. Phot,amult,ipli~r power supply 2% Asl,irator burner-with forusing lens and mirror, and filter hol&r 24. hlngnetie s t i r r ~ r 25. Illt~xvioletsomre-mercury vapor lamp and Dower supplv
constrr~rtion. I.'igure 2 shows side views of a light sourre a i d a gas phot,otu~bereceptor, and a top view of a test tuhe holdrr. The bracket a t the right in t,he test tuhe holder is the holder for 2-in. square filters. Only a few of t,he experiments t,hat may he performed hg students using these modules will be discussed with the realization that instructors will vary in their emphasis of functional principles and will use the modules t,o illustrat,e t,heir presentation. A manual of esperiment,s performed by our students is in preparation for puhliration. Optical Experiments
hlodrlles 1 and 2 may he connected to form a simple phot~orle(:tric phot,ometer. This will int,rodoce the student to the self-generating barrier-layer photocell, and the instrument may he used wit,h a met,er stick and a 10 wat,t. incandescent bulh to verify t,he relationship of distance to light int,ensity. The st,udent will observe non-linearit,y at. high intensit,y levels, due t,o t,he transit. time of current,-carriers in t,he semi-conducting layer, and a backgromd current a t low intensity, due to room illuminat,ion. He may then proceed to the use of modules 14, 18, 19, and 20 which, with module 2, will form a photometer having a wider range of linearity. hlodrdc 19 supplies power to module 18, and module 20 supplies a variable potential to module 14. Modules 3 and G may t.hen be connected to each other and to module 1, by their simple keyway connectors, t,o form, wit,h module 2, a simple photoelectric absorption instrument, with power supplied by either module 4 or 5 , which may be used for practical turbidmetry or colorimetry measurements. Figure 3 shows a student inserting a sample t,uhe in the colorimeter which he has 74
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Journal o f Chernicol Educofion
assembled from modules 4, 3, 6, 1, and 2 (reading from left to right). The principle function of this instrument is to illustrate inst,rument.alprinciples such as the import,ance in ahsorptimetry of having a source of constant radiant flux, and of the spectral response of the receptor. The effectiveness of a constant-voltage light source supply transformer is checked by first, connect,ing a variahle-voltage transformer (Variac or Powerstat,) between the 115 volt source and the unregulated light source supply transformer (module 4) and taking a series of photocurrent readings a t various settings of the Variac, then replacing module 4 by module 5 and repeating the same measurements. The linearity of the receptor is easily checked hy using a series of Kodak Wrat,ten neutral density filters in the filt,er holder of module 6. The spectral response of the source-receptor combination may be checked by the nse of a set of calibrated interference filters, if these are available, or this experiment may be performed later wit,h the grating monochromator after calibration. An alternative method of compensating for changes in the source flux,and a met,hod for directly observing the ratio of absorhanre of two samples, is to split the source flux into two beams and use the output of a second recept,or as a reference. This may be accomplished by placing module 8 between modules 3 and 6 of the previously desrrihed colorimeter, and then connecting another receptor, either directly or through another module G , to the right angle exit port of module 8. The out,puts of the tn.0 receptors are compared with a simple bridge circuit, and their ratio is the input to module 18. We use tn.0 identical receptors to correspond to prartical commercial inst,rumentat,ion, hut unlike reccptors may be used mithin their linear response regions if the expense of duplicating receptors is a factor. The limitations of compensation by ratio comparison will hecomr evident to the student during t,he experiment. The grating monochromator, module 11, is completely open to inspection by the student,. After interconnecting modules 3, 5, and 11, the student may trace the beam, using a small strip of white paper, from the small right-angle mirror through the ent,ranc.e slit, from which it goes to the collimating mirror as a diverging beam of decreasing intensity, from the collimating mirror toward the grating as a parallel beam of constant intensity, from the grating as dispersed radiation, and from the collimating mirror to a focused spectrum a t the slit. Is'igure 4 shovs two students experimenting with the grating monochromator. The studenl, on the left is adjust,ing the position of the collimating mirror, while the student on the right is controlling the posit,ion of the grating with his left hand to
Figure
3.
Calorimeter orrembled from moduler.
presrnt a. desired port,ion of the spectrum on t,he exit slit while t.raaing the rays wit,h the small strip of white paper in his right hand. Modules 2, 6, 14, 18, 19, and 20 may be attached to form a complet,e grating spectrophotometer. The monochromator is wavclength-calibrated by replacing modules 3 and 5 by modulc 25 and using thc mercury lincs of that source. The oalihrated monochromator is then ready for nnmemus experiment,^, including plot,ting the transmission curve of a Wratten fiker and
Figure 4.
Adjusting the grating monochrometer.
t,he determination of t,he spectral sensitivity and linear.. ity of various rereptors. At this point the student may question the linearit,y of the amplifier. He is then provided with a decade resist,ance box and a mercury reference bat,tery, with which he can accurately det,ermine its linearity and input resistance. Other Experiments
Module 23 (aspirator burner) may be used with othrr modulcs to assemble a flame photometer, with int,ernal standard operation if desired. Figure 5 shows st,udents preparing to study flame photometry. The student a t the left is filling the aspirator with the solut,ion to he studied. Light from the flame will pass through a filtcr in module 6, and be received by module 11 (vacnum phototube). The student on the right is
completing t,he clectriral connertions between modules 2,18,19, and 20 to measure the photocurrent produced. The interronnection of module 17 with modules 2 and 19 forms an electrometrir inst,rnment which may be used t,o det,ermine pH with student-construrted calomel and qninhydrone electrodes, or d h a commercial glass indicator electrode and either the studentconstructed or a commercial calomel electrode. When module 24 is used with module 7, tit,rations may be performed with the endpoint (colorimetrir, spectrophot,ometric, nephelometric, t,urbidimetric, or fluorimetric) detected photometrically, or with an electrometric endpoint ( p H , potrntiometric, redox, or amperometric) by using the appropriate additional modules. Coulometric titrations and polarography are other elert,romat,ic methods that can be studied using these modules. Table 2 is a list of instruments which may be assembled from t,hese modules. I t should hr noted that our students become a(,quainted with electrical measurements, and p o w r supply, dc amplifier and ar amplifier circuitry during the first third of the semester. We have used the excellent elcrtronirs rhapter in "Experiments for Instrument,al Methods" by Reilley and Sawyer as a text, and electronic modules derived from that text. The last two-thirds of the semester are devoted to experiments with instruments ronstructed from the modules desrrihed in this paper. Stndent response and arhievrment have heen excellent, and we believe that the nse of the modules has helped them achieve understanding of the instrumentation principles which will permit them to nse complex instrument,^ of the present and future xith confidence and success. The assistanre of several undergraduate students in the design and construction of these modules is gratefully acknowledged, with special credit to David Ash for his many signifirant contributions to the projert. Table 2.
Instruments Which M a y Be Assembled From the Modules
I. Simple photoelectric photometer-modules 1 and 2 11. Gcnernl purpose photoelcrtrir photometer-modulcs 2 (14, 15, or 16),18, 19, and 20 111. .High .. sensitivity photoelectric photometer-modules 2, 21, Bnd 24 IV. Turbidimet~r-modules 3, (4 or 5 ) , ( 6 or T), and instrument ( -T -. o r-n-), V. Nephelimmtte-mmdulie 3, (4 or 5), ( 9 or lo), and instrument ( I 1 or 111) VI. Colorirnetcr-moddrs 3, (4 or 5), ( 6 or 71, and instrument \
I T T T nr T T T )
VII. Compensating turbidimeter, nephclometer, or colorimeter-module R and appropriate instrument (IV, V, or T I ) with an additional module ( 14, 15 o r 16) VIII. Spertrophatometer-modulpc 3, (4 or 5), ( 6 or i ) ,( I I , 12, or la), and instrument.(II or 111) IXL__,Fluunrmetcr-m
