Reversible Chromatic Thermosensitivity EUGENE W. BLANK Research and Development Department, Colgate-PalmolivePeet Company, Jersey City, New Jersey
This paper directs attention to the frequent occurrence in chemical substances of reversible color changes as a result of a change in temperature. Some of these color changes are explainable on the basis of allotropy; others are of unknown mechanism. Among the substances chromatically sensitive to heat are the borax beads, microcosmic salt beads, copper ( I ) and silver ( I ) tetraiodomercuriate ( I n , the chromates, various oxides and sublimates, and the hexacyanoferrates (II), (III). The colors of the hexacyanoferrates (II), (III) have been found to be remarkably sensitive to changes i n temperature. N o explanation of the phenomenon i s available at the present time. There is little i n the literature on the subject and much work remains to be done in thisfield.
INTRODUCTION
A
GREAT many chemical substances change color upon being subjected to a change in temperature. As a result the temperature of the heated object may often be qualitatively estimated from its color. This fact is of value in quantitative analysis as a rough approximation of ignition temperatures (1). However, of all the many chemical substances that change color on being heated, only a very few revert to their original color when the temperature change is reversed. The behavior of the latter is the subject of this paper. Mercuric iodide furnishes a typical example of a reversible color change for which an explanation is available (pseudo-morphism). Upon heating red mercuric iodide, yellow rhombic crystals can be obtained by condensation of the vapor.' After a short time these spontaneously transform to the red tetragonal modification without alteration of external shape, despite complete internal rearrangement. If the yellow iodide persists a t ordinary temperatures, i t rapidly changes to the red form when rubbed with a glass rod. Bloxam (2) has described a very striking demonstration experiment based on the above transformation. The double iodides, Cuz(Hg14) and Agz(Hg14),find a limited industrial use as beat-indicating paints (3). Thus copper (I) tetraiodomercuriate (11) on heating turns from scarlet to black rather sharply a t 71°C.2 The corresponding silver salt, silver (I) tetraiodomercuriate (11) changesfrom yellow to red a t approximately 45'C. but the transition is not so sharp as in the case of the copper salt. It is unfortunate, but in a great many cases no explanation is available as to why a particular substance should undergo a reversible color change when subjected to changes in temperature.
REVERSIBLE CHROMATIC THERMOSRNSITIVITY OF OXIDES AND SUBLIMATES
A chromatic reversibility under the influence of heat is shown by a number of substances encountered in qualitative analysis. Table 1 lists the data given by McAlpine and Soule (4), Senter (5), Warren (6),Kraus and Hunt (7), and Ephraim (8). TABLE 1
RBVBIIS~BLB COLOR CHANOBS OF OUDBS AND S~bxlnrcs TiO*
cr,o,
SWLIWATBS ON H&&TIND
Color Kol
Color Cold White
Yellow
MnO MmO, Pel01 NiO
cu.0 zno
Strong yellow White Dark orange In.08 Weak yellow SnOl Yellow SbrSsO (Kermesitc) Dark red Te01 White HKO Strong red HgaCI, White HgCls Whtfe Tho Yellow PbO Weak yellow Pbr01 Strong red BiaOa Pale yellow Ass8
MOOS CdO
Black Light yellow
Dark brown Brown Brown Black Light yellow Black Yellow Yellow Red Strong yellow Strong purple to black Orange t o dark reddish brown
Ferric oxide gives a yellow glaze on porcelain and is used by the Chinese to produce the colored glaze known as Imperial Yellow. This glaze is very sensitive to temperature changes and i t is possible to discern changes in color of this glaze when i t is used on teapots and other vessels subjected to heat. The reversible yellowing of ordinary procelain laboratory ware a t high temperatures enables one to distinguish between a hot and a cold vessel by appearance. As shown in Table 1, red mercuric oxide on heatine 'Colors are designated by 1.S.C.C:N.B.S. color names. See becomes black in color and regains its original red BIEPELD AND GRIPPING, Txrs JOURNAL, 19, 282, 307 (1942). color on cooling. The yellow modification of mercuric 2Compounds are named according to the rules given by the International Union of Chemistry for Reform of Inorganic oxide on heating becomes red and then black. On cooling it reverts first to a red color and then to yellow Chemical Nomenclature. 1940. J. Am. Chem. Soc. 63. 889 (1941). , 171
-
-
REVERSIBLE CHROMATIC THERMOSENSITIVITY
OF BORAX
BEADS
The difference in color between a borax bead when hot and when cold is of significant value in qualitative analysis. complete tables of borax bead colors in both the oxidizing and reducing flame can be found in most manuals of qualitative analysis (9, 10, 11). Microcosmic salt beads behave in a similar fashion (12). the borax and m~crocosmic beads that demonstrate reversible color changes on heating and cooling are listed in Tables 2 and 3. The colors with microcosmic salt are not in every instance identical with those of borax. In general the tests obtained with the borax flux are more delicate, while the microcosmic salt fusions yield a greater variety of colors. The difference in color of the beads in the oxidizing and reducing flame is due to valence change. Thus, for instance, the light reddish purple color of the manganese bead in the oxidizing flame is due to tervalent manganese; manganese in the colorless bead formed in the reducing flame is in the bivalent state (13). The change in color of the copper borax bead from blue t o green is due to reduction of Cu++ to Cu+ (14)
brown. Silver chromate exhibits a change from dark red to black. All of these color transformations are Lee (17) has also described the reversible thermo sensitivity of the mineral gillespite (FeO.Ba0.4Si02) from Fairbanks, Alaska. On heating this mineral, the color changes from a light purplish red to deep purplish blue. Upon cooling, the reverse color change occurs. The explanation of the phenomehon is not known. The mineral Cinnabar (HgS) becomes black on heating but the red color returns on cooling. If the mineral is heated above 410' it remains black on cooling. The labile character of the black form is shown by the fact that, in accordance with Ostwald's rule, i t i t always the first form to be produced in a reaction. On grinding the black form in a mortar it is converted into the red form. REVERSIBLE
CHROMATIC THERMOSENSITIVITY HEXACYANOFERRATES (11)
As early as 1925 it was noticed by the writer that the color of zinc (11) hexacyanoferrate (11) is extremely sensitive to changes in temperature. Recently the TABLE 3
TABLE2
M I C R ~ C O S MSALT X C BEADTESTS
B O R A X BE*D TESTS
(Only Reversible Color Changes Tabulated)
(Only Reversible Color Changes Tabulated)
El#"rrrl Antimony Bismuth Cerium C'aromium Copper Iron
IXld Manganese
Oxidiaiilg Fioms Cold Hot
Colorless
Yellow
Colorless Pale yellow Moderate yellowish green Blue Nearly colorless Colorless Light reddish ~"'Ple
Yellow Yellow Yellow Green Red
Cold
Hoi
.....
..... Nearly color-
Pale green
Elemenl
cerium
Colorless
Yellow
Chromium Copper
Green
Olive
Blue
Green
Iron
Yellow
Reddish brown
Manganese
Light reddish purple Colorless
Tungsten
Yellow Coloderr Colorless
Moderately reddish purple weak greenish ~rllow Red Yellow Pale yellow
Uranium Vnnadium
Green Pnle yellow
Yellow Yellow
Molybde-
Yellow Strong purple
11rown
Moderate purple
Nearly color-
Pale yellow
I'ranium Vanadium
less Colorlesr Weak yellow green
Yellow Yellow
Oridieiltl Florllr Cold Hot
Reducirlg Florir
less
Nickel Titanium
OF THE
num
....
.....
Light reddish purple Colorless Green
Very pale purple Pale green P~leolive
REVERSIBLE CHROMATIC TRERMOSENSITIVITY OF
CIIRO-
MATES
Lee (15) has found that a t elevated temperatures the normal chromates of the alkaline earths of the type RCr04 change from their respective shades of greenish yellow to yellow, to orange and red shades, the original colors reappearing in reverse order on cooling. The color changes are best demonstrated with electric heat to prevent reduction of chromate to chromite. The extent of the color change on heating increases with diminishing atomic number of the alkaline earth whose normal chromate is heated. The effect of heat on some of the metallic chromates is quite similar. The color of lead chromate changes from deep yellow t o dark red (16). Cadmium chromate is transformed from a strong yellow to deep red color and zinc chromate changes from a deep yellow to
Nickel
Titanium
Rcducin%Flollir Cold Hol
.... Green Pale blue Colorless
....
.....
Olive
Weak ).el low green Yellow to red
.....
Green
Olive
Vellow
Red
Purple
Yellow
Pale blue
Moderate blue Pale olive Paleolive
Green Green
color changes on heating of various other hexacyanoferrates (11) were investigated. The results found are shown in Table 4. The observation that the hexacyanoferrates (11) are chromatically reversibly thermosensitive appears to have escaped other investigators. Perret and Gislon (18) do not mention having observed any reversible color changes during the course of their comprehensive study of the thermal decomposition of various hexacyanoferrates (11). Table 4 lists those hexacyanoferrates (11) that were found to be reversibly thermosensitive together with the color changes they undergo and the temperatures corresponding to the transition from one color to another. The salts were heated in porcelain crucibles in an electric muffle and the resulting color compared with that of some of the unheated salt contained in a similar
Bismuth ( I l l ) heracyanofcrrate (11) Csdminm (11) heracvanoferrate (11) Cobalt (11) helneynnoferrate (11) Lead (11) heracyanoferrate (11)
Light yellow
Manganese (11) hcraeyanoferrste 111)
Weak yellow green
. .
Pale olive Weak olive green Light yellow
Nickel (11) hexncyanoferrate Light green (11) Zinc (11) hexaryanoferr~te(11) Very light olive
121° Dark yellow R 93Pale yellaw R 66w e a k green R 121° Pale yellow NR 121' Moderate bluish olive R 93' Dark yellow R 177O Light green R
149' Dark olive N R 149' Light blue NR o*o ""
s t r o n g green R 232' Light z r r o i s h ;dl& NR 204Moderate bluish olive R 149O Dark yellow R 232' Moderate olive % , D
. A . *
Zirconium (IV) heraeyanofcr- Light greenish yel93' rate (11) low Light green R
121° Dark green R
177" Dark green R *204° Dark blue R t12ID Dark green R tt20O' Deep yellow R
204' Dark green R *232" Dark blue R t204° Dark green R tt288O Deep yellow R
260' Deeomoares
....
177204O Dark brown R Dark brown R 260' 288' Moderate olive Pinkish white x,n
.A..
149O Strong blucgreen N R
260' Decomposes
316' Vivid pink NR
NR 177" Deep blue R
260° Deep blue R
* R e v c r m t o light blue. t Reverses t o pale orange-pink. it Reverses to light bluish gray.
crucible. If there was a perceptible differencethe color changes from white to pale purple while bismuth (111) was recorded according to the 1.S.C.C.-N.B.S. color hexacyanoferrate (11) appears to become slightly system. The muffle was equipped with a pyrometer deeper yellow. All of these color changes are reversible, reading in degrees Fahrenheit. and in the case of zirconium (IV) hexacyanoferrate The colors were compared and recorded a t every (11) the color transformation is particularly pro50°F. rise in temperature but for purposes of tabulation nounced. the temperatures have been calculated t o degrees Centigrade. In the tables the letter R stands for REVERSIBLE CHROMATIC THERMOSENSITIY~TY OF THE HEXACYANOFERRATES (111) "reversible"; the abbreviation NR for "nonreversible." Frequently upon heating the hexacyanoferrates In general, the hexacyanoferrates (111) do not show (11), there is an initial non-reversible change in color as marked changes on heating as the hexacyanoferrates as a result of the loss of water of hydration. At higher (11) nor in the case of those changes that do occur are temperatures the reversible color changes are produced. the color transformations as vivid as those demonInasmuch as many of the hexacyanoferrates (11) strated by the hexacyanoferrates (11). are highly colored a t room temperature and change In a dry ice and acetone bath bismuth (111) hexacolor on being heated, strong cooling of the salts cyanoferrate (111) becomes a deeper red than a t room should reveal further reversible color changes. Ac- temperature. Upon allowing to warm to room temcordingly small amounts of various hexacyanoferrates perature the normal color is regained. With the ex(11) were placed in stoppered test tubes immersed in ception of the bismuth salt, the hexacyanoferrates a cooling bath of acetone and dry ice. (111) listed in Table 5 do not show any color changes I t was observed that on cooling, zirconium (IV) upon cooling in a dry ice and acetone bath. hexacyanoferrate (11) changes from green to strong LITERATURE CITED blue. Molybdenum (IV) hexacyanoferrate (11) . . (which . SMITH, "Analytical processes," 2nd ed., Edward Arnold does not show therm%romatic properties on heating (1) & Co., London, 1 9 4 0 , ~431. . above room temperature) changes from a reddish brown (2) BLOXAM, '8ChemiStry: and organic," Sth ed,, to a dark red. Manganese (11) hexacyanoferrate (11) Blakiston's Son & Co., Philadelphia. 1907, p. 502.
Conpmund
.
Color of Air-Dried Preci9.
Bismuth (111) hexacy- Light brown anoferrate ( I l l ) Cadmium (11) heracy- Light yellow anoferrate ( I l l ) Manganese (11) hexacy- Light hmwn anoferrate (111) Mercury hexacyaoofer- Pale blue rate ( I l l ) ~ i "( I n heraeyanoterpale blue rate (111) Zinc (11) hexacyanofcr- Light yellow rate (111) * Reverses t o pale green. 7 Reverses to light blue. t t Reverser to very light brown.
Succcsrivc Color T~onrfoimolions and Corrrspondinp Te,r#perolurrr-------93Dark brown R 121" Light brown R 121° Pale yellow R 93O Light bluish green R 93' Pale green N R 121' Brilliant yellow N R
121' Dark brown R 149' Moderate olive NR 149O Light yellow R 121" I.ight bluish green R t121Pale green R ttl49" Liahf ~ r c e oR
149O Dark olive NR *I??' Moderate olive R 177' Moderate yellow R 14ge Light bluirhgreenR 1490 Pale green R 177' Dark n e e n R
.,.. 204" Dark olive R 204' Strong olive NR 177Deep blue black NR 1770 Light bluish green R 204" Dark areen NR
....
232' Dark bluish gray S R
.... . ... 204' Light blvish green S R
....
G n . Elec. Reu., 29, 521 (1926). (3) ANDREWS, (4) MCALPINEAND SOULE,"Qualitative chemical analysis." D. Van Nostrand Co., Inc., New York, 1933, p. 386 and 6%
"U".
( 5 ) SENTER,"A Textbook of inorganic chemistry," 3rd ed., Methuen & Co., Ltd., London, 1916, p. 474. (6) WARREN,"Deteminatiw mineralogy," 1st ed., McGrawHill Book Co., Inc., New York, 1921, p. 79 and 81. , "Minemlogy,'' 2nd ed., McGraw-Hill (7) KRAWS~ r r r HUNT, Book Co., Inc., New York, 1928, p. 169. (8) EPHRAIM."Inorganic chemistry." 3rd English ed. by THORNEAND WARD,Gurney and Jackson, London, 1939, p. 444. (9) TREADWELL AND HALL,"Analytical Chemistry: Qualita-
tive analysis," 6th ed., John Wiley and Sons. Inc., New York, 1927, p. 453. MCALPINEAND SOULE,1 0 ~ .it., p. 590. BANcRoar, "Applied colloid chemistry," 3rd ed., McGrawHill Book Co.. Inc.. New York, 1932, p. 442, el. sep. TREADWELL iuro HALL, 1m. cit., p. 453. BANCROWT AND NUGENT,3. Ph?.?, Cham.. 33, 481 (1929). AND NUGENT. ibid.. 33. 729 (1929). BANCROFT
LEE,l&.
