spontaneous hydrolysis and the larger difference between excitation and emission wavelengths. ;in advantage of the resorufin esters is the greater fluorescence of resorufin, which permits the assay of lower substrate concentrations (10-8.11 for resorufin acetate compared to 2.5 x 10-8 for indoxyl acetate). The sensitivity of either for assay of cholinesterase is about the same (Table IV). Since these substrates are attacked by a number of different’ enzymes, the procedures described are not’ truly
specific. Hence the identity of other enzymes present in the sample to be determined must be established. LITERATURE CITED (1) Ellman. G. I,.. etal.. Bzochem. Pharmacol. 7, 88 (1961). ’ (2) Gal, E., Roth, E., Clin. Chzm. Acta 2 , 316 (1957). ( 3 ) Gehauf, B., Goldenson, J., ASAL. CHEM.29, 276 (1957). ( 4 ) Guilbault. G.. Kramer, D. Pi., Ibid., 36, 409 (1964):
( 5 ) Heilbronn, E., Scand. J . Clin. Lab. Invest. 5 , 305 (1953).
(6) Kramer, 1). S . , Cannon, P. L., Guilbault, (+. G.,A X A L CHEM. 34, 842 (1962). ( 7 ) Kramer, 11. X., Gelman, C., C.S. Arnip
Chem. Res. and Ilev. Lab. Reports, Edgewood Arsenal, ;\Id., p. 541 (1!l6O). (8) Kramer, 11. X . > Guilbault, G . G., ANAL.CHEM.36, 1662 (1864). (9) Lowry, 0. H., J . Biol. C h e m 173, 677 (1948). (10) ;\lendel, R., Rudney, H., Riochem. J . 37, 59 (1943). (11) Seligman, A,, Barnett, It., Science 114, 579 (1959). RECEIVEDfor review July 8, 1964. Accepted October 20, 1964.
Composition of Basic Ferric Formate Precipitate from Homogeneous Solution JOHN
V.
GOODE, Jr.,I and CHARLES T. KENNER
Department of Chemistry, Southern Methodist University, Dallas, Texas ,Composition of the basic ferric formate precipitate formed in homogeneous solution b y the method of Willard and Sheldon varies with the heating time after precipitation. The composition changes from (FeOOHh 6(FeOCI)1 d F e O C H 0 2 h 0dH20h o a t 3 minutes after precipitation to I ( F e 0 0 H h JFeOCI 11 86( F e O C H O Z )OO(HzO)s6 a t 150 minutes after precipitation. The probable mechanism for this precipitation i s the initial formation o f a ferric oxychloride hydrosol which hydrolyzes at elevated temperatures t o pFeOOH and reacts with formate ions to form FeOCH02. The FeOCHOz slowly decomposes to p-FeOOH and the pFeOOH changes to anhydrous cyFe20a with extended heating.
P
STUDIES ( 7 , 9) of the properties of basic ferric formate formed by the precipitation of iron from homogeneous Solution by the method of TTillard and Sheldon (16) show that the pi ecipitate contains chloride, formate, and @-ferricoxide and that the composition varies a i t h the final p H of the precipitation medium. Xore consistent results are obtained by measuring the time of heating after precipitation rather than the pH> even though these two values are related. REVIOUS
EXPERIMENTAL
Procedure. The original method of Willard and Sheldon (16) was modified ?lightly in that nitric acid was used instead of hydrogen peroxide to prevent reduction of the iron(II1). The character of the precipitate depends on the anions present and the initial pH. For this work an initial pH of 1.5 Present address, The Dow Chemical
Co., Freeport, Texas
7i541
75222
to 1.6 produced precipitates which were dense and easy to filter. The precipitating solution was prepared by dissolving 9.2 grams of FeCl3.6H20in 2 liters of water, adding 150 grams of KH4C1, 150 grams of (XH&CO, 15 ml. of HSO3 (3TYGw.’ v . ) ~ 20 ml. of HCOOH (goyo w./v.)] and diluting to 3 liters. A11 chemicals were reagent grade. This solution was adjusted to an initial pH of bet,ween 1.5 and 1.6 by the addition of XH,OH ( 1 : l ) or HCI (1:1),, covered, and heated on an electric hot plate to 100” C. This temperature was maintained throughout the required heating interval. Precipitation occurred a t a pH of 2.05 and was essentially complete after 5 minutes. The precipitate was allowed t,o remain in contact with the heated solution for various lengths of time. The times used were plotted on the X-axis of all composition graphs. The precipitate was then filtered while hot on a sintered glass crucible, washed with 600 ml. of 0.0511 ammonia solution, dried in a vacuum desiccator for a t least two days, and further dried for 12 hours at 100’ C. Effect of Anions Present. The effect of the nature of the anion on the precipitate was investigated by repeating the precipitation procedure with different anions. NH4Br!”Br and N H 4 S 0 3 1 ” h ‘ 0 3 were used in the same molar concentrations as the respective chlorides in the original method. I n all of the precipitates formed in this manner the x-ray diffraction patterns indicat,ed the only crystalline form present was cu-Fe,Os, and infrared analysis showed no formate present. The precipitates were not gelatinous, but the particle size was sufficiently small to clog a medium porosity sintered glass filter. The precipitation in the presence of chloride was repeated with no formic acid present with various formate concentrations but no precipitation
occurred with concentrations above 1.0-11. X-ray and IR data showed that t’hose precipitates which did form in the presence of formate were eshentially identical in composition. The precipitation in the presence of chloride was also repeated using hydrofluoric acid instead of formic acid. X-ray anal! that the material formed was p-FeOOH, but the crystal size was much larger than samples precipitated in the presence of formate. The precipitate filtered better than other samples with no formate present, but was not so easy to filter as the precipitates formed in the presence of formate. Ferric Oxychloride Reactions. FeOCl was prepared by the Brauer method ( I ) and its role as an intermediate in the formation of the precipitate froin homogeneous solution was investigated by placing some of the solid in a boiling solution containing the usual concentrations of SH,Cl, (XH2)2CO, HCOOH, and H X 0 3 . After 30 minutes of heating, the solid was filtered and washed in the usual manner and the x-ray and I R data showed the presence of both p-FeOOH and formate, Beta ferric oxide was prepared by the Milligan method (f4)and was used to prepare solid mixtures with FeOCl and a-FezOa for use as standards in x-ray analysis studies. Analysis. The iron content and the amount of acid required to dissolve the sample were determined according to Starke (12) and the chloride was determined by the usual gravimetric method. The formate was determined by both infrared and differential thermal analysis. The infrared analyses were made on a neckman IR-5 instrument using the KBr pellet technique and measuring the area under the absorption peak. The differential thermal analysis equipment was a modification of the design of Pakulak ( I f ) in that Chronielhlumel thermocoupler were used with a modified Sargent SR recorder and a VOL. 37, NO. 1 , JANUARY 1965
123
heating time and are shown in Table I. The milliequivalents of base per gram of sample for these formulas were calculated and agreed with the values found by the Starke method (12).
i'
endothermic
DISCUSSION
M
V
exothermic
I I lhl 1 I l , $ I
l & l
q O 1
I 1
I
I
12&
TEMPERATURE Figure 1 .
Differential thermal analysis scans
water cooling coil was provided to eliminate the long cooling period between runs. The exothermic decomposition with loss of CO a t about 200' C. (Figure 1) was measured. Because x-ray analysis of the sample after this decomposition indicates only the presence of a-Fe& the most probable equation for the decomposition is:
+
2 FeOCHOZ-+ a-FezOa 2 CO
+ HzO
separate samples by the dichromate method. The values for milliequivalents of acid to dissolve the sample are equal to the milliequivalents of base in the sample. These values varied from 26.8 meq. per gram for the 3-minute sample to 30.7 meq. per gram for a sample heated 150 minutes. Calculation of Composition. The mole fractions of FeOOH, FeOC1, F e O C H 0 2 , and HzO were calculated from the percentages of iron, chloride, and formate as read from the graphs in Figure 3 and are shown in Figure 4, with the water being obtained by difference. T h e formulas a t each heating time were determined by dividing each mole fraction bv the mole fraction of FeOCHOz a t that
(1)
RESULTS
The variation of pH of the precipitation medium with time of heating after precipitation is shown in Figure 2 and the results of the analyses in Figure 3. The values for iron by the Starke method (12) agreed with results on
Composition. After formation, t h e precipitate slowly decomposes with FeOCl and FeOCHO:, hydrolyzing to P-FeOOH and the P-FeOOH changing to a-Fe203. This is shown by t h e x-ray diffraction patterns in Figure 5 which represent typical patterns of samples together with the patterns of various ferric oxides and FeOCl. The 3-minute sample shows only the peaks of 8-FeOOH with a trace of FeOCl (the small peak a t 8.2 A. corresponds to the strongest peak of FeOCl). The FeOCHOz is either amorphous or not present in sufficient amounts to give detectable peaks. The 150-minute sample shows the small peak a t 8.2 A . and also all the strong peaks of both a-Fe?Oa and pFeOOH. These were strong enough to attempt a quantitative estimation of the p-Fe00H/a-FeZO3 ratio. Solid mixtures of a-Fez03 and p-FeOOH were intimately ground and the x-ray patterns of the mixtures observed. From these data it was estimated that the 150minute sample was 25 h 10% a-Fe203 and the 120-minute sample was 20 f 10% a-Fe203. Samples heated for 7 hours in the precipitation solution contained only a-Fez03. Beta Ferric Oxide. This compound, which constitutes the bulk of the precipitate, was first reported by Weiser and Milligan (16) who formed it by slow hydrolysis of ferric chloride solutions a t elevated temperatures. They noted the difficulty in the removal of chloride and assumed that the chloride is adsorbed and that it stabilizes the crystal lattice. 0 Iron
59.0
0 Chloride
A Formate by IR
A Formate by DTA
57.0
,[:I
,
,
,
,
,
,
I
,
,
, , ,
,
, 1.2
Jd 0
0.0 10
0
Figure 2.
124
20
30
40
50 6 0 70 8 0 90 100 110 120 130 140 150 HEATING TIME (minutes)
pH of sample as a function of heating time
ANALYTICAL CHEMISTRY
Figure 3. time
IO
20
3b
40
5b
6 0 70 80 90 100 110 H E A T I N G TIME (minutes)
, 120 1 3 0
153.8
1 4 0 150
Percentage composition as a function of heating
Table I.
Sample heating time, minutes 3
a
because equilibrium conditions can be established more readily while the second reaction is favored by rapid decrease in hydrogen ion concentration. In the absence of chloride a t pH 2, the dominant, form present is the dimer [Fe(H20)r(OHj2Fe(H20)4]+4 ( 2 ) which evidently forms a-FeyO3 HeO by slow hydrolysis or a-Fe203by rapid hydrolysis (15). The work of Hofer, Pe&les, and Dieter (4) on Fischer-Tropsch catalysts further substantiates the fact that there must be different mechanisms of formation and differences in structure of the precipitates formed in the presence and in the absence of chloride because catalysts prepared in the Iresence of chloride were not effective. They concluded that reduction of the P-FeOOH in the Fischer-Tropsch process produced a reduced phase with some different structural feature and that any sample which contained p-FeOOH at the time of formation would be ineffective as a catalyst whether it contained fl-FeOOH after washing and drying. After formation, the FeOCl hydrolyses rapidly to FeOOH and condenses
Chemical Composition of Precipitate as a Function of Heating Time
Composition
20 30 40 60 900 1205 150" Iron oxide is calculated as FeOOH even though appreciable amounts of Fe203are
present. Kolthoff (8),however, believes the chloridc is carried down as isoniorphous FeOCl because solid 0-FeOOH continuously removes chloride from chloride solutions but establishes equilibrium rapidly with bromide solutions. Weiser and llilligan (16) and Heller, Kratky, and Sovotny(3j obtained either a-Fe203or a-Fe203 H 2 0 by hydrolysis of F'e(13r)3, Fe(SC1V)3, Fe(r\'Oa)s, Fey(SO,)3, and various organic iron salts. In this study, all precipitations made from homogeneous solution in the absence of chloride produced a-Fe203. Rapid addition of base to FeC13 also produces a-FeyOs even though the precipitate is usually amorphous until aged for some time (IS). Thus it appears that 0-FeOOH is formed only in the presence of chloride by slow of FeCI3 (15), by slow addition of carbonate (4)to FeCI3 solutions, by precipitation of iron from homogeneous solution containing chloride, and by hydrolysis of solid FeOCl in the usual precipitation from homogeneous solution medium a t elevated temperatures. Mechanism of Precipitation and Aging. T h e first step involved in the formation and aging of the precipitate probably is the formation of a hydrosol of frrric oxychloride which reacts with water to form colloidal particles of fl-FrOOH-FeOCI, which are positively charged, and also reacts with formate ion or formic acid to form colloidal FeOCH02 particles which are negatively charged. In support of this step, the formation of a hydrosol is evidenced by the appearance of a dark brown color in the solution just prior t o the formation of the light yellow precilitate which occurs a t pH 2. According to constants given by Cotton and Wilkinson (2))the monochloro complex [either FeCl+* or Fe(OHj2C1] is the dominant form in the solution a t this pH and chloride ion concentration. If ,the pH is raised rapidly through this region (as by direct addition of base), a-Fee03or a-Fen03 H 2 0 is formed and the usual red brown hydrous oxide precxipitate or so1 results ( I S ) . If, however. the pH is raised slowly through this region (as by slow hydrolysis or precipitation from homogeneous solu-
tion), FeOCl i b formed and 0-FeOOH precipitates. In the presence of chloride, the competition evidently is between two reactions R hich may be represented as
+
Fe(OH)>Cl+ FeOCl H20 1HD L+ P-FeOOH
+ HC1 (2)
+
Fe(OHj2C1 + a-FeOOH HC1 ' HpO L+ a-Fe20s nHyO
(3)
The first reaction is favored by J o w decrease in hydrogen ion concentration
J
2
8
0.36
2
A
I
H20
0 FeOOH 0 FeOCl 0 FeOCHOt
0.32
7
-
0.28
t
0.24
ii
Figure 4. Mole frac6 tion composition as a g C function of heating e$ 1 time
0.20 0.16
6u
0 . 12
s
0. O B
0.04
0.40 10
0
-
,
I
I
I
e
,
20
. . , I
,
30
60 7 0 80 90 100 110 I20 1 3 0 140 1 5 0 H M T I N G TIME (minutes)
50
40
,
,
I
I , , , )
I
I
I
, l , l , I j l
d Fe,O,'H,O
c( F e
,g *
I. .zi
II
I I
I
p,
I
150 Minute
I
3 Minute
,
P)
2 4F I
I
I
I _ ,
I
I
I
I
I t
I .
. . . I. I , I
, , .
I
.
BFeOOH
FeOCl I
.
,
.
,
.
I
, 1 , , . 1 , ,
VOL. 37, NO. 1 , JANUARY 1965
125
with it to form positively charged colloidal particles by the following reaction: FeOCl
+ H20
-j
p-FeOOH
+
+ HC1
zp-FeOOH yFeOCl --f [ (8-FeOOH).(FeOCl),-,](FeO +)n nC1-
(4)
+ (5)
Thus the charge can be considered to be derived either from the ionization of FeOCl on the surface or from the adsorption of FeO+ ions in the primary layer ( 5 ) . In either case the chloride ion acts as counter ion in the secondary layer. This reaction is somewhat similar to that of Jirginsons and Straumanis (6) for the alpha-ferric oxide sol. I n the presence of formate ion, some of the FeOCl reacts to form FeOCH02. FeOCl
+ C H 0 2 - -+ FeOCH02
+ C1-
ANALYTICAL CHEMISTRY
Figure 6. a.
(6)
The formate must react with the FeOCl because if it were precipitated directly, it should form as easily in solutions containing anions other than chloride as well as in chloride solutions. Formate, however, was found only in those precipitates which were formed from homogeneous solutions containing chloride. Also, the solid formed when solid FeOCl was hydrolyzed in the precipitation medium contained formate in approximately the same amount as the precipitates from homogeneous solution heated for the same length of time. The FeOCH02 probably does not form mixed crystals with FeOCl or FeOOH because of the size of the formate ion. However, since the negative charge of the formate radical can be distributed over a greater volume, the negative charge density of the FeOCHOz particle would be less than the negative charge density of the p-Fe00H-FeOC1, and the FeOCH02 particle would have less tendency to adsorb FeO+ ions. As a consequence, the FeOCH02 particles would tend to adsorb negative ions in the primary layer. Another fact>or which indicates that the FeOCH02 particles adsorb negative ions in the primary layer is the adherent film of precipitate which is deposited on the glass during precipitation from homogeneous solution containing formate (16). This film can be removed only by washing with acid. No such film is formed in the absence of formate. Nysels (10) indicates that glass is negative because of silicates so that the film formed must be caused by ndsorption of the glass silicates by FeOCH02 particles formed in the immediate vicinity of the glass. Because the film is held so tenaciously by the glass, the adsorption is probably in the primary layer. The FeOCH02 particles formed in the body of the solution probably 126
1
b. c.
'
'
1
I
I.1
' I
3 4 5 X-ray diffraction patterns
1 ' 1 ' 1 I'll 6
7
89011h
Sample precipitated in the presence of formate 6-FeOOH Sample precipitated in the absence of formate
adsorb chloride ions to become negatively charged. X-ray diffraction patterns of 8FeOOH, prepared according to Reiser and Milligan ( 1 4 ) ,and of the precipitate formed from homogeneous solution in the presence and absence of formate are shown in Figure 6. The sharpness of the lines for the precipitate in the absence of formate indicates much larger crystals of p-FeOOH than those formed by slow hydrolysis or in the presence of formate. However, the particles formed in the presence of formate are much easier to filter than those formed in the absence of formate, which indicates that the particles formed in the presence of formate are probably aggregates of many small crystallites. This further substantiates the probability that the particles of FeOCH02 are negatively charged because the better coagulation in the presence of formate may be explained by the mutual coagulation of oppositely charged particles. The chloride initially present in the precipitate is probably caused because FeOCl is formed faster than it hydrolyzes to P-FeOOH and some is carried down with the p-FeOOH &s mixed crystals. The rate of formation of FeOCl is relatively slow, however, for it is formed in solution only under conditions of slow rise of pH. The decreasing chloride and formate concentration in the precipitate with time of heating is caused by the continued hydrolysis of these two compounds and the replacement of chloride in the crystal lattice. The conversion of p-FeOOH to aFe203a t room and higher temperatures has been observed by many investigators (3, 4 , 1 4 ) . In this work, the crystals formed when the transforma-
tion took place in aqueous solution at 100' C. gave sharp x-ray diffraction patterns while those formed by heating during the differential thermal analyses were essentially amorphous until aged above 165" C. for several hours. LITERATURE CITED
(1) Brauer, Georg, "Handbuch der Prae-
parativen Anorganischen Cheniie," p. 1121, Ferdinand Enkl Verlag, Stuttgart, 1954. (2) Cotton, F. A,, Wilkinson, G.;, "Advanced Inorganic Chemistry, pp. 715-6, Interscience, New York, 1962. (3) Heller, W., Kratky, O., Novotny, H., Compt. Rend. 202, 1171 (1936). (4) Hofer, L. J. E., Peebles, W. C., Dieter, UT.E., J . Am. Chem. SOC.68, 1953 (1946). (5) Jirginsons, B., Straumanis, M. E., "A Short Textbook of Colloid Chemistry," 2nd rev. ed., p. 149, 11ac;llillan Co.. New York. 1962. (6j Jirginsons, B., Straumanis, ?*I. E., Ibid., p. 309. (7) Kenner, C. T., ANAL.CHEM.2 5 , 1933 (1953). (8) Kolthoff, I. XI., Xloskovitz, B., J . i l m . Chem. SOC.58, 777 (1936). (9) Lloyd, Selson A , , Thesis, Southern Xlethodist I'niversity:' 1951. (10) hlysels, K. J., Introduction to Colloid Chemistry, ) ' p. 203, Interscience, New York, 1959. (11) Pakulak, J. M.,Jr., Leonard, G. W., ANAL.CHEM.31 , 1037 (1959). (12) Starke, Kurt, Ibid., 35, 1310 (1963). (13) Weiser, H. B., XIilligan, W. O., Chein. Rev. 2 5 , 1 (1939). (14) Weiser, H. B., hIilligan, W. O., J . A m . Chem. SOC.57, 238,(1935). (15) Weiser, H. B., Milligan, W. O., J . Phys. Chem. 39, 25 (1935). (16) Willard, H. H., Sheldon, J. L., ANAL. CHEY.22, 1162 (1950). ?
-
~
~~
RECEIVED for review August 17, 1964. Accepted October 12, 1964. Taken in part from the thesis submitted by John 1.. Goode, Jr., to the Graduate School of Southern Ilethodist I-niversity in partial fulfillment of the requirements for the degree of master of science.
