Colorblindness and Titrations with Visual Indicators Harvey Diehl. Dennis C. Johnson, Richard Markuszewski. and Robert M. Moore Iowa State University, Ames, IA 5001 1
The patter that a sharp teacher keeps going while demonstrating a titration can well include the story of the discovery of colorhlindness bv Dalton. Of the two versions of the discovery (which are covered in more detail below), the authors prefer the one telling of the refusal of Dalton's mother, a devout Quaker, to wear the scarlet stockings Dalton had brought her as a gift. The demonstration of titrations is a good time to request students to call for aid if they cannot see the color changes a t the end-points being shown. Curiously, far fewer students respond than expected from the known incidence of color hlindness, and this in spite of any explanation that the deficiency is inherited and is neither uncommon nor any untoward reflection of an individual. Apparently colorblind students prefer to call on other students for aid in locating end-points in their work, and rather than to admit a deficiency and attract attention, quickly become skilled in recognizing the change in intensity that accompanies a change in color. Just as often the student will be unaware of his colorhlindness, and the sympathetic instructor will have to bring the news to him in such a manner as to ~ r e c l u d eDroducine a shock. The fact remains, however, that among l06white males, three or four can he expected to have trouble with the colorless-to-pink change of phenolphthalein and five or six with the vellow-red chance of methvl red. We are derelict toward our students in fail& to give Hpecific aid to even this small group. Achromatism, that is, total color hlindness, is very rare; in 30 years we have encountered only two cases, and one of them was a woman. Yellow-blue colorhlindness is rare. Hed-green colorhlindness is common but varies runsiderahly in degree; a student hat,ing this hlindness will &en have dif!icultv with the phenolphthalein end-point and almost always have grrat difficulty with the methyl red end.puint. We haw found that thymulphrhalein, with a color change from culorless to blue o\,er the pH range 11.4 to 10.6, servesaia substitute for phenolphthi~l(.in,and t)n)mcresolgreen, with a color change from yellow to hlue over the pH range 3.8to 5.6, aguod >uhstitutr ior methyl red. The difiiculty students have with thr phenolphthalein end-point urill usually shou, up as n great loss of prerisiun in a aeries of titmtions of a standard acid; even after considerable ex~eritncewith ~ h e n o l ~ h t h a l e ian colorblind , student will ushally obtain higher precision using thymolphthalein, Table 1. A colorblind student will often report the color change of phenolphthalein as colorless to pink, having been brought up to believe what he was seeing was pink although the actual
perception may have been quite different from that of the noncolorhlind person. Dalton, himself, dwelt a t length on this aspect of the problem. This was hrought out clearly by one of the present authors (D. C. J.) who clearly distinguishes red from gray hut who as an undergraduate student obtained poorer results than his classmates, because his end-point always required several additional drops of standard alkali. After calling attention to the colorblindness problem during a lecture demonstration, it is perhaps best to allow the student to discover for himself a colorhlindness hy making available in the laboratory a set of colorhlindness charts. Such a set is contained in the small hook "Isbihara's Tests for ColourBlindness," published in Tokyo hut available in the United States ( I ) ; this is a set of 14 charts with short but adequate explanations with particular reference to the red-green color deficiency. The one-column treatment of colorhlindness in the "Encyclopaedia Britannica" has a good explanation of some of the more technical terms. There is a great variation in the degree of red-green deficiency, and the number of departures from the normal on the 14 Ishihara charts is a good measure of the degree. Thisvariation shows up in titrations by students: the greater the deficiency the more wildly the color change with methyl red will he missed; thus, a student with two or so deficiencies will usually overrun the end-point by a matter of a few tenths of a milliliter, hut one with 10 deficiencies may not see the change a t all and so badly overrun the end-point as to attract comment from neighboring students. We discovered, however, in the course of gathering some statistical work for this paper, that as soon as the colorblind student is given proper instruction and motivated to use extreme care, his results on a series of titrations will almost equal those of a skilled operator with normal vision (Table 2, compare the methyl red columns of A, B, and C with D), and his data may he only somewhat poorer than that which he obtains with hromcresol green (Table 2, compare columns of A, B, C and D, again). His relief though in using hromcresol green is enormous, or as one student expressed it: "With methyl red the change was barely perceptible and I had to work with great care, slowly and with absolute attention, but with hromcresol green it was like turning on a light bulb in a dark room." Certain incidental ohservations about the data of Tahle 2 are of interest: (1) All participants agreed that better color changes are obtained using relatively large volumes, 150-200 ml in a 500-ml conical flask, so that the light traverses a fairly lengthly volume of solution; this, of course, is contrary to the
Table 1. Precision in the Titration of a Weak Acid by Colorblind Students Using Phenolphthalein and Thymolphthalein A
n Hi-Lo
-X 6 A .M B
C
B
Phth
mYPh
7 0.097690.09654 0.09715 5.8 ppt
6 0.097600.09741 0.09754 0.7 ppt
Phth
WPh
Phyth
0.097800.09750 0.09766 1.1 ppt
6 0.097620.09740 0.09754 0.7 ppt
6 0.097930.09733 0.09761 0.9 ppt
6
T ~ Y P ~ 6 0.097750.09744 0.09751 0.5 ppt
CoIOTblM: C1-"
CaI~uIstednarmal cancentration of sadium hydmxlde salutlan based on titration of Min = Number of titrations H i l o = HlohsJt and lowest results ?= Average 6 =Relativestandard devlatlon m pans per thousand
sampler 10.5 to 1.0 9.) of potasslum hydrogen phthslate
Volume 62 Number 3 March 1985
255
Table 2.
Preclslon in the Titration of a Weak Base by Colorblind Persons Uslna" Methvl Red and Bromcresol Green B
A
Bromcre~ol
Methyl
n
7
6
1.05751.0549 1.0559 1.02 ppt
n Hi-b
6 1.1942-
-
1.1865 1.1900 2.13 ppt
-X
Bmmcresol
Red
Green
Green
Red
Hi-Lo
Methyl
7 1.0551-
8
6
1.18931.1876 1.1884
1.18791.1876 1.1875 0.23 ppt
1.0540 1.0544 0.341 ppl
0.60 ppt
5
6
6
1.19051.1671 1.1891 1.14 ppt
1.05931.0582
1.05751.0549 1.0561 0.97 ppt
D
C
X
6
1.0589
0.48 ppl
A. 6, and C colwblind: D "am1
Successivetinstion resula (in Ratio of Volumes: ml~THAMlml.HCi)of 50.00-ml.
aliqvn~of appaximately 0.1 Ntrishydroxyms~yiaminomethhnn~ITHAM) wi* aDaOXimatelV 0.1 NhYdmChlaiC acid.*,. ..
Svmbalr have the same meaning as in Table 1 me so ,!,m a rhaM r s r acha maae up by weght a t a m r a 2nd of tovat on of pr mav 3 ~ - a &am nwyr o nc
,
me hyorocnlorc acd was mas mlo-1
on togemor
usual instructions to keep down the total volume and miuimize the amount of indicator added. (2) I t is important to standardize the standard solution in exactly the same manner in which the determination is made (color change or hue adooted as the end-ooint.,total volume. liehtine conditions): see 'Table 2, compare the volume ratio; 01 operators A (col: orblind) and D (normal) who worked with the same solutions of^^^^ and hydrochhrir acid. Yote though that operators Hand C . huth colorhlind and workinr! with the same set of solutions (different from those of A and D) and working with exceptional care, obtained agreement within 2 ppt on all four standardizations. (3) Operator D, normal vision, obtained better results with methyl red than with bromcresol green. There was present in the titrations with bromcresol green a disconcerting self-filtering effect, which was complicated by shadows from the ring stand, buret holder, and the operator's left m.Much depended on the lighting, and a change in color with dilution, a consequence of the self-filtering phenomenon, proved very disturbing and was evident in the results in Table 2. (Comoare the standard deviations in the two columns under D , ' ~ a h i e2.) The work iust reoorted was done under the liehtine .. .. conditions normally fuund in a teaching lahorarory, a combination of daylight from windows and "soft white" fluorescent lights. There is certainly a big field to be explorrd in the d ~ r e c t of h selective liehtina with si~crificwrtions of the spectrum, and it may weE he also thai the response to the ~ i h i h a r acolorblindness charts is determined in part by the illumination used. Althoueh - .oerfect substitutes are thus available for the common indicators used in acid-base chemistry, no satisfactory indicator for the EDTA titration of calcium plus magnesium is known. Practically all colorhlind persons miss completely the red to blue change of Calmagite (or of Eriochrome Black T). Our experience with this titration has been
.
256
Journal of Chemical Education
poor that we now immediately assign an alternative deterniination, usuall\fthe titration of chloride withsilvrr nitrate using sodium chromate indicator. This presents an interesting challenge to the research worker in organic reagents: Can the o,o'-dihydroxyazo molecule of Calmagite and Eriochrome Black T he modified in such a manner as to retain the chelating properties toward calcium and magnesium hut have the color change shifted to yellow-to-blue?
SO
Note Regarding Dalton and the Scarlet Stocking Story The storv of Dalton's mother and the scarlet stockines " are the opening paragraphs of a 19-page treatment of Dalton's work on colorhlindness in Roscoe's biography (2) of Dalton. The story is also in Millington (3).Most recent biographies (4.5) of Dalton ienore the storv, the exception being that by Greenaway (6),who tracked down the orjgin of thestory. 1t first appears in a very early biography of Dalton, by Henry ~ foundthat there were three ~ o n s d a i e(7). ~ r e e n a w aalso papers mentioning what we now call colorblindness antedating Dalton's 1794 paper, one of which, by Huddart (81, has a story of a colorblind child, a scarlet stocking, and a Quaker lady. Greenaway is inclined to accept Dalton's own version, that his attention was attracted in the course of his work in botany and in particular by his observation that a geranium had a different color under sunlight ("sky blue") than under candlelight (''. . . it was astonishingly changed, not having any hlue in it, but heing what I called red, a colour which forms a strikine contrast to hlue"). Whatever the circumstances of l)almn7s discovery of hts culorhlin(lnrss,all h~ographersagree that his thoruurh treatment oiculorhlindness w3s a classic bit of science. ~ a l t o found n that his brother was also colorblind, that colorblindnessran in families, that the parents were never colorhlind, and that the deficiency was transmitted by the women, who were, however, vew, vew seldom colorhlind. He relattd in detail his and the deficiencies of others to the spectrum ohrained by dishursing white light with a prism and recorded the terms used for colors h\, normal and hv colorhlind persons of varying degree of colorblindness. Because of his stronp adherence to Newtonian atomism he refused to interpret cis findings in terms of the then new wave theory of light of Thomas Young. He attributed his deficiency to absorption of the red end of the spectrum by a blue pigment in the fluid of the eye and clung to this belief even though no less aperson than the astronomer John Herschel clearly showed him this could not he correct. Curiously, to this day, colorhlindness is referred to on the European continent as Daltonism. For what Dalton saw under candlelight and under sunlight and a detailed but clear treatment ofthe physiology of cdorblindness, see the paper of W. D. Wright in a recent centenary volume on Dalton (9). Wright reminds us, too, that we as teachers are remiss in not detecting colorhlindness in students early on and giving them the necessary aid. ~~~
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Literature Cited (1) lahihara. S.,"Lhiharais T ~ t n f oCalour-Btindn-." r l h s h i n k Foundatim, T o b , 1972. Distributed in the United Ststea by Houseof Vision, 1980UniversityLane. Lisle, IL
London. 1895. pp. 70-89. (3) Millington. J. P., "John Dalton." J. M.Dent & Co., London, 1906, p. 51. (4) Pettomon, E. C.,"JohnDalton andthe AtomieTheary."Doubleday& Company.Garden
pp. 99-106. (7) Londale, H.,"WorthieaofCurnbrlsnd. JohnDalton."G.RoutledgeandSona.London, 1874. p p 99-125. (8) Huddart, J..Phil Tmns., 61.260 (1777). (9) WrkhC W. D., "John Dalton tod ththPv- of SddceOMan~anhhteteU u i i i t q P Manchester. Great Britain, 1916,Chap. 19, p. 309.
