July 18, 1944. H, R NEUMARK ­ y 2,353,782 I ELECTROLYTIC PREPARATION OF SODIUM FERRICYANIDE Filed May 2, 1942

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ATTORNEY Patented July 18, 1944 '2,353,782

UNITED STATES PATENT­ OFFICE 2,353,782 ELECTROLYTIC PREPARATION OF SODIUM FERRICYANIDE Hans R.. Neumark, Forest Hills, N. Y., assignor to General Chemical Company, New York, N. Y., a corporation of New York Application May 2, 1942, Serial No. 441,500 9 Claims. (Cl. 204-91) This invention relates to manufacture of so I have »found that the objects of the invention, dium ferricyanide. With respect to removal of sodium hydroxide im It has been proposed to make sodium ferri purity contained in a sodium lferricyanide solu cyanide by electrolytic oxidation of sodium fer tion, maybe accomplished by treating the anolyte rocyanide. Such an operation may be carried out liquor containing sodium hydroxide with certain in a cell equipped with an anode and a cathode agents which function to convert the sodium of surrounded by a porous ceramic diaphragm form the sodium hydroxide to sodium iron-cyanide ing the cathode chamber. On passage of current compounds which are soluble and remainl in so thru the cell, production of sodium ferricyanide lution in the treated liquor, and to convert the is understood to take place as follows: hydroxide component of the sodium .hydroxide to an iron hydroxide precipitate which may be removed from the treated solution by simple fil tration. Thus, I am enabled to substantially According to this equation, it will be noted that completely neutralize or lower the sodium hy as oxidation proceeds, for every mol of sodium. droxide content of anolyte liquorsto any desired ferricyanide formed one mol of NaOH is also extent, to -suit purity requirements of the sodium produced. Approximately half of this NaOH is ferricyanide product, and to transform the so retained in the cathode chamber while the bal diumconstituent of the sodium hydroxideto4 a ance of such NaOH Works thru the porous Walls soluble sodium iron-cyanide and remove from of the diaphragm into the anolyte solution Which,­ the system the hydroxide radical of the sodium on completion of electrolysis, is withdrawn from hydroxide so neutralized, all Without introducing the cell. into the operation any material Which adversely In an operation of this nature, it is not feasible affects purity of the­ ultimate product, and With to carry electrolysis to a point Where substantial out causing loss of sodium or cyanide values by ly all of the sodium ferrocyanide is oxidized to liquor bleed-off as is often necessary in chemical ferricyanide. Hence, a so-called ñnished sodium processes. ferricyanide anolyte solution contains as an im With regard to removal of lsodium ferrocyanide purity, in addition to a substantial amount of so impurity still present in the -sodium ferricyanide dium hydroxide, a generally equal quantity of liquor after the desired neutralization ofsodium unoxidized sodium Íerrocyanide. To my knowl hydroxide, I iind that there are certainherein edge substantially pure sodium ferricyanide, . after described conditions according to> which either in liquid or solid form, has not been com the sodium ferricyanide solutionmay be concen mercially obtainable. Basis for this diñiculty is trated in such a Waythat it becomes> possible to the very high solubility of sodiumlferricyanide precipitate-out of the solution, in a readily sep coupled with the relatively high solubility of the arable form, substantially all or any desired lesser sodium hydroxide and sodium ferrocyanide im >amount of the sodium ferrocyanide impurity, and purities. to accomplish this separation VWithout any sub The principal object of this invention is pro stantial formation of» solid ferricyanide, i. e‘., sub vision of a process for making sodium ferricyanide stantially all of the latter being held in solution. of controllable degree of purity. The invention ~ Briefly, `the process» of the invention ’comprises aims to provide for electrolytic manufacture of electrolytically oxidizing a solution offs'odíum fer sodium ferricyanide by procedure according to ­ rocyanide to form a sodium ferricyanide liquor which it is possible to remove, substantially com -f containing sodium hydroxide ‘and sodium ferro pletely or to any desired lesser extent, the sodium cyanide as impurities, treating'th'e liquor With hydroxide impurity Without introducing extra- > certain `neutralizing agents to convert'the hy neous impurities into the sodium ferricyanide droxide component of the sodium hydroxide to liquor and without requiring losses of sodium or cyanide values. A further object lies in the pro iron hydroxide, removing the latter from'the vision of a process by practice of which it is pos system, concentrating resultingl sodium 4ferri sible to similarly remove, substantially complete 50 cyanideliquor still containing soluble ferrocy ly> or to` any desired smaller degree, the sodium anide impurity according tocerta'inïprocedural ferrocyanide impurity inherently present in a so steps to effect precipitation of sodium f_errocy dium ferricyanide anolyte formed by commercial _, anide impurityy as crystals',,separatingthe latter ly feasible electrolytic oxidation of sodium ferro from the sodium ferricyanide liquor, and'then re cyanide. covering from `the residual. _sodium ferricya-nide 2 2,353,782 solution, substantially pure sodium ferricyanide of the group consisting of ferro and ferri cyanic optionally in liquid or solid form. acids, such salts being reactable with the so In carrying out the invention, in apparatus dium hydroxide contained in the liquors treated such as illustrated diagrammatically on the ac to form soluble sodium iron-cyanide and to pre companying ñow sheet, when starting operation cipitate iron hydroxide. Examples of suitable a. sodium ferrocyanide solution may be formed neutralizing or treating agents are Prussian in make-up tank IU, by dissolving in water blue, f understood to be ferrie ferrocyanide, Na4Fe(CN) 6.101-120 crystals in quantity to form Fea(Fe(CN) s) s; Turnbull’s blue, understood a substantially saturated solution. A sodium fer to be ferrous ferricyanide, Fes(Fe(CN)ß)z; and rocyanide solution as fed into the anolyte cham-Y a more or less white iron salt, understood to be ber II of an electrolytic cell I2 should contain a ferrous ierrocyanide, FezFe s. certain amount of sodium hydroxide, the function The most satisfactory treating or neutraliz of which is primarily to prevent corrosion of the ing agent is Prussian blue which when added to anode and secondarily to promote conductivity a sodium ferricyanide solution containing so of the anolyte solution. Hence, a desired amount dium hydroxide reacts with sodium hydroxide in of sodium hydroxide from an external source accordance with the following: may be added to the sodium ferrocyanide solu (2) 12NaOH-I-Fe4 (Fe (CN) s) 3*) tion in tank IU, and a typical starting solution 4Fe (OH) 3+3Na4Fe(CN) s may comprise from 200 g. p. l. to 300g. p. l. of Na4Fe(CN)s and from one to 5 g. p. 1. of NaOI-I. Thus, the sodium of the neutralized sodium hy The starting solution is ru-n into the anolyte ' droxide is converted back to soluble sodium fer chamber of the cell provided preferably with a rocyanide which is the essential constituent of a nickel anode and a steel cathode surrounded by starting solution fed into an electrolytic cell and a porous ceramic diaphragm I3 forming a cath ' which ferrocyanide may be eventually recycled ode chamber I 4. Circulation of solution in the thru the process by being returned to tank I0. anolyte chamber, and solution temperature of The hydroxide content of the neutralized sodium the order of 40-50° C. may be_ maintained by hydroxide is converted to ferrie hydroxide which any convenient means. Ordinarily, during the may be ñltered out of the more or less neutral progress of oxidation, temperature should not ized liquor as by ñlter I8. exceed~50° C. in order to prevent decomposition The amount of treating agent to be used de of ferricyanide to iron hydroxide, and the tem pends upon the purity requirements of the ñnal perature maybe kept as high as say 40° C. to sodium ferricyanide product. In some in avoid crystallization Vof sodium ferricyanide. stances, product containing a relatively small ' The .cathode chamber is filled with a weak solu amount of sodium hydroxide may be unobjec tion of NaOH which functions primarily as a tionable, and in other instances it may be de conductor between the cathode and the walls of sired to form a product of best feasibly obtain the surrounding diaphragm. When the current able purity. The NaOH neutralization step of is .turned on', reaction proceeds in accordance the’present process is flexible and facilitates with Equation (1), and sodium ferricyanide is 'either substantially complete elimination of formed.` As previously explained, approximate 40 NaOH or removal of NaOH to any lesser extent ,y ly half of the NaOH formed during the course ’ desired. The amount of NaOH present in an of reaction lWorks its way thru the porous wall anolyte liquor may be, found by analyses and _ of _the diaphragm and into the anolyte solution. ,the theoretical quantity of treating agent needed The electrolytic oxidation operation may be ' to effect the desired degree of puriiication may carried out so as to fomn an anolyte solution 45 be readily calculated. In usual practice of the relatively concentrated with respect to sodium ' process, I find that an ultimate ferricyanide ferricyanide. YThis may be accomplished by ad ' product of acceptable purity with respect to dition to the anolyte liquor, from time to time NaOH content may be obtained when the quan as_oxidation proceeds, of further sodium ferro tity of treating agent employed is such as to cyanìde crystals in quantity to keep the anolyte lower pH of the treated anolyte solution to a solution substantially saturated with respect to value not higher than, 10. In this situation, sodium ferrocyanide up to a convenient point sodium ferricyanide product containing not priorïto completionV of cell operation. A typical more‘than _about 0.2% NaOH may be obtained. finished anolyte solution may contain from 300 ' In the more'` satisfactory embodiments of the ' to 350 g. p. l. of Na3Fe(CN) s', from 20 to 30 g. p. l. invention, it is preferred to control amount of of NarFe(CN)s,-and fromf20 to 30 g. p. l. of ' treating agent used so that at the end of the NaOH.` According to, one example of practice treating step, pH of the anolyte solution is 10W of the process, the anolyte solution at the ,end ` ered to a value not in excess of 8.5. Ordinarily, of electrolysis may analyze 332 g. p. l. of it is preferred to use in excess of about 5 to 10% Na3Fe(CN)s, 24 g. p. l. of -Na4Fe(CN)s, and 23 (30 by weight of treating agent over the calculated gfp. l. oi NaOH. A feature .of the `present proc ' theoretical requirements. Degree of neutrali ess is directed toward partial or substantially ` zationA maybe regulated‘by addition to the complete removal from the system of the sodium ' anolyte liquor being treated o_f successive incre hydroxide which is contained in the finished Yments of ltreating agent and by analyses along anolyte. solution and which was produced during î' towardv the'end point of neutralization. I-Iow oxidation. " To this end, on completion of elec ' ever, I find that the indicated pH control is the trolysis, the anolyte solution is run into tank I6 most satisfactory and practicable mode of ad „ i'or treatment. " .iusting neutralization and securing the sought , Practice of Vthis’ purification stepr comprises ` for degree of purification. Generally speaking, I treating .a.liquor, of the type described andcon in the case of use of Prussian blue as neutraliz _ taining sodium lhydroxide with a treating agent ing agent satisfactory results are obtained when reactable with Ysodium hydroxide to vform sol "using about 2.4-2.5 parts by weight for every uble sodiumV iron-,cyanide and to ‘precipitate iron ‘ part of NaOH to be neutralized. ‘ h_yclroxide.V VMore_particularly I und that'suit- . ' Y To further facilitate the neutralizing reaction ‘ ableftreating agents are iron salts,‘o£ an 'acid 75 .in tank I5 and obtain a satisfactory ñlterable iron 2,353,782 3 hydroxide precipitate, the treating operation facilitates the subsequent separation of sodium should be carried out at temperatures from about ferricyanide and sodium ferrocyanide. Thus, in 35° C. to not more than 50° C. Since the anolyte the following described operation for separation liquor discharged from cell I2 is ordinarily within of ferrocyanide from ferricyanide, I ordinarily this temperature range preliminary heating of the use a ferricyanide solution which has been neu liquor in tank I5 is unnecessary although this tralized to a pH value not exceeding 10 and pref tank may be provided with a heating coil or other erably not exceeding 8.5. means suitabie to keep the temperature during In accordance with one important feature of the neutralizing operation Within the preferred the invention, I have found that sodium ferro« 35° C.-50° C. range. The neutralizing agent is cyanide contained in the filtrate of filter I8 may added to the liquor in tank I6 and the mass is be separated from sodium ferricyanide by con agitated from say 10 to 20i minutes during which centrating the filtrate, at temperature preferably time the reaction of Equation (2) takes place and less than that causing appreciable decomposition proceeds to completion. Thereafter, the liquor of ferricyanide, to a ferricyanide content such is filtered in filter I8 to remove the >iron hydroxide that on cooling of the concentrated solution to which is discharged from the system at I9, the about room temperature, substantially all or a filtrate being run into a vacuum evaporator 2I. desired lesser amount of the ferrocyanide crystal A representative neutralized and filtered solution lizes out in crystals of readily separable form. as fed into evaporator 2i may contain from 300 to My investigations show that a final sodium ferri 350 g. p. l. of Na3Fe(CN) s, fro-m 70 to 90 g. p. l. of cyanide product of low ferrocyanide impurity Na4lï’e(CN)s, and from 0.1 to 0.5 g. p. l. NaOH. content may be made by concentrating a solution In the particular example of operation of the such as a filtrate of filter I8 to a sodium ferri process, the quantity of Prussian blue (contain cyanide content of not less than 450 g. p. l. To ing 6% H2O) used to effect neutralization of prevent any substantial decomposition of sodium N aOI-I in tank I6 is such as to lower the pH of the ferricyanide with the resultant formation of so anolyte solution to between 8 and 8.5, and is dium ferrocyanide Vand iron hydroxide, the con approximately equivalent to 2.4 parts of Prussian centrating operation is carried out at temperature blue per part of NaOH neutralized. The neu not in excess of about 80° C., such temperature tralized and filtered anolyte as fed into the evap being obtained by operating evaporator ZI under orator 2i may analyze 324 g. p. l. of Na3Fe(CN) s, 30 sufficiently reduced pressure, e. g., 28-29 inches of 78 g. p. l. of Na4Fe(CN) a and 0.3 g. p. l. of NaOH. Hg. When proceeding under these conditions, When Turnbull’s blue or ferrous ferrocyanide there is formed a solution from which, on cooling are used as neutralizing agents, the reactions tak to around room temperature, sodium ferrocyanide ing place are much the same as previously indi crystallizes out to such an extent that following cated and apparently >proceed respectively as separation of ' such ferrocyanide crystals there follows: may be obtained a sodium ferricyanide solution in which the sodium ferrocyanide impurity con tent ordinarily does not exceed about 0.02%. In the better embodiments of the invention, concen 40 tration in evaporator 2| is preferably carried to a sodium ferricyanide content of not less than 500 However, it is preferred to use Prussian blue as it g. p. l., and where the commercial requirements has been found that the resulting iron hydroxide are such as to call for a sodium ferricyanide prod precipitate is more readily ñlterable. The next uct of best feasibly obtainable purity, evaporation overall most satisfactory treating agent is Turn is proceeded with to a sodium ferricyanide con bull’s blue. centration of 550-575 g. p. l. While any sodium The sodium ferricyanide filtrate of filter I9 ferricyanide precipitated during this concentra contains in solution the sodium ferrocyanide tion stage Would not constitute a process loss, which Was not oxidized during electrolysis plus such sodium ferricyanide would have to be re the sodium ferrocyanide formed in treating tank À cycled. In order to avoid any appreciable forma I6 by the reaction represented by Equation (2). tion of solid ferricyanide before separation of Another feature of the invention consists in the ferrocyanide, it is not desirable to concentrate in provision of procedure by which it is possible to evaporator 2! to a ferricyanide content in excess remove, from the sodium ferricyam‘de solution, of about 600 g. p. 1. ' substantially all of such sodium ferrocyanide or On completion of concentration, the ' evapo of whatever lesser amount needs to be removed to rated liquor isrun into a cooler and crystallizer form a ferricyanide product satisfying particular 23 in which the solution is cooled down to about commercial requirements as to ferrocyanide im room temperature, e. g., 15-30" C. Such cool purity content. ing crystallizes sodium ferrocyanide as While even in alkaline solution, solubility of 60 sodium ferricyanide is high, I have observed that such solubility decreases appreciably in the pres These crystals are separated'out in filter 24 and ence of increasing amounts of sodium hydroxide, are returned thruline`25 to sodium ferrocyanide and that the presence of substantial quantity of i make-up solution in tank I 0. YAccording to this sodium hydroxide in the sodium ferricyanide solu procedure, the sodiumY ferrocyanide which is un tion lowers the solubility of sodium ferricyanide » oxidized in cell I2 and also the sodium ferrocy and correspondingly enhances the difñculty of anide produced in tank I6 is recovered in a form making an effective separation of ferricyanide which maybe recycled thru theprocess, thus and ferrocyanide. Hence. the neutralizing treat avoiding any loss of sodium or cyanide values> ment in tank I6 affords not only removal from the 70 A typical filtrate discharged‘frcm fiiter 24 system, as iron hydroxide, of hydroxide compo . contain from 500 to 600 g. p. l. of Na3Fe(CN)s, nent of the sodium hydroxide but also the added 0.2 to one g. p. l. of Na4Fe(CN)e, and 0.2 to 0 5 important advantage of removal of sodium Vhy gip. l. of NaOH.` In the illustrative example droxide from the solution, which correspondingly given, the solution in evaporator 2I 'is concenn increases solubility of sodium ferricyanide and 75 trated attemperature not .exceeding 70° C; un 4 2,353,782 der vacuum of about 28-29 inches of Hg to an comprising electrolytically oxidizing a solution Na3Fe(CN)e concentration of'about 550 g. p. 1., of sodium ferrocyanide to form a sodium ferri and the concentrated solution cooled to about cyanide solution containing sodium hydroxide 15° C. The filtrate of filter 24 may analyze 550 and sodium ferrocyanide, treating the solution g. p. l. of Na3Fe(CN)s, 0.2 g. p. l. of Na4Fe(CN)s, with an iron salt, of an acid of the group con and 0.3 g. p. l. of NaOH. Although not essen sisting of ferro- and ferricyanic acids, reactable tial, the sodium ferricyanide solution may be fu; - with sodium hydroxide to form a soluble sodium ther ñltered again in a clariñer 21 to produce in iron-cyanide and to precipitate iron hydroxide, line 28 a crystal-clear solution. Part or all of the quantity of said salt being such as to lower this sodium ferricyanide solution, drawn off thru pH of the treated liquor to a value not higher line 28, constitutes one of the products of the in than 10, separating iron hydroxide from the vention, and may be marketed at the sodium fer treated solution, concentrating the solution at ricyanide concentration thereof or diluted with temperature not in excess of 80° C. to a sodium water to reducethe sodium ferricyanide concen ferricyanide content of not less than 450 g. p. l., tration, e. g., to a 40% solution of Na3Fe(CN)e. cooling the solution to crystallize sodium ferro vIf solid sodium ferricyanide product is desired, cyanide, separating sodium ferrocyanide crys part or all of the clear solution is run thru line tals from the solution, further concentrating the 29 into a second evaporator 30 in which the solu ­solution, and recovering solid sodium ferricya tion is further Yconcentrated to a ferricyanide nide therefrom. „ »strength such that on cooling to say 15-30° C. a i 3. The process for making sodium ferricyanide good crop of sodium ferricyanide crystals is ob comprising electrolytically oxidizing a solution tained. Ordinarily, the solution is concentrated i of sodium ferrocyanide to form a sodium ferri in evaporator 30 to a sodium ferricyanide con cyanide solution containing sodium hydroxide tent of about 650 g. p. 1. It is more important to - and sodium ferrocyanide, treating the solution prevent decomposition of sodium ferricyanide in with material of the group consisting of Prussian evaporator 30 than in evaporator 2|. While `blue and Turnbull’s blue, reactable with sodium concenerating temperatures in evaporator 30 may hydroxide to form a soluble sodium iron-cyanide be as high as 80° C., preferably concentrating and to precipitate iron hydroxide, the quantity temperatures should not exceed about 65-70° C. of said material being such as to lower pH of The pressure during concentration in evaporator 30 the treated liquor to a value not higher than l0, 30 -may be about the same as in evaporator 2|, separating iron hydroxide from the treated solu e. g., 28-29 inches of Hg. On completion of con tion, concentrating the solution at temperature centration, the solution is run into the cooler not in excess of 80° C. to a sodium ferricyanide and crystallizer, cooled down to about 15-30° C., content of not less than 450 g. p. l., cooling the and crystallized sodium ferricyanide is sepa solution lto crystallize sodium ierrocyanide, and rated out in filter 33. Wet crystals, separating sodium Íerrocyanide crystals from the solution. ' 4. The process for making sodium ferricyanide are dried in a drier 35 at preferably maximum comprising electrolytically oxidizing a solution of temperature of 60° C. The sodium ferricyanide 40 sodium ferrocyanide to form a sodium ierricya solution filtrate of ñlter 33 is returned to evapo- ` nide solution containing sodium hydroxide and rator 30. According to this Vmode of procedure, `sodium ferrocyanide, treating the solution with `all of the sodium ferricyanide liquor run into Prussian blue, reactable with sodium hydroxide evaporator 30 is eventually recovered as crystal to form sodium ferrocyanide and to precipitate product. VIn the particular example under con iron hydroxide, the quantity of Prussian blue be sideration, in the evaporator 3U, solution con ing such as to lower pH of the treated liquor to a taining 550 g. p. l. of NaaFe(CN)e, 0.2 g. p. l. of value not higher than 10, separating iron hy Na4Fe(CN)e and 0.3 g. p. l..of NaOH is evapo droxide from the treated solution, concentrating rated to ferricyanide concentration of about 630 the solution at temperature not in excess oi 80° ` g. p. 1., and the final product comprises about » VC. to a sodium ferricyanide content of not less . 99.4% of the Na3Fe(CN)s.2HzO crystals,_ 0.3% l than 450 g. p. l., cooling the solution to crystallize of NafiFe(CN)s, and 0.01% NaOH. - sodium ierrOCyanide, separating sodium ferro My copending application Serial No. 441,499, cyanide crystals from the solution, further con ñled May 2, 1942, is directedymore particularly centrating the solution, and recovering solid ferri to a process for making potassium ferricyanide. r cyanide therefrom.

Iclaim: . . 5. In the process for making sodium ferricy 1. The process for making sodium ferricyanide anide by procedure involving electrolytic oxida comprising electrolytically oxidizing a solution of tion of a solution of sodium ferrocyanide to form sodium ferrocyanide to form a sodium ferricy a sodium ferricyanide solution containing sodi anide solution containing sodium hydroxide and um hydroxide and sodium ierrocyanide, and at sodium ferrocyanide, treating the solution with least partial neutralization of sodium hydroxide, an iron salt,'of an acid of the group consisting the steps comprising concentrating the solution, of ferro- and. ferricyanic acids', reactable with having pH value not higher than 10, at tempera sodium hydroxide to form a soluble sodium iron ture not in excess of 80° C. to a sodium ferricy cyanide and toY precipitate iron hydroxide, the anide content of not less than 450 g. p. l., cool quantity of said salt being such as to lower pH . ing the solution to crystallize sodium ferrocya of the treated solution to a value not higher nide, and separating sodium ferrocyanide crys than 10, .separating iron hydroxide from the tals from the solution. treated solution, concentrating the solution at 6. The process for making sodium ferricya temperature not in excess lof 80° C. to a sodium 70 nide comprising electrolytically oxidizing a solu ferricyanide content of not less than 450 g. p. l., tion of sodium ferrocyanide to form a sodium cooling the solution to crystallize sodium ferro " ferricyanide solution containing sodium hydrox cyanide, and separating sodium ferrocyanide . VideY and sodium ferrocyanide, treating the solu crystals from the solution. Y » Y tion‘with an iron salt, of an acid of the group 2. The process for making sodium ierricyanide consisting of ferro- and ferricyanic acids, react 2,353,782 5 able with sodium hydroxide to form a soluble and sodium ferrocyanide, treating the solution sodium iron-cyanide and to precipitate iron hy with material of the group consisting of Prus droxide, the quantity of said salt being such as sian blue and Turnbull’s blue, reactable with to lower pH of the treated solution to a value sodium hydroxide to form a soluble sodium iron not higher than 8.5, separating iron hydroxide cyanide and to precipitate iron hydroxide, the from the treated solution, concentrating the solu quantity of said material being such as to lower tion at temperature not in excess of 80° C. to a pH of the treated solution to a value not higher sodium ferricyanide content of not less than 500 than 8.5, separating iron hydroxide from the g. p. l., cooling the solution to about room tern treated solution, concentrating the solution at perature to promote crystallization of sodium temperature not in excess of 80° C. to a sodium ferrocyanide, and separating sodium ferrocyanide ferricyanide content of not less than 500 g. p. l. crystals from the solution. and not more than 600 g. p. l., cooling the solu '7. The process for making sodium ferricyanide tion to about room temperature to promote crys comprising electrolytically oxidizing a solution tallization of sodium ferrocyanide, and separat of sodium ferrocyanide to form a sodium ferri ing sodium ferrocyanide crystals from the solu cyanide solution containing sodium hydroxide tion. and sodium ferrocyanide, treating the solution 9. The process for making sodium ferricyanide with an iron salt, of an acid of the group con comprising electrolytically oxidizing a solution sisting of ferro- and Íerricyanic acids, reactable of sodium ferrocyanide to form a Sodium ferri with sodium hydroxide to form a soluble sodium cyanide solution containing sodium hydroxide iron-cyanide and to precipitate iron hydroxide, and sodium ferrocyanide, treating the solution the quantity of said salt being such as to lower with Prussian blue, reactable with sodium hy pH of the treated solution to a value not higher droxiole to form sodium ferrocyanide and to pre than 8.5, separating iron hydroxide from the cipitate iron hydroxide, the quantity of Prus treated solution, concentrating the solution at sian blue being such as to lower pH of the treated temperature not in excess of 80° C. to a sodium solution to a value not higher than 8.5, sepa i’erricyanide content of not less than 500 g. p. l., rating iron hydroxide from the treated solution, cooling the solution to about room temperature concentrating the solution at temperature not to promote crystallization of sodium ferrocya in excess of 80° C. to a sodium ferricyanide con nide, separating sodium ferrocyanide crystals` 30 tent of not less than 500 g. p. l. and not more from the solution, further concentrating the solu than 500 g. p. l., cooling the solution to about tion, and recovering solid sodium ferricyanide room temperature to promote crystallization of therefrom. sodium ferrocyanide, separating sodium ferrocy 8. The process for making sodium ferricyanide anide crystals from the solution, further concen comprising electrolytically oxidizing a solution . trating the solution, and recovering solid sodi of sodium ferrocyanide to form a sodium ferri um ferricyanide therefrom. cyanide solution containing sodium hydroxide HANS R. NEUMARK.