-4 PRELIJIIN.4RY S'I'IJDY O F LEAADCHROhlATE BY E. E. FREE
This work was undertaken in the endeavor t o determine if possible in what way and t o what extent the character of precipitated lead chromate varied with the conditions of precipitation. If satisfactory results were obtained b y purely chemical methods i t was intended next t o study the electrolytic precipitation of lead chromates, the work forming one step in an extended study of the electrolytic precipitation of pigments, which is in progress a t Cornell. No work has been done on electrolytic precipitation and only a beginning made on the study of the chemical precipitation. As, however, there is no possibility of my being able t o carry the work further, it has seemed desirable to publish the results (obtained in the spring of 1906) in the hope that they may prove interesting and suggestive to other investigators.
I.
THE CHROMATES OF LEAD N eu t r a 1 Chroma t e--P b CrO The neutral chromate is formed by the reaction of any neutral lead salt Irith an acid or neutral solution of any chromate or bichromate. It is ordinarily prepared b y the precipitation of a lead salt--nitrate or acetate-by potassium chromate, and so formed it is a brilliant yellow, cryptocrystalline powder practically insoluble in water' and decomposed by strong acids' or alkalis.' \\'eak solutions of alkalis or alkaline carbonate5 change it more or less completely into one or other of the basic salts. Lead chromate occurs natix7e as crocoite or crocosite
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I-ellom-powder is very striking. This difference seems cntirely due t o the difference in size of particle, for n-hen crocoite is finely powdered (as for instance in taking the streak) it shows a yellow scarcely less brilliant than t h a t of the precipitated compound. The behavior of the basic chroinates is quite analogous---the larger crJ-stals corresponding t o t h e deeper and redder shades. lT7hen chrome red is ground it takes on a decided yellow tinge.’ The streak of the dark Icelanochoite is a brick-red.’ Chromates prepared by the action of chromate solutions on lead hydroxide are redder when t h e h!.clroside is crj-stalline.’ This behavior of pigments on grinding is probably t o be explained as the effect of the light reflected froin the external surfaces of the crystals in the ground pon-der. “ T h e reflection spectrum of pigments arises from tivo distinct sources: ( ( 2 ) light reflected from the surface of the substance; i b ) light reflected from the interior faces. The light reflected from the surface is nearly \vhite. * * * * It is t o the light internall>- reflected t h a t the pigment on-cs its color.”” The colored light reflected from a layer of pigment is t h a t part of the incident light Tvliicli has entered a t least one of the cr>-stals a n d 1)een reflected from the posterior surface of t h a t crystal, or from the surface of some other crystal situated more decplJ- lvithin the mass. The colored light is, as above quoted, light reflected from .’ interior faces.” Haliicli: Dingl. l’i!lyt, j i i u r . . 140,
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544 ( ~ F i ) [ i ) . liersch: .\lnnuf:icturc 31iwr:iI 1;uss: Ertlinann’s J i ~ u r . 18, , 330 ( I
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G I I I I C I : Cllexii. Z t g . , 2 3 , I ) i g ~ i ~ c i 1). ~ t s1 ,% ( I ~ o I : ,
Dana, System i d 31iiieralii Redlicli, German P a t e n t , 1 1 7 , i p J a n . I ( I , I O O I ; we CIWIII. % c u t . , 1901, I , 28s. I t is p(sssil>let h a t this i m y I)e :ti1 eiicct o t :I higher lnsicity tlle crystallized hydrate. 5, Sichols a n d Snow: Phil. 3I:iq. [ ~ i j3,2 , 423 118[)1). The ci!in~~:iratively rare phenomena o f ’ ’ surface color ‘ire‘, course, excluded f r o m consideration, ”
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I n a more finely powdered pigment there will be obviously more surface per unit of mass, and the first of these '' interior faces" will be on the average nearer the external surface of the layer; t h a t is, the layer of colored material through which the internally reflected ray must pass, is less in the finely powdered material. It must therefore happen t h a t the selective absorption which takes place in this ray will be relatively weaker or in common phrase, the '' color ' ' itnparted to the ray will be less marked. I n the terms of the curve expressing the relation between intensity and wave length in the reflected light, what happens may be expressed as a flattening of the curve or a decreasing of the accentuation of the maximum (or maxima) to which (with the physiological amendments below noted) the visual color is due. In the simplest case, therefore, the effect of pulverization of pigment should be simply to weaken the color or dilute it with white.' The natural deduction is t h a t the more finely a substance is powdered the more nearly white it will become, though a limit will finally be reached beyond which the individual particles beconie comparable in size t o the wave length of light and hence change their optical behavior. hlany colored substances are actually white or nearly uhite when finely pulverized,2 and near117 all colored bodies show a tendency in this direction. Lead chromates, however, show a more complex behavior. The original reddish orange of crocoite changes on grinding to a clear yellow with no apparent trace of red, and this clear yellow is characteristic of all finely divided samples of PbCrO,. Xone of them become white. This seems t o I t is perhaps unnecessarv to point o u t t h a t visual white and spectral white are not necessarllq the same thing To the retlna, red and yelloic are the opposites of green and blue respecti\eely, dnd each pair of opposites IS algebraically summed A light is visuallv nliite \\hen the led IS of equal physiolog~c intensity tu the green and the iellon equal to the blue This is n o t necessaril! the same as the (ideal) spectr,tl n h i t e in nliich the light is of equal intensity in all Rave lengths For instance, colored quartzes and fluorspai S , garnet, obsidian, cobalt glass, manganous salts, nickel-amn~nq~um salts, etc , etc.
A P Ieliwzitiai y Study of Lcud C'ht oiiiate
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indicate not onlj- a weakening of the total color as described, b u t an actual change in color, a shifting of the (visual) maximum from the red or orange into the yellow. The explanation of this effect requires a reference t o the spectrum of the light reflected by lead chromate. This has been investigated by Kichols,' who obtained the curve given in Fig. I , where the abscissae represent wave lengths and the ordinates intensities in terms of the spectrallj- uniform ideal white.
Fig.
I
Spectruin of light reflected from lead chroniate. Ordinates indicate relative intensities i n terms of ideal !illite After Xichols.
The curve may be legitimately considered as the representation of the light which is allowed to pass by a thin plate of lead chromate. It is the reverse of the ordinary absorption curve which indicates the parts of the spectrum in which the light is retained by the body under examination. Considering the curve in this way it is apparent that lead chromate absorb5 practically all the blue and violet, most of the green .and a coiisiderable fraction of the yellow, but allows the red (and probably the infra-red) t o pass in much greater degree and in increasing proportion with increasing wave length. I n other ~~
' Am. Jour
SCI.[ 3 ] ,28, 345 (1b8q)
words, there i i a n a1)sorption x c ‘ t or * ‘ baiid ” extendinq out of the dtra-violet into the 1 i ipcctruin contiguous on either side. This iprcadinq is not all\ :L\ i svinnietrical b u t is tnost frequentl! approsiinateli i o , m t l i n t h e caw under discussion, since definite data arc. lacll! eriiplo! ed i n neutralizing in the red Tvhicli \ \ ~i retina the excitation due t o t l : ~px5cnce of tlic green. If, on tlic other hand, thi thichtieii of the absorbing plate be decreased, the aliborption ai-ea 11 ill retreat t o n a r d the violet, t l i c high part of tlic cur1 c indicating mort complete transmisLion) will 11101 c toxwrtl the riqlit, and, successively the !-ellon, the grecn, ,iiitl the, blue 11111 be alloned to pass. In the limiting case, \ ~ h i c l i i prohbi! not actually realizable, ~
the absorption area would move altogether beyond the visible spectrum, making the plate xrisually transparent. I n intermediate cases the retreat of the absorption area and the progressive transmission of the green will progressivelj- weaken the red by phj-siological neutralization, and thus make the yellow more and more prominent in the apparent color of the transmitted light. ;It the same time the yellow is further strengthened bj- its own progresiively more complete transmission. It is therefore evident t h a t with decrease in thickness of the plate of pigment the reddish tones of the apparent color will disappear and will be replaced b y clear yellows n‘hen the plates are very thin, the yellow itself is in part physiologically neutralized bj- the blue which begins t o be transmitted, and the apparent color changes, never back t o red (for there is always more green transmitted than blue), but t o a paler and paler >-ellow and ultimately (in the limiting case) t o white. -411 this concerns the light traizsmitted through thin plates, but the application t o the case in 1% hich light is reflecfcd from a non-homogeneous layer of pigment is easy and obvious. It has already been pointed out that the colored fraction of the light reflected from (nearly) all bodies is light which has passed through a greater or less thickness of the material and suffered selective absorption. However, in the light reflected from a mass of powdered pigment there does come into play another factor-the reflection of white light from external faces of the crystals, as alreadj- discussed. The only effect of this is to dilute the color with white and render color differences lesi pronounced and less easily perceptible. Practicallj- then we should expect just \vhat occurs. The larger crystals should be redder and the smaller more purel!yellow. I n very finely powdered samples the color should be much diluted with white and therefore very “clear.” I t i h even conceivable that pulverization might be pushed to a point where the body would become white, though whether this is practically realizable is problematical. Of the facts a s stated there is no question, but the explanations advanced can have
no certain \xlitlity in the abseiice of direct experiiiiciital evidence. If the al)o\-e discussion is correct, it is possible t o draw the practical cleductioii that the occasional cases of turning, which are apparentlj. not due to increase of basicity, may possibly be caused b y cliange in the coarseness of the precipitate. IYitli increase of crystal size, the precipitate will take on a darker, redder, anti less brilliant color.' The color is practically tlie same as t h a t caused by a slight basicity, and is exactly that olwrvcd when the pigment ' I turns." The differences in crj-stal size ma>-be caused by variations in the concentration or temperature of the precipitating solutions,' or may tie due to secondarj. changes which take place when the precipitate is a l l o \ ~ t~o lstand in contact with the mother liquor. I t is ~vellknown that a11 crj-stalline precipitates do tend to increase in coarseness when allon-ed t o stand in this way. Tlie discussion of t lie color changes, which correspond to changes in size of c.r!.stal iri tlie case of tlie basic chromates, is iinpossiblc in the a1)sencc of reliable determinations of the spectra of thesc. liodies. 'I'heir 1,cliavior is probably quite analoxous t o that of the neutral chromate. It is known that tlie yelloiv tinge sliowti I)!. finely ground chrome red is rcally present in tlie color at all times, for it makes its appearance \\lien this pigment is coinbined lvitli others which wholly or partiall!- ncutralize the red component. On account of this yello\v tinge chrome red has been highly recominended for the production of flesh tints in painting.4
Color and Temperature I,cad chroniate is one o f the man!- so-called ( ' metacliroI n tcch~iicalIilirasc, a "dirty color." I t \\-ill lie slio\\-ti in tlie exlicriinerit:il 1i:ii-t i d tliis ixi1)er that such variations (10 l 1 : i ~ e:in iiillucnce {in tlie cryst:il size. j I- h a w been more fully and more accurately invest ig a t e d . The practical applicatioil of these results to the a r t of pigment manufacture depends upon tlie fact t h a t a chrome yellon of sniall cry5tals possesses a clearer color and, other things equal, a greater covering power, than one of large crystals. Hence the best conditions for the manufacture of chrome yellow, other thing5 being equal, should be cold and concentrated solutions mixed with rapid stirring under conditions which would most iuccessfully prer ent the formation of basic salt.' U-liat these last conditions are can only be settled by further investigation. It is a n interesting question whether the presence of glue in the solutions would have any disadvantageous efiect. The matter of oil absorption I t is possillle tlmt the rccoiiiineiit1:itii~n~ol concentrated solutions and the prevention of the formation of Ixisic salt nre iiiutually incompatible. It may lie t h a t the use of concentrated scilutions ~ r o u l din itself cause the preci1)itnte to become partinlly Insic. I t is cl:tiriied 1)y m a n y pigment exllerts t h a t this is the case. If so, i t would be necessary for Imcticnl purposes to reach n compromise between, on the one liund, tlle tendency o f dilute solutions to produce coarse precipitates; and o f ~the utlicr, the tendency of concentrated solutions to induce basicity. To this cluestion, as to others, n cciinplete ansxrer cannot lie given ~i-itliciutfurther investizatioii of the I)roductioti c,i tlie basic salts. The d a t a oi 'l'aliie 1 n.iiulti seem til indicntc that vcry ccincentrnted solutions are n o t , lion-evei-. inconllintihle v i t h l o x Ixisicity.
is another important question which must some day be taken UP. The results so far obtained must, of course, be considered as tentative and as suhject to rex-ision. The methods of testing the character of the precipitate leave much t o be desired, and the variables affecting the character are not yet controlled adequately. Some means should be devised whereby all the precipitations could he made under exactly the same conditions of mixing of the two solutions, of stirring, etc. If we had such means, 11-ork on the character of the precipitates formed from other lead salts or from solid lead compounds, or in the presence of colloids. organic liquids, solid particles, etc., etc., ought t o be both interesting and valuable. n'hile the net results of this investigation have not been large, the paper will have served its purpose if it calls attention t o the interesting possibilities in\-ol\-ed in a systematic stud>- of the conditions affecting the production and properties of pigments. The investigation \vas suggested b y Professor Rancroft and carried on under his direction. I ani glad t o have opportunit:. of acknowledging in!. great indebtedness to him not alone in this matter b u t in man>- others. (,'urj?l.// 1 . j ?
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