COMPOSITION OF IRON BLUE
ELMER R. IHNE’ AND EUGENE W. KANNING Indiana University, Bloomington, Ind.
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The data presented indicate that the precipitate of ferrous ferrocyanide formed by the reaction of sodium ferrocyanide and ferrous sulfate is contaminated with adsorbed sodium ferrocyanide. The ratio of the two ferrocyanides in the precipitate varies continuously with the concentration ratio of the reacting compounds. The composition of the iron blue pigments prepared in the investigation depends upon the method of preparation of the ferrous ferrocyanides (“white pastes”) and is affected somewhat by the nature of the oxidizing agent employed in the oxidation of the ferrous compound to ferric ferrocyanide. From the data presented, it is possible to obtain some insight as to a definite control in the preparation of uniform iron blue pigments.
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ANY reports concerning the composition of Prussian blue precipitates have appeared in the literature in recent years. For the most part, investigators have assumed that the blue pigment, commonly called “iron blue,” was composed primarily of complex compounds consisting of ferric ferrocyanide and variable proportions of ions from which the precipitate was produced. According to a number of investigators, the method of preparation is of some importance in producing a uniform pigment. Tliis problem is of considerable importance to the producers of iron blue paint pigments and is of extreme interest from a fundamental standpoint. The simple compound, ferric ferrocyanide, Fec[Fe(CN)&, is formed, according to Muller (6),only if the solutions of ferric iron and ferrocyanide are of definite concentrations. In most cases the precipitate consists of a mixture of this compound and KFeFe(CN)s. Woringer (9) states that the commercial iron blue is a complex compound, Fe”’ Fe”s[Fea(CN)e]s,which is formed as the result of the mutual oxidation and reduction of the ferric and ferrocyanide ions. Schmidt and Rassow (7) are of the opinion that iron blue is a ferric ferrocyanide in which a part of the iron is replaced by potassium and two molecules of water in intermolecular combination. Davidson and Welo (1) state that the formulas for the “soluble” and “insoluble” blues are KFe [Fe(CN)B]and Fe4[Fe(CN)6lS, respectively. Justin-Mueller (4) suggests that the formula for the blue pigment is Fe”’[Fe”’i (CN)6:FeIs, ferric triferrocyanide. Concerning the composition of the intermediate product in the production of iron blues-i. e., the ferrous ferrocyanides (“white paste”)-Muller and Treadivell (6) state there are three ferrous ferrocyanides, KzFe”Fe (CN) 6, KaFef’dFe(CN)~Is, and %eKzFe(CN)B, the solubilities of which increase in the order mentioned. Grove (2) concludes that the precipitate formed by the reaction of sodium ferrocyanide and ferrous sulfate possesses unless there is a conthe formula F~ZF~(CN)~.~F~N~~(CN)~,, siderable excess of other sodium salts present; under such conditions the formula approaches FeNazFe(CN)6. In the presence of potasdum chloride a t the time of precipitation, the formula will be FeKzFe(CN)6. The products in the reaction of ferrous chloride with sodium ferrocyanide, according to Itzkovich and Shmulgan (S), is FezFe(CN)6. Upon oxidation this compound yields ferric ferrocyanide, Fer[Fe(CN)&. Stender and Semenov (8) report that upon electrolytic oxida1
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Preaent address, Sherwin-Williams Company, Chicago, Ill.
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tion, ferrous ferrocyanide yields the complex salt, (KFe)Fe(CNh. In view of the conflicting data concerning the composition of Prussian blue pigments and also that of the intermediate product, white paste, this investigation was undertaken to study certain factors which influence the production of the precipitates and to attempt to explain the manner in which the primary precipitates are contaminated with the ions from which they are prepared. The effect of the concentrations and the temperatures of the solutions of the reactants in the precipitation of the ferrous ferrocyanides was investigated. Experiments were carried out in an effort to determine the relative effects of various oxidi~ingagents in the oxidation of the ferrous compounds to the blue ferric compounds.
Preparation and Analysis of White Pastes The ferrous ferrocyanides were prepared by adding a solution of sodium ferrocyanide (in all the experiments, 10.5 grams of c. P. NaJ?e(CN)6.10Hz0dissolved in 50 ml. of distilled water), which had been previously heated to the desired temperature, to a solution of ferrous sulfate (the indicated weight of c. P. FeS04.7H20 dissolved in 250 ml. of distilled water) which had also been brought to the desired temperature. During the addition of the ferrocyanide solution, and also for 10 minutes following, the solution was agitated by means of a mechanical stirrer. The mixture was then allowed to cool to room temperature and stand for 2 hours to permit the precipitate to settle. The film of solution above the precipitate prevented the oxidation of the ferrous compounds by the action of the air. After settling, the precipitates were filtered through two layers of filter paper in a Buchner funnel. The precipitates were washed free of the sulfate ion with distilled water, the filtrates and washings being collected in volumetric flasks. Precipitates of the white pastes were prepared by the addition of the NarFe(CN)e.lOHzO solutions, indicated above, to 250 ml. portions of solutions containing from 8 to 18 grams of FeS04.7Hz0. Concentrations ofless than 8 grams of ferrous sulfate in 250 ml. of solution produced colloidal precipitates which were difficult to filter even by means of collodion membranes. The composition of the precipitates of the white pastes was determined by the analysis of the filtrate. This method of procedure was found necessary since the precipitates were readily oxidized by the action of the air and were insohble in common reagents. The analysis of the filtrate consisted of the determination of all the possible constituents present-
JANUARY, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
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i. e., ferric and ferrous iron, ferrocyanide, TABLE I. COMPOSITION OF WHITEPASTES and sulfate. Bv difference the exact amount Na&Fe(CN)e of each constkuent in the white ferrous Combined FeSOI.- FesO,,ferrocyanide precipitates was calculated. 7+0 7HaO with 1 in Used in Fe(CN)s---Fe++ N a + FeeFe CN)e N&Fe(CN)s Mole Ferrous iron was determined volumetrically sO1n. Pptn, in Ppt. in Ppt. in Ppt. in hpt. in Ppt. FaFe(CN)a by titration with potassium permanganate. Urams Grams Urams Urams &am Urams Urams Millimoles The sulfate content of the filtrate was deFerrous Sulfate and Sodium Ferrocyanide Solutions Heated to 55" and 100' C., Respectively 1.9753 427.6 0.5980 4.9147 18 8.4449 4.5959 1:6966 termined gravimetrically as barium sulfate. 2.0624 456.4 0,6244 4.8220 16 8.2855 4.5959 1.6646 When present the ferrocyanide ion was 14 8.1648 4.7516 2.1285 476.2 1.6403 0.6444 4.5959 2.2613 517.5 0.6846 4.6103 12 7.9217 4.5959 1.5915 estimated by a volumetric titration with 10 7.7009 561.1 4.4817 2.3815 0.7210 4.5959 1.5471 656.5 4.2378 2.6108 9.7904 4.5959 1.4629 zinc sulfate solution. In all of the experi8 7.2819 Both Solutions Heated t o 35" C. before Mixing ments reported, the qualitative test for 373.4 5.1074 1.7943 18 8.7760 4.5959 1.7631 0.5432 the ferrocyanide ion was negative. 16 8.7386 1.8147 382.1 0.5494 5.0856 4.5959 1.7556 14 8.7013 5.0649 1.8352 384.6 4.5959 1.7481 0,5556 418.3 12 8.5055 4.9501 1.9422 4.5959 1.7088 0.5880 Preparation and Analysis of Iron 4.7682 2.1130 469.4 10 8.2319 4.5959 1.6460 0,6397 Blues 8 7.6259 0.7332 4.4391 2.4218 583.9 4.5959 1.5324 Both Solutions Heated to 25O C. before Mixing The iron blue precipitates were prepared 18 5.1818 1.7245 356.2 8.9038 4.5959 1.7888 0.5221 by first precipitating the white pastes as 16 8.8498 4.5959 364.8 5.1502 1.7539 1.7779 0.5310 14 8.7958 1.7838 373.4 4.5959 1.7671 0.5400 5.1190 previously described and then oxidizing 12 8.5834 4.9954 1.8996 409.1 4.5959 1.7244 0,5751 2.0807 461.8 10 8.2708 4.5959 0.6267 4.8134 1.6616 these ferrous ferrocyanides with various 572.5 4.4713 2.3912 8 4.5959 1.5435 0.7240 7.6831 oxidizing agents, according to common practice, The oxidation was carried out before the precipitates were filtered, by ferrous sulfate from which the white pastes were produced the addition of the oxidizing agent with constant agitation and by varying the temperature during the precipitation. In until an excess was present. The oxidizing agents employed all, eighteen different pigments were prepared and analyzed. were sodium chlorate, potassium dichromate, and potassium permanganate. After complete oxidation the blue precipiIn the analysis of the iron blue pigments, 1-gram samples of tates were washed by decantation several times with water the dried material were boiled for about 5 minutes in a mixture and then filtered by means of a Buchner funnel. After the of 50 ml. of water and 30 ml. of sodium hydroxide solution (2.5 normal). After standing on a steam bath for several hours precipitates were washed free of all soluble ions, they were allowed to dry a t 85' C., after which they were ground to a to allow the precipitates of ferric and ferrous hydroxides to powder and again dried a t the same temperature. Blue pigsettle, the solutions were allowed to cool and were filtered ments were DreDared auantitativelv: the DreciDitates were washed free of soluble . . bv varvinp: " - the concentration of the ibns. The p&cipitates ;ere dissolved in hydrochloric acid and treated with a slight excess of stannous chloride to reduce 650 the iron, and then titrated with a standard solution of potassium dichromate. This titration represents the quantity of iron combined with the complex ferrocyanide ion. A quantity of the filtrate was acidified with sulfuric acid and titrated with potassium permanganate to determine the 600 amount of ferrocyanide ion present. Another portion of the W filtrate was analyzed for the sulfate ion. Small amounts of this ion were found in all the pigments even though it should have been completely removed from the precipitates by wash550 ing. The filtrate from the separation of the hydroxides of u iron was also tested for ferricyanide, which was found to be cl 0 4 absent in all cases. From the analyses of the blue pigments prepared according 500 to the described procedure, the composition is calculated on the basis of total content with respect to each constituent ion.
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Data and Results 450
The data in Table I show the results of the analyses of the white pastes prepared under the indicated conditions of concentration of the reactants and the temperature of the solutions. I n all the experiments reported, 10.5 grams of Pu'arFe400 (CK)6.10H20 were used in the precipitation of the ferrous ferrocyanides, which was insufficient to react with the ferrous ion. Therefore, an excess of ferrous sulfate was found in all the filtrates. The values in the second column of Table I, indicating the grams of ferrous sulfate used in the precipitation, 350 RATIO NqFe(CN)e.lOH20 TO Fe904.7H20 were obtained by subtracting the grams of ferrous sulfate found in the filtrates from the ferrous sulfate contained in the 0.5 0.7 0.9 1.1 '*@ original solution. From the data indicating the composition FIGURE 1. MILLIMOLES OF SODIUM FERROCYANIDE COMBINED of the precipitates with respect to the ferrocyanide, sodium, WIT* ONE MOLE OF FERROUS SULFATE USED IN THE PEEPAand ferrous ions in the precipitate, the proportion of sodium RATION OF THE PRECIPITATES ferrocyanide and ferrous ferrocyanide in the white pastes was I. Ferrous sulfate heated to 55" and sodium ferrocyanide t o 100' C. calculated and expressed as millimoles Of the sodium Salt 11. Both solutions heated to 35O c . : III. Both solutions heated to 25O c .
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INDUSTRIAL AND ENGINEERING CHEMISTRY
combined with one mole of ferrous ferrocyanide. The curves in Figure 1 show the millimoles of sodium ferrocyanide combined with one mole of ferrous ferrocyanide as a function of the ratio of sodium ferrocyanide to ferrous sulfate used in the preparation of the precipitates. Curve I represents the relation for the precipitates prepared from the solutions of ferrous sulfate and sodium ferrocyanide heated to 55" and 100" C., respectively; curve 11, for those precipitates prepared from the two solutions heated to 35" C.; and curve 111,for those precipitates obtained by mixing the two solutions heated to 25OC. Table I1 gives the data obtained from the analyses of the iron blue pigments prepared as previously described. The blue ferric ferrocyanides were prepared by the oxidation of the ' white pastes formed in the same manner as in the preirious experiments on the determination of the composition of the ferrous ferrocyanides. In all cases 10.5 grams of NadFe(CN)G10HzOwere mixed with the various weights of FeS04.7H20 in forming the white pastes. From the analysis of each of the blue pigments produced, the data shown in Table I1 were calculated. These data indicate the ratio of the ferric iron (external iron) to the iron contained in the ferrocyanide complex (internal iron). The percentage oxidation by sodium chlorate was calculated on the basis of the fact that, for complete oxidation, the ratio of external iron to internal iron would be 1.3333. The curves in Figure 2 indicate the ratio of external iron to the iron in the ferrocyanide complex as a function of the ratio of sodium ferrocyanide to ferrous sulfate employed in the preparation of the white paste.
1.9
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1.8
3
1.6
l.s
0
RATIO Na4Fe(cN)~.lOH20
TABLE 11.
C o n w o s r r r o N OF
IROX BLUEPIGMENTS
a The oxidation of the white pastes was accomplished by the addition of 2 ml. of HzSOa (sp. gr., 1.84)and 2 grams of NaC10a.
The data in Table I11 indicate the effect of the nature and concentration of the oxidizing agent in the oxidation of the white paste t o the iron blue pigment. Potassium permanganate, which is not included, was found to cause the oxidation not only of the ferrous ion but also of the ferrocyanide ion.
Conclusion From the data in Table I and the curves in Figure 1it may be concluded that the precipitate produced by the reaction of sodium ferrocyanide and ferrous sulfate consists of ferrous ferrocyanide combined physically with 'Odium ferrocyanide' The ratio of sodium salt to the ferrous ferrocyanide appears to vary continuously with the ratio of the reactants, which in-
To FeS04.7H20
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0.5
FeS0a.Ratio, 7Hz0 External in External Iron in Iron/Iron Soln. Iron Fe(CN)e---Sulfate in Complex Oxidation= Grams Gram Gram Gram % Ferrous Sulfate and Sodium Ferrocyanide Solutions Heated t o 55' and 100°C., Respectively 0,0332 1.7006 44.91 18 0.2289 0.1346 0.0232 1.6842 47.37 0.1346 16 0.2267 0,0146 1.6318 55.26 14 0.2250 0.1379 0.0123 1.5062 74.07 12 0.2178 0.1446 0.0106 1.3422 98.66 10 0.2077 0.1485 8 0.2016 0.1502 0.0082 1.3409 98.86 Both Solutions Heated to 35' C. before Mixing 18 0.2195 0.1251 0.0358 1.7546 36.81 0 0309 1.7204 41.94 16 0.2178 0.1266 14 0.2136 0.1280 0 0295 1.6641 56.38 12 0.2077 0.1335 0.0233 1.5558 66.62 10 0.2044 0.1407 0.0136 1.4527 82.09 8 0,1954 0.1457 0.0113 1.3411 98.83 Both Solutions Heated t o 25' C. before Mixing 0.0313 1.9324 10.14 18 0.2372 0.1228 0.0290 1.8718 19.23 16 0.2351 0.1256 0,0272 1.7762 33.72 14 0,2290 0.1290 0.0250 1.6826 47.61 0.1301 12 0.2189 0.0212 1.5511 67.33 10 0.2139 0.1379 S 0.2066 0.1468 0.0122 1.4074 88.89
VOL. 31, NO. 1
0.1
0.9
1.1
1.3
FIGURE 2. RATIOOF EXTERNAL IRON TO IRON IN FERROCYANIDE COMPLEX AS FUNCTION OF RATIO OF SODIVM FERROCYANIDE TO FERROUS SULFATE USSD IN THE PREPARATION OF WHITEPASTES I. Ferrow sulfate heated t o 55' and sodium ferrocyanide to 100' C. 11. Both solutions heated t o 35' C.; 111. Both solutions heated t o 25O C.
dicates a physical rather than a chemical combination. The mechanism of the combination may be explained on the basis of the coprecipitation of the sodium ferrocyanide with ferrous ferrocyanide by the process of adsorption. The evidence on behalf of adsorption occurring in the precipitation is substantiated by the fact that the quantity of sodium ferrocyanide combined with ferrous ferrocyanide increases as the particle size of the precipitate decreases, the particle size decreasing with an increased ratio of sodium ferrocyanide to ferrous sulfate used in the preparation of the white paste. This behavior is in agreement with the principles of adsorption. The fact that the adsorption of sodium ferrocyanide by ferrous ferrocyanide increased with increased temperature is explained by the fact that the colloidal precipitate formed [FezFe(CN)a]is thermally irreversible. Since the sodium ferrocyanide is not in excess in the solution from which the precipitate separates, the adsorption rate is the controlling factor and results in a permanent increased adsorption. Because
TABLE 111. COMPOSITION OF IRON BLUEPIGMENTS" AS AFFECTED BY THE NATUREAND CONCENTRAT~OK OF THE OXIDIZING AGENT Oxidizing Agent per 25 Ml, Soln. 2 NaClOa NaC103 2 KzCrzOl 4 KzCrz0,
External Iron Uram 0.2128 0.2122 0.2177 0.2177
Iron in Fe(CN)s---Gram 0.1379 0.1379 0.1284 0.1284
Ratio, External Iron/Iron in Comulex
Oxidation
% 1.5432 1.5388 1.6958 1.6955
68.52 69.17 45.67 45.67
a The white pastes from which the iron blue pigments were prepared were by mixing 10.5 grams of sodium ferrooysnlde w,th 10.0 gram8 of ferrous sulfate 250 c.
