spectroscopic, chromatographic and visual investigation of organic dyes

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T H E C H E M I C A L E D U C A T O R h t t p : / / j o u r n a l s . s p r i n g e r - n y . c o m / c h e d r

© 1 9 9 7 S P R I N G E R - V E R L A G N E W Y O R K , I N C . 1 0 . 1 0 0 7 / s 0 0 8 9 7 9 7 0 1 0 8 a

L a b o r a t o r i e s a n d D e m o n s t r a t i o n s

Spectroscopic,Chromatographicand VisualInvestigation ofOrganic DyesDALE RUSSELL*, CURTIS OLSON, SUSAN SHADLE, ANDMARTIN SCHIMPFDepartment of ChemistryBoise State UniversityBoise, ID [email protected]

This experiment

provides…

experience for

students to learn

about color and

the interaction

of dyes with

electromagnetic

radiation.

n introductory level laboratory experiment ispresented in which students learn about colorusing spectroscopy and chromatography. Thepedagogical approach is discovery-based; students

are given only enough background information to enable themto take good data. Commercially available dyes are dissolvedin water to make concentrated stock solutions, which studentsthen dilute to prepare solutions of primary, secondary andtertiary colors. The class works as a team to study coloredsolutions representing a range of concentrations andcombinations of the three primary colors: cyan, yellow, andmagenta. Students record transmission and absorption spectraand compare the results with human perception. They show thenumber of components in each solution by paper

A

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© 1 9 9 7 S P R I N G E R - V E R L A G N E W Y O R K , I N C . S 1 4 3 0 - 4 1 7 1 ( 9 7 ) 0 1 1 0 8 - 4

chromatography. They explore the filter nature of dyes, the effects of concentration,and the cumulative effects of mixing dyes. From this information they deduce theprinciples of color printing.

The experiment is best performed with a photodiode array spectrophotometer;alternative approaches include spectrometers and simple spectroscopes which thestudents make from cereal boxes. The experiment can be performed in either a singlethree-hour laboratory period, or alternatively, three one-hour sessions.

BackgroundSeveral clear and well-written papers concerning color phenomena have appeared inthe chemical education [1–6] and physics education [7–10] literature. The scientificliterature abounds with theoretical and applications-oriented discussions of colorscience [11–14], reprographics [15], and dye chemistry. [16] Spectroscopy in generaland visible spectroscopy in particular remain topics of current research interest. Thisinformation is suggested as background for the instructor, but in keeping with our“guided inquiry” approach, we do not provide it to the student until after completion ofthe experiment.

Intended ApplicationWe use this experiment in our first semester general chemistry course for majors. Theexperiment is suggested for use in other general chemistry programs as well, includinghigh school chemistry (regular or honors), concepts of chemistry, and nonmajorscourses. The level of difficulty can be adjusted by the instructor as appropriate;modifications indicated below render the experiment suitable for these settings.

ExperimentalSafetyThis experiment is generally nonhazardous, however, it is absolutely necessary for thestudents to wear appropriate eye protection in the laboratory at all times, a conditionassumed in the following discussion. The hazards in this experiment include handlingthe dry powder dyes (inhalation hazard: nontoxic nuisance dust; combustible material),

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which we recommend be done by the instructor or stockroom personnel. The dyes arenot known to be hazardous to humans, however, some people may be concerned aboutthem. If the FD&C (Food, Drug and Cosmetic) dyes are chosen, the perceived risk isless, and this may be a consideration in some cases. The students may prepare capillaryspotting pens which requires heating and pulling glass (open flame hazard) and thenbreaking it apart (eye hazard). Students should be instructed to dispose of the glass inappropriate receptacles. If the cereal-box spectroscopes are made by the students,handling sharp implements presents a laceration hazard. The developing solution forpaper chromatography (dilute salt water) is chosen partly because it is completelynonhazardous and nontoxic. Disposal should, as always, be in accordance with stateand local regulations, however, many areas permit small amounts of dye to be disposedof in the sewage system, in some cases after decolorization with hypochlorite(household bleach) [17,18].

TimeThe experiment is written for a single three-hour laboratory session, however, it worksequally well in three one-hour sessions, which is usually the case for high schoollaboratories. In that case, the activities in each session would be:

1. solution preparation

2. paper chromatography and swatch dyeing

3. spectroscopy

If access to instruments is limited, students could be divided into two groups thatperform parts 2 and 3 on alternate days. For classes without access to instruments,simple spectroscopes can be made to view the color bands. They require only a cerealbox, black electrical tape, and inexpensive plastic gratings, and work reasonably wellif a qualitative result is acceptable. The spectroscopes permit observation of the filtereffect of the different colored solutions. Detailed descriptions of how to make thespectroscope have been previously published [19, 20]. Additional time should beallowed for this.

Dyes and Dye SolutionsWater soluble, fiber reactive dyes were obtained from the sources indicated in Table 1.The only colors chosen in this laboratory experiment are cyan, yellow, and magenta to

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TABLE 1. Dyes names, suppliers, and amounts required.

Color Supplier Dye name Amount for 1 L, Abs. = 2

Cyan Dharma #25, Turquoise 0.14

Aldrich Reactive Blue 15

Rainbow Chemicals FD&C Blue No. 1

Spectra Colors FD&C Blue No. 1

Warner-Jenkinson FD&C Blue No. 1

Yellow Dharma #1, Lemon Yellow 0.06 g

Aldrich Reactive Yellow 2

Rainbow Chemicals FD&C Yellow No. 5

Spectra Colors FD&C Yellow No. 5

Warner-Jenkinson FD&C Yellow No. 5

Magenta Dharma #13, Fuschia Red 0.12 g

Aldrich Reactive Red 4

Rainbow Chemicals FD&C Red No. 40

Spectra Colors FD&C Red No. 40

Warner-Jenkinson FD&C Red No. 40

correspond to the colors used in three-color printing processes. Instructors can, inprinciple, choose any dyes they wish, however, these three behave approximately asblock filters and none of them absorbs appreciably in the regions of maximumabsorbance for the other two.

The suppliers are Dharma Trading Company (P. O. Box 150916, San Rafael, CA;phone 1-800-542-5227) , Rainbow Chemicals Co. (910 Sherwood Dr., Unit 19, LakeBluff, IL 60044), Spectra Colors Corp.(25 Rizzolo Rd. Kearny, NJ 07032), Warner-Jenkinson Co. (2526 Baldwin St., St. Louis, MO 63106), and Aldrich (1001 W. St.Paul Ave., Milwaukee, WI 53233).

Table 1 also indicates the mass per liter of solution used in our work to obtain anabsorbance of 2.0 at the λmax indicated. These numbers are necessarily approximateand will vary with dyelot. Commercially prepared dyes are blended with salts and

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other excipients in amounts up to 90 % by mass or more, and quality control varieswith the supplier. Each batch must be tested when received, and the values in Table 1should be considered guidelines.

We prepare the primary dyes in relatively concentrated solutions. The students usethese concentrates to prepare all of their diluted mixtures. We do not advise having thestudents work with the powdered dyes because they are very fine particles, which formairborne suspensions that can get into the students eyes and lungs. The airborne dustseems to get everywhere, and clean-up can be quite extensive. The instructor orstockroom personnel should wear a particle mask, gloves, and work in a fume hoodwhen measuring and dissolving the dyes. Secondary and tertiary mixtures in solutionphase could also be prepared in advance. This has the double advantage of savingstudent time, if that is a consideration, and also keeps secret the composition of thesecondaries and tertiaries until the spectra are evaluated.

Stock solutions are prepared by placing the desired mass of the powdered dye samplein a one-liter volumetric flask. The sample is dissolved in about 100 mL of distilledwater and then diluted to the one-liter mark. No other preparation is needed. All of thedyes dissolve in the 100 mL aliquot without stirring or warming in 30 min or less.

The yellow primary has a strong single peak whose absorbance can be used todetermine the dilution necessary for the sample solutions. The magenta has a doublemaximum. The fiber-reactive cyan dye is a copper phthalocyanine and has acharacteristic double peak with the more prominent absorption band at about 700 nm.Students should be advised to base their solution calculations on the lower, broaderpeak at about 625 nm, because this peak is in the range of human visual sensitivity andcorrelates better to human color perception. The larger peak is in a region wherehuman sensitivity to light approaches zero, so its contribution to perceived color alsoapproaches zero.

SpectroscopyThis experiment is intended to be performed using a spectrophotometer orspectrometer. If no instrumentation is available, simple spectroscopes that students canmake have been described in the literature [19, 20].

Absorption and transmission spectra were recorded using a Hewlett-Packard 8453 UV-visible spectrophotometer controlled by a Hewlett-Packard Vectra XM computer and

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operated through resident UV Chemstation software. This photodiode-array-detectorspectrophotometer records the entire spectrum at once and, because of its speed,allows all 24 students (our laboratory limit) a chance to record spectra during the timeframe of one class period. A single scanning instrument will not accommodate thispurpose; however, if several scanning instruments are available, they could be shared.Alternatively, if each student had access to a manually-set instrument, such as theSpectronic 20 (Bausch and Lomb), spectra could, in principle, be prepared and λmax

determined for each sample.

Inexpensive plastic cuvettes, supplied by Fisher Scientific, are adequate for operationin the visible regime. Our experiment is more qualitative than quantitative; eachstudent in the classroom used a different cuvette, and we did not take precautions toensure matching of the optical behavior of the cuvettes. If more quantitative results aredesired, matched cuvettes should be used. The spectrophotometer “blank” isestablished using deionized water, and stored in the spectrophotometer for thebaseline.

Paper ChromatographyThe students perform paper chromatography on the solutions to demonstrate whichones are single dye components and which are dye mixtures. The spotting of the paperfor the paper chromatography should be done after the concentrated stock solutions areblended for each student’s assigned mixture and BEFORE they are diluted up to themark. This is necessary because the colors are somewhat pale, and after dilution forreading on the spectrophotometer, the color spotted on paper is invisible to the humaneye. Also, the students should be told to spot the same location 4–5 times, allowing afew moments for drying before applying another “coat”. Whatman chromatographicpaper is the stationary phase. The most satisfactory mobile phase we found, overall, is0.1% NaCl in water. This mobile phase is nontoxic, nonhazardous, and it separates allof the dye mixtures into their visibly-distinguishable component dyes within 20 min.The experiment is conducted on a piece of the Whatman paper about 4 cm by 11 cm indimension. A pencil line is drawn parallel to and about 1 cm from one short edge ofthe paper. Open-ended capillary tubing (1.5 × 100 mm, Kimax) is heated in a flameand drawn to a fine thread, which is broken off to make two eye-dropper-like spottingpens, according to Figure 1.

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FIGURE 1. PREPARING THE SPOTTING PENS.

Dilute solutions of the dyes are spotted on the pencil line using the capillary pens. Thepaper is then accordion-folded so that it will stand up in the developer tank (Figure 2).The folds should be made away from the dye spot, not through it or near it, because thespot may separate at the fold into two distinct color regions, one running up each sideof the fold. Usually students have difficulty interpreting such a chromatogram. Thedeveloper tank is comprised of a 400 mL beaker, a layer of the 0.1 % NaCl solution(< 1-cm deep), and a square of polyethylene food wrap to seal the beaker. It isimportant to cover the developer tank during the experiment so that water vapor will

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FIGURE 2. SPOTTING AND FOLDING THE CHROMATOGRAPHIC PAPER.

be in equilibrium with the solution phase; this prevents the stationary phase fromdrying out as it approaches the top of the paper. If drying were to occur, movement ofthe mobile phase up the paper would stop, as would the process of dye separation.

Color Swatches for Human Color PerceptionEach student prepares a colored swatch of his or her dye solution on a section ofWhatman paper. Any size could be used, but to save money and time and to allow all24 to be displayed on a poster board, we use swatches about 3 × 5 cm. The swatchesshould not be much smaller than this because human perception of color changes withthe solid angle of viewing. A small dot will not appear the same color as a larger area,even though both are painted with the same colorant. The paper should be wellcovered with the student’s solution and allowed to dry before displaying it. It shouldalso be labeled (in pencil) with the sample type (primary, secondary, or tertiary) andsample number.

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The Student’s ProcedureThe following procedure can be modified in several ways to accommodate larger orsmaller classes; 24 students is about average for our classes. Each student is assigned aunique solution of dye(s) to analyze. The 24 solutions are listed in Table 2 andsummarized below.

The students prepare the solutions from primary (and if desired, secondary) dye stocksolutions which have a known absorbance of ~2.0 at the appropriate λmax, using aBeer’s Law assumption about the correspondence between concentration andabsorbance (A = abc) where A is the spectrophotometrically determined absorbance forthe solution, a is the absorptivity of the dye at the wavelength of maximum absorbancein units of reciprocal length and concentration, b is the cell path length ( 1.00 cm forcommercially available plastic cuvettes), and c is the concentration in the units ofchoice.

A

A

C

C1

2

1=2

The A1 and C1 refer to the stock solutions and the A2 and C2 refer to the student’sdilution of these. Students must compute how much of each stock to add, consideringthe dilution effect of the other primaries to be added, if a mixture is being prepared.

The unit of concentration here is grams of powdered dye per liter. Students calculateand then prepare the assigned solution using glassware appropriate to the level ofprecision desired (graduated pipette, buret, or graduated cylinder). Students recordtheir absorption and transmission spectra using the photodiode arrayspectrophotometer. Spectra are displayed for all students to inspect. Students whoprepare the primary and secondary colors will also record their data in a table on thechalkboard or on a piece of poster board, which provides them with the informationnecessary to prepare the Beer’s law plots of the primaries and to answer the questionsabout the secondaries in their report. Displaying the data in a table that can be seen bymany students at once eliminates the bottleneck of trying to allow each student toinspect all of the individual spectra; however, each student is expected to examine theraw data for the tertiary colors and draw some conclusions, as discussed below, in thereport section.

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TABLE 2. Student Solutions

Sample Type Perceived Color Abs. Cyan Abs.Yellow Abs. Magenta

1 primary cyan 0.8 0 0



4 primary yellow 0 0.8 0

5 primary yellow 0 0.4 0

6 primary yellow 0.8 0.2 0

7 primary magenta 0 0 0.8



10 secondary blue 0.8 0 0.8

11 secondary green 0.8 0.8 0

12 secondary red 0 0.8 0.8

13 secondary purple 0.4 0 0.8

14 secondary orange 0 0.8 0.4

15 secondary yellow green 0.4 0.8 0

1 tertiary olive drab 1 0.6 0.8 0.2

1 tertiary olive drab 2 0.4 0.8 0.4

1 tertiary brown 1 0.4 0.6 0.8

1 tertiary brown 2 0.4 0.8 0.6

1 tertiary neutral gray 1 0.8 0.8 0.8

1 tertiary neutral gray 2 0.6 0.6 0.6

1 tertiary burgundy 1 0.4 0.2 0.8

1 tertiary burgundy 2 0.2 0.4 0.8

1 tertiary burnt orange 0.2 0.8 0.4

Students perform paper chromatography of their sample and make a paper “swatch” oftheir dye color by using a paintbrush to coat a piece of the Whatman paper. These twosteps are performed using the measured quantities of the dye solutions before dilutionto the final volume. This is done so that the colors of the chromatographic spots and

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the swatches will be visible to the human eye. After spotting the chromatographicpaper and painting the swatch, the mixture should be brought to the full volume andthe spectra run. The paper chromatographies and corresponding painted swatches arelabeled and displayed for all students to see by mounting them on a large piece ofposter board. Students are expected to refer to the chromatographic data to determineif the color is generated by one or a mixture of two or more individual dyes. They referto the painted swatches to determine the human perception of the color (Figure 3).

The timing and organization as presented works well for this experiment because thestudents preparing the primary and secondary color solutions perform the easiercalculations and preparations, but they have the extra task of recording their data in thelarge table. All students have the tasks of preparing a color swatch andchromatographic separation and of putting this data in presentation format. Because ofthe normal variation in the time it takes students to perform these tasks, there is notlikely to be an operational bottleneck, once the primary-color solutions have beenobtained from the stock-solution bottles. To minimize the bottleneck involved indispensing the primary solutions, we provide several bottles of each.

A comment about color-blind students is appropriate. About 8% of males and 0.5% offemales have some color-vision impairment [12]. Also, older people usually have ayellowing of the cornea, which impacts on the perception of color. Instructors shouldbe aware that nontraditional older students may not perceive color the same way theyounger students do. For questions involving human perception of color these studentsshould be encouraged to work with a partner who has normal color vision. It ispossible that in the course of this experiment a student may discover for the first timethat s/he has impaired color vision. It is imperative for the instructor to be sensitive tostudent’s feelings about this discovery.

Student ReportsAll students answer certain questions about the dyes and their chromatographic andspectroscopic behavior, regardless of which sample they prepared. The concepts weexplore are summarized below. An actual student report form is available in thesupplementary material.

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FIGURE 3. CHROMATOGRAMS AND COLOR SWATCHES.

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1. For each of the three primary colors students plot the measured absorbance as afunction of concentration (calculated absorbance) on a piece of graph paper. They areto observe the Beer’s law relationship. If the instructor wishes the students to derivethe Beer’s law relationship, these dyes are well suited to the task; however, moredilutions should be prepared in order to generate a meaningful least squares line withany certainty, and this requires more time than suggested here. The Beer’s lawderivation could, however, be performed as a separate experiment prior to this one.

2. Students are to determine the region of the visible spectrum that is absorbed andtransmitted by each of the three primaries. Students need to be given the criteria usedto determine the absorption region, else many of them think only the single wavelengthof maximum absorbance counts. We give the students color names and bandwidths innm for the bands: red, green and blue. This is in keeping with the convention in colorscience and reprographic technology.

3. Students are to examine the transmission and absorption spectra and are expected torecognize an “inverse” relationship. (Figures 4–9 are examples of student data,showing this relationship.)

4. Students examine the color swatches for the primaries (cyan, yellow, and magenta)and compare them to the absorption and transmission spectra. They are expected todetermine which region(s) of the spectrum correspond to the color perceived by ahuman observer for each of the primaries, in terms of both the perceived color bands:red, green, and blue, and their corresponding bandwidths in nm.

5. Students are to comment, qualitatively, on the impact of dye concentration on theperceived color. Comments should be based on visual examination of the colorswatches and inspection of the spectra. Students will note that color intensity isreduced when the dye concentration is lower, because more light of all wavelengths istransmitted through the solution or reflected from the paper.

6. The primaries behave as light filters, removing some light in a region. Studentsexamine the spectra to determine if the dyes filter all light in their characteristic regionto the same extent, and they are expected to observe the roll-off at the sides of the

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FIGURE 4. ABSORBANCE SPECTRUM FOR THE YELLOW DYE.

FIGURE 5. TRANSMISSION SPECTRUM FOR YELLOW DYE.

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FIGURE 6. ABSORBANCE SPECTRUM FOR THE CYAN DYE.

FIGURE 7. TRANSMITTANCE SPECTRUM FOR THE CYAN DYE.

absorbance peaks. They are expected to recognize that the peak shape means not allwavelengths in the band are absorbed equally. Students are asked to consider what an“ideal” dye spectrum would look like by comparison to an “ideal” filter. They are alsoto conclude an ideal dye would look like a square wave, not a sine wave or a gaussianpeak.

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FIGURE 8. ABSORBANCE SPECTRUM FOR THE MAGENTA DYE.

FIGURE 9. TRANSMITTANCE SPECTRUM FOR THE MAGENTA DYE.

7. Students examine data for the three secondaries and determine the number andidentity of dyes required to make the secondary colors. The six secondaries are madeso that the same two dyes are combined in different ratios. Students are to recognizethat each secondary consists of two primaries and that an increase in the amount of a

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primary color blocks more light in its absorbance region. The perceived color shiftsaccordingly.

8. Students are asked to consider the following color “logic” statements and to explainwhy they are true. Students are to recognize the filter nature of the primary colors [10].

_ _ _C = R Y = B M = G

A bar over the top of a letter means “NOT”. The single letter abbreviations stand for:cyan (C), yellow (Y), magenta (M), red (R), green (G), and blue (B).

9. Students examine the data for the neutral grays and for at least two other tertiarycolors and evaluate them as follows:

(a) Students determine of the identity and relative amounts of each primarypresent. (Note: The primaries are chosen so that to a first approximationabsorbance of each is zero in the regions of high absorbance of the other two.)

(b) Students explain the impact on human perception when light is, at leastpartially absorbed, through the entire visible region. The tertiary colors tend toappear “muddy” even if they have a strong primary color present. Students are torecognize that some light is absorbed all across the spectrum because all threeprimaries are present.

10. Students apply what they have discovered to explain three-color printing. They candiscuss the filter characteristics of the dyes and the independent and cumulative effectof mixing the dyes. Students may also be asked to deduce the optimum absorptionbands for each dye and the fact that they should not exhibit any overlap.

ConclusionThis experiment provides an enjoyable experience for students to learn about color andthe interaction of dyes with electromagnetic radiation. They learn to operate a modernspectrophotometer, perform paper chromatography, and calculate and prepare solutiondilutions. The experience of drawing together data from three sources: the spectra, thechromatogram, and their own human visual perception helps them understand aninteresting phenomenon, and they use their observations to deduce the operational

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principles of three-color printing processes. Relating the spectral effects to the humanperception of color puts the data in terms that students can readily understand. Becausethe experiment is very colorful, there is a minimum of (“boring”) theoretical inputprior to the laboratory work. Students are asked to perform a subjective evaluation(color perception) and students, who are not usually excited about chemistry, seemgenuinely enthusiastic. Students with a background in fine arts or printing technology,who may have never “warmed” to chemistry before, find this experiment especiallyinteresting. Most students have normal color vision and enjoy the aesthetic aspects ofthis experiment, while learning important concepts and techniques in solutionmanipulation and spectroscopy. Color-blind students may come to a betterunderstanding of color vision. Most students seem to appreciate relating what theyhave learned to common consumer products like printed-color materials and clothingdyes.

ACKNOWLEDGEMENT

This work was funded by grant DUE-9550993 from the National Science Foundation.

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12. Hunt, R. W. G. Measuring Colour, 2nd ed.; Ellis Horwood Limited: Chichester, West Sussex,England, 1991.

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14. Hunter, R. S.; Harold, R. W. The Measurement of Appearance, 2nd ed.; John Wiley and Sons:Somerset, 1987.

15. Gregory, P. High Technology Applications of Organic Colorants; Plenum Press: New York,1991.

16. Gordon, P. F.; Gregory, P. Organic Chemistry in Colour; Springer-Verlag: New York, 1987.

17. Salisbury, C. L. “Disposal of Laboratory Wastes Down the Drain”, Textile Chemist and Colorist,1994, 26(7), 223.

18. Salisbury, C. L., “Disposal of Laboratory Wastes Down the Drain”, The Referee, 1994, 18 (4), 10.

19. Cortel, A.; Fernandez, L. J. Chem. Educ. 1986, 63, 348.

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spectroscopic, chromatographic and visual investigation of organic dyes

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