The Duboscq Colorimeter and Its Inventor John T. Stock University of Connecticut, Storrs, CT 06269-3060
Before the emergence of optics a s a recognized branch of science, certain factors concerning the appearance of common, usually aqueous, solutions were self-evident (I). The tint (ex.. -,, red). is a snecific urooertv . of the solute (2). The intensiry ofthc tint increases with the concentration of the h the solutim. sdutiori ( 3 , .The lonaer thc lirht ~ a t through the more intense tKe tint. ?his can be demonstrated by looking down into a measuring cylinder that is standing on a white surface while steadily pouring in any faintly colored liquid. Any description of the tint depends upon the perceptive judgment of the observer. Quantitative assessment depends on two facts. The visible spectrum covers a quite narrow waveleneth ranee of electromannetic radiation (approximately 460 to 707) nm), and the &en solution selectivelv absorbs onlv - .art to anextent that varies with the wavelength. ~~~
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The History of Optics
Color-Matching by Path Length The history of the study of the concentration and pathlength factors has been described by Malinin and Yoe (I). In 1852, August Beer (1825-1863) gave examples of the proportionality between the extent of light absorbance and the amount of solute. From the experiments on light path length reported in 1729, Pierre Bouguer (1698-1758) concluded that, if a given thickness of glass intercepts half of the light from a source, then the insertion of a second, identical, thickness into the light path reduces the emergent light to one-quarter. ~ o h a n n~ e i n r i c hLamben (1728-1777, mathematically examined the path-length fkctor in 1760, and stated If the particles which capture light are evenly distributed, the logarithm of the diminution of light is equal ta the product of the opacity of the medium and the length of the pathway.
Nowadays, the combined effect of the three factors is commonly referred to a s Beer's Law, which is expressed as
Figure I . Duboscq colorimeter (from Chem. News. 1870) (1772-1842) estimated iron and nickel in cobalt ore by this type of approach (2). Balancing methods involve the determination of b$b,. Strictly, eqs 1 and 2 apply only to a monochromatic light source. However, when comparisons are made by eye, monochromaticy does not matter. I n practice, with white light (real or artificial daylight).Beer's Law usually holds within the limits of visual acuity. Beginnings and Modifications of the Duboscq Colorimeter
where A is the absorbance; PJP is the power ratio of the incident to the transmitted light; E is the molar absorptivity of the solute; b is the path length in cm; and c is the molar concentration of the solute. Until the development of photoelectric devices, colorimetry involved matching, by eye, the absorbances of the sample solution (x) and a standard solution (s) of the same solute. Then eq 1reduces to
Thus, if the ratio bJbXis determined, the concentration c, of the sample solution can be found. One approach is to make a set of standard solutions of differing concentrations (or to quantitatively dilute a single standard) until a match to the eye with the sample solution is obtained. In 1838, Wilhelm August Lampadius
The simplest approach involves the use of Nessler tubes (3).These, used in pairs, are identical, glass, flat-bottomed graduated cylinders. One tube contains a suitable column length b, of the sample solution. The standard solution is progressively poured into the companion tube to the length b, a t which the tints match when observed through the lengths of the respective columns. Mechanical adjustment of thr path lengths makes man i ~ u l a t ~ oofnthe solutions unnecessary The Duboscq colorimeter, a major development in thi8approach, was the A description of the insubject of a brief note in 1868 (4). strument, with illustrations, appeared in 1870 (5).These illustrations are reproduced in Figure 1. The solutions to be compared are placed in flat-bottomed vessels CC' that contain smaller but empty vessels TT'. These can be independently raised or lowered by rackand-ninion svstems (not shown) t h a t have .eradnated . scitl~s.One scale indicates the distance from the undersidt~ of the bottom oST to the inner side of the buttorn of C. The other scale provides the corresponding information for the Volume 71 Number 11 November 1994
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Figure 2. Modification for pH determination
system T'C'. The scale readings thus give the respective path lengths directly. Light (usually daylight), suitably reflected by mirror M, passes through the solution lengths that are defined by the respective positions of T and T'. After undergoing double reflection i n Fresnel rhombs PP', t h e emergent light beams are brought together to produce a split-circle image. Then either T or 'I" is adjusted until the two half-images appear to be identical. The Duboscq colorimeter was popular for many decades, but was, of course, subject to various modifications and improvements. For example, a detachable cover for the cell compartment prevented the lateral entry of light. Patents for some of these improvements were still being issued until the mid-1940's (6).
pH Determination One modification, diagrammed in Figure 2, was developed for pH determination with the aid of a n appropriate acid-base indicator. For example, thymol blue (pK = 8.9) might be chosen for samples having pH values in the approximate range 8.2 to 9.5. Fixed plungers A,A' are associated with two cups B,C and B',C'. The four cups can be raised or lowered independently, their positions being indicated by separate scales. Auxiliary acidic and alkaline solutions (e.g., about 0.1 M HCI and about 0.1 M NaOH) are also required. Equal concentrations of indicator are added to these and the sample solution. The sample solution should acquire a tint intermediate between the extremes (e.g., yellow and blue) produced by the auxiliary solutions. The bottoms of cups B and B' are set a t a fured distance y from the plungers. Cup B holds the sample solution, and B' is filled to approximately the same level with the alkaline solution. The acid solution is placed in cup C', and the empty cup C i s raised to touch plunger A, a s shown. Cup C' is then raised or lowered to a distance x from plunger A' to ihtain a color match in the eyepiece. The arrangement allows the ratio of the extremetints to be varied while keep968
Journal of Chemical Education
Figure 3. Jules Duboscq (1817-1886).
ing constant the light-path length through the auxiliary solutions. The pH of the sample is obtained from
Ify is set a t a round number such as 10,15, or 20 units, the value of the logarithmic term can be read from tables (7). Duboscq's Inventive Work with Optical Instruments Color-matching by path-length variation is simple in principle. However, the design of a n instrument t h a t makes such matching convenient is another matter. Viewing two light paths through a single eyepiece requires the exercise of optical expertise. It is not surprising that the Duboscq colorimeter was invented by a member of a firm noted for its optical instruments (8). Jules Duboscq (Fig. 3) was born on March 5, 1817, in Villaines, Seine e t Oise, France, and died in Paris on December 24, 1886. He became the son-in-law of the Paris instrument maker Jean Baptiste Soleil(1798-1878). Their association with him began in 1830. When Soleil retired in 1849, Duboscq took over the business (8).The firm, which specialized in optical instruments, later developed a world market, including the United States (9). Stereoscopes and Daguerreotypes In 1838, Charles Wheatstone (1802-1875) introduced the stereoscope to illustrate his theory of binocular vision
' Dag~errefelt .nab e lo aescrloe nls word lo tne Acaaem e . SO me acc0.m ( i t , Has gwen by DomlnqLe Francos Arago ,1786-1853) Prev1o.s y. Dag-erre naa per?-aaen Arago lo present !he process lo the French government, which granted Daguerre a life pension, provided that an account was published and no patent or other claims were made.
agent, they coated the plates with a n ether-alcoholic ammonium iodide solution. to which a n amber-based varnish had been added. After Sensitization in acidified silver nitrate solution. the olates were dried in the dark. Robiauet and Duboscq also used these plates for the printing ofgiass ~ositives.that is. trans~arencies. u s i n g systems incorporating prisms and birefringent crystals that were r a .~ i d.l vrotated. Dubosca made ~ e r s i s t ence-of-vision expt!nmrnts to colored, rinKiikt: imaces (15,. He also obtaiued the illusion of denth in an image using the various deviations and brightnesses of red and violet rays produced by a prism (16). Overhead Projectors
Figure 4. Overhead projector of 1876
(10). This required pairs of pictures that were slightly different and virtuallv imnossible to oreoare bv hand. Fortuws's just bein'g bbrn. 1; the following nately, ~hotoeranhic~rocess'invented bv Louis Jacaues "vear.. the . Daguerre ( l 7 8 k i 8 5 l j w a s announced, and the pictires thus produced were known a daguerreotypes (11). The process involved the exposure of a polished, silverplated, metallic plate to iodine vapor, thus forming silver iodide on the surface. After exposure in a camera, the plate was developed with mercury vapor. Fixing, which originally involved boiling in strong sodium chloride solution, was later carried out by immersion in hypo (sodium thiosulfate) solution. I n 1849, David Brewster (1781-1868) perfected a more convenient form of stereoscope. The invention of the daguerreotype was a boon, because a twin-lens camera, simulating human binocular vision, could produce the necessary pairs of pictures. Being unable to arouse much interest in England, Brewster made contact with Duboscq, who quickly began the commercial production of the Brewster stereoscope (12). Duboscq also began producing daguerreotypes. In fact, Figure 3 was made from a n enlargement of one partner of a stereoscopic pair of self-portraits, taken around 1850. Moving Pictures During the 1830's, Joseph Antoine Plateau (1801-1883) discovered the phenomenon of persistence of vision and invented a device to illustrate his discovery. The device, which ~roducedthe illusion of movine ~ictures.reauired a set of drawings representing the seq&ntial pisitions of a moving subject. At Plateau's suggestion, Duboscq replaced drawings by sets of photographic images and thus may be considered a pioneer in the area that developed into cinematography (13). Working with the pharmacist and physicist Henri Esme Robiquet (1822-18601, Duboscq improved the process for making the collodion-based glass plates that were inuse in the 1850's (14). They noted that, after the plates were withdrawn from the sensitizing bath and rinsed, the silver iodide coating existed a s a myriad of disconnected globules. In wet-plate photography, the water bound these into a continuous active surface; long exposures were needed if the plates were allowed to dry. Reasoning that a dry film of a suitable varnish might act a s a n alternative binding
Demonstrations with overhead projection are by no means new. Following a first version constructed in 1866, the overhead projector shown in Figure 4 was described by Duboscq in 1876 (17). Light from a suitable source is reflected upwards through a large lens, on which the object or system to be demonstrated is Dlaced. An achromatic objective and a total-reflection produce the image on the screen. Various transparent accessories were used for demonstrating magnetii and electrochemical effects. A separate paper (18) describes the construction of a n astatic galvanometer with a transparent case, intended for use with the projector. Powerful light sources are needed for uses such a s optical projection. Duboscq was therefore naturally interested in the electric arc lamp and had described a regulator for this light source in 1850 (19). He later worked with Jean Bernard Leon Foucault (1819-1868) to produce a n improved form of regulator. Presumahlv a n interest in t o ~ o m a ~ caused hv Dubosca to submit a n &count of the photo-graphomhtre (20). he' clockwork-driven viewine head of this device is directed towards salient points & t h e horizon, which is slowly scanned to produce a photographic record. .
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Magic Mirrors The magic mirror, long known in the Far East, is a thin metal disc, the concave front surface of which is highly polished. Ornamentation, such a s lettering, is engraved on the back of the mirror. If a strong light is reflected by the front surface onto a screen, a n image of the invisible ornamentation is seen, or may appear if the mirror is warmed. Engraving the back locally distorts corresponding regions on the front, rendering them planar. Parallel rays from these small planes are stronger than the feeble illumination provided by the reflection from the remainder of the disk. Dubosca was involved in the renrodudion of such mirrors. He s;spected that warming'the mirror distorted it slightly, bringing it to its active state (21). Reasoning that a similar distortion might be obtained in the cold by application of pressure, he made the mirror to be one face of a brass box that could be pressurized with a hand pump. Beautiful images were reported with a pressure of about 2 atm. A device for studying the intensities of various portions of a spectrum appears to have been Duboscq's last investigation (22). White light from a single source was split into two identical spectra, portions of which could be superposed to give a mixture of colors. Triple lens-iris systems allowed the two spectra and the mixture to be viewed on a small screen so that the intensities could be varied and assessed. Acknowledgment Part of this work was carried out under the Research Fellowship Program of the Science Museum, London.
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Number 11 November 1994
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Literature Cited 1. Mslinin, D. R;Yoe, J. H.J. Chem. Educ. 1961,38,129-131. 2. Lampadius, W A J PmkL. Chsm. 1838,13,385397. 3. Nessleq J.Chem. Goz. 1856,13.446. 4. Duboscq, J:M*ne,C. Compl. Rand. 1886,67,1330-1331. 5. Anon. "Dvboseqb New Calorimeter"; Chem. News 1870,21,3142. 6. Snell. F D.; Sneli, C . T ColorimdricMethaL. ofAnolysis,3rd ed.:Vsn Noatrand: New York.1948;Vol 1,pp 4652. 7. Ref 6.pp 210-211. 8. l,Indual~ie F m n p i s . &s Insfrumnls, 1901-1902; Syndicat der Conshmteurs en Instruments d'optique et de Prkiaion. 1902; p 188 (facsimile reprint; Main B"eur: pans, 19801. 9. Brenni. P.T h e SoleilLDuboscq-Pellin Dynasty" In "Reporton the llthhtematianal Scientific Instrument Symposium, Boloma, Septembe~1991";Bull. Sci Insfrum. Soc 1991.1311.16.
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10. Wheatstane, C.Phil Tram. Roy Soc.1838,128,371494. 11. Compt. Rand. 1839.9.250-267, 12. Duboscq, J. Campl. R e d . 1850.31.69~96. 13. Buelger, J. E.Fmneh Doguermolypa; UnivemityofChicago: Chicago, 1989;pp 108. 14. Robiquet, E.;Duboseq, J. Campt. Rend. 1856,43,ll9&1196. 15. Duboacq, J. J. &Phys. 1877.6.213-216. 16. Duboscg, J. J.& Phys. 1817.6.216. 17. Duboseq, J. J &Phys. 1676,5,216218. 18. Duboscq, J. J de Phys. 1876,5,216219. 19. Duboaeq, J.Compf.Rend 1860,31,807-809. 20. Duboscq, J . Compt Rend. 1867.64.573-574, 21. Bern", A,;Duboseq, J.A"". Chirn. Phy3. 1880,20,143-144 22. Paenaud,-; hrhoscq, J.J. &Phys. 2 n d 1885.4.271-273. ~ ~
