Analyst, April, 1968, Vol. 93, $9. 237-243 237 The Determination of Ferrocyanide and Related Compounds in Commercial Sodium Chloride BY R. F. ROBERTS AND R. H. WILSON (Research Department, Imperial Chemical Industries Limited, Mond Division, Northwich, Cheshire) Ferrocyanide [hexacyanoferrate (I I)] in commercial sodium chloride can be determined spectrophotometrically as its iron complex in the range 0.013 to 50.0 p.p.m. of [Fe(CN),14-.. The iron complex is concentrated from a large volume of sample solution by filtration on kieselguhr, and a repro- ducible Prussian blue colour formed in a small volume under controlled conditions. Aquopentacyanoferrate can be determined simultaneously, and the amounts of each complex present are found by a simple calculation. Some interference is caused by carbonylpentacyanoferrate, which only partially reacts under the conditions of the procedure but the amount present can be determined and allowance made.The precise determination of carbonylpentacyanoferrate is carried out by using a similar principle of concentration, but with different reagents to develop the iron complex. No interference is caused by the presence of other stable iron - cyanogen com- plexes, or by the usual impurities and additives in commercial salts. FOR several years alkali-metal ferrocyanides and, to a lesser extent, other complex cyanides, have been in general use as anti-caking agents and crystal-habit modifiers for commercial sodium chloride.192 Although an estimate of the amount of complex present could be found by a determination of total cyanogen, it was considered desirable to have a method for the specific determination of hexacyanoferrates (11) and (111).A preliminary study of the recorded reactions of ferrocyanide indicated that the formation of Prussian blue was the most sensitive procedure for our purpose; ferrocyanide in molasses has been determined with iron (I I I) chloride .3 EXPERIMENTAL Early experiments showed that when a solution of iron(II1) chloride was added to an acidified brine containing 6 p.p.m. of [Fe(CN),I4-, a trace of blue colour formed only very slowly, whereas iron( 11) sulphate gave a relatively intense blue colour almost immediately. Iron(I1) ferrocyanide is white when formed, but is quickly oxidised by atmospheric oxygen to a Prussian blue colour.The shade and intensity of the blue colour is influenced by the amount of alkali metal present and the ratio of iron(I1) to iron(II1) in the molecule. Various means of controlling the reaction were considered, and, finally, an effective procedure was developed, in which the oxidation was controlled by using iron(II1) ions to oxidise the initially formed ferrous ~omplex,~ the whole operation being conveniently carried out with a single reagent solution containing 2-85 per cent. of Fez+ and 0.29 per cent. of Fe3+ ions. The colour produced has an absorption that is linear between 0.1 and 5-0 pg of [Fe(CN),J4- per ml of final solution. A direct spectrophotometric determination can be carried out on a solution of salt containing 4 to 50 p.p.m. of [Fe(CN),I4-, provided the solution contains no interfering colour or turbidity.With smaller amounts of ferrocyanide, the iron complex is separated and concentrated by dissolving a large amount of salt in water and vacuum-filtering on to a kieselguhr pad in a crucible. The iron complex, which can be seen as a blue layer on the kieselguhr pad, is then decomposed in a small volume of alkali-metal hydroxide, the solution acidified and the Prussian blue formed under controlled reproducible conditions, the optical density being measured at 700 mp. This procedure is suitable for the determination of ferrocyanide in amounts as little as 0.10 p.p.m. in salt, which is well below the normal range of concentrations effective for anti-caking purposes. If required, however, the limit may be extended to 0.013 p.p.m. by using a solution of copper sulphate, in place of the iron(I1) - iron(II1) reagent, to concentrate the [Fe(CN),]*- 0 SAC and the authors.238 ROBERTS AND WILSON: DETERMINATION OF FERROCYANIDE AND [Analyst, Vol.93 from a larger volume of solution, then following the described procedure, and finally forming the Prussian blue in a small volume with the iron(I1) - iron(II1) reagent. Aquopentacyanoferrate is also quantitatively concentrated and converted into a blue complex by the procedure described, provided the ratio of ferrocyanide to aquopentacyano- ferrate is no less than 1 : 2. It has only rarely been encountered in samples of salt and, 011 those occasions, an excess of ferrocyanide was present. This is to be expected, because it is unstable in solution, being converted into ferrocyanide.By measuring the optical density at 860mp, as well as at 700mp, the individual amounts of ferrocyanide and aquopenta- cyanoferrate present can be determined by means of a simple calculation. Some interference may be caused by carbonylpentacyanoferrate, but its presence is also unusual, and it has been found only on a few occasions. The iron(II1) complex is incompletely formed by the iron(I1) - iron(II1) reagent, except when large amounts of ferrocyanide are present. Its colour is violet, and even small amounts impart a violet tint to the Prussian blue precipitate on the kieselguhr pad. A correction for its presence can be made by a further calculation after taking an additional optical density measurement at 550 mp, and at 950 mp, where the aquopentacyanoferrate reading is not much reduced but the interference by carbonylpentacyanoferrate considerably less.The absorption spectra of the iron complexes of ferrocyanide, aquopentacyanoferrate and carbonylpentacyanoferrate formed by the iron(I1) - iron(II1) reagent are shown in Fig. 1. 8 I 1 I I I I 500 600 700 800 900 lo00 Wavelength, mp Fig. 1. Absorption spectra of, the iron com- plexes of ferrocyanide, aquopentacyanoferrate and carbonylpentacyanoferrate obtained with the iron(I1) - iron(II1) reagent: curve A, Fe(CN),]'-; curve B, [Fe(CN),H,0l8-; curve C, [Fe(CN),CO]a- The precise determination of carbonylpentacyanoferrate in the presence of ferrocyanide and aquopentacyanoferrate is best accomplished by using the same general procedure of concentration with copper sulphate, as described for ferrocyanide, and finally forming the iron(II1) complex quantitatively with an iron(II1) nitrate reagent and measuring the optical density at 530mp.METHOD FOR DETERMINATION OF FERROCYANIDE REAGENTS- All reagents should be of analytical-reagent grade. Dilute sulphuric acid, ap@oximately 0-5 M. Dilute potassium hydroxide solution, approximately 0.05 M. Sodium chloride solution, 20 per cent. w/v-Prepare from salt that is free from complex cyanides and filter.April, 19681 RELATED COMPOUNDS IN COMMERCIAL SODIUM CHLORIDE 239 Kieselguhr, white. Iron(l1) - irout(Il1) solution-Dissolve 200 g of ammonium iron(I1) sulphate, (NH,) 2S04. FeS0,.6H20, and 25 g of ammonium iron(II1) sulphate, (NH,) 2S04.Fe2(S04), .24H20, in distilled water to which lo0 ml of dilute sulphuric acid have been added.Make up to 1 litre, filter and store the solution in a dark bottle. Copper suulphate, CuS0,.5H20, solution, 20 per cent. w/v. Standard ferrocyanide stock solution-Dissolve 0-9964 g of potassium ferrocyanide, K4Fe(CN),.3H20, in water containing 5ml of dilute potassium hydroxide and make up to 1 litre with freshly boiled-out distilled water. Store the solution in the dark. Standard dilute ferrocyanide solution-Dilute 50 ml of standard ferrocyanide stock solution with freshly boiled-out distilled water containing 5 ml of dilute potassium hydroxide and make up to 1 litre. 1 ml of solution = 25 pg of [Fe(CN),I4-. PREPARATION OF CALIBRATION GRAPH- To a series of 100-ml graduated flasks, add standard dilute ferrocyanide solution in volumes covering the range 0 to 500 pg of [Fe(CN),I4-.Then add, in the following order, mixing after each addition, 50ml of the 20 per cent. sodium chloride solution, 10ml of 0.05 M potassium hydroxide, 5ml of 0 . 5 ~ sulphuric acid and, finally, 5ml of iron(I1) - iron(II1) reagent. Add water to the mark, mix well and allow to stand for 15 minutes. Determine the optical density of each standard at 700 mp, with 4-cm cells and distilled water as reference. Correct for the blank and construct the calibration graph. PROCEDURE FOR RANGE 10 to 500 pg OF [Fe(CN),I4- (NOTE 1)- Dissolve a suitable weight of commercial sodium chloride, usually 100 g, in about 450 ml of distilled water. Add 10 ml of 0-5 M sulphuric acid and 25 ml of iron(I1) - iron(II1) reagent, shaking after each addition.Allow to stand overnight, but 15 minutes is long enough if the Fe(CN),I4- content is greater than 1OOpg. Fit a sintered-glass crucible, No. 1 porosity, 15-ml capacity, on a vacuum-filtration flask and add about 1 g of kieselguhr. Fill the crucible with water, stir, and allow to stand about 15 seconds before applying the vacuum. Press the pad down firmly with a flat-ended glass rod and wash with about 20ml of 20 per cent. sodium chloride solution. With the vacuum still applied, start the filtration (Note 2). When the filtration is complete, wash the sample-solution container and crucible twice, each time with about 15 ml of distilled water. Remove the crucible and fit it into a 100-ml filter flask.While under gentle vacuum, add about 10ml of 0 . 0 5 ~ potassium hydroxide to re-form the soluble alkali-metal ferrocyanide, and wash the crucible with a few millilitres of distilled water. The solution is usually slightly turbid at this stage and should be filtered through a sintered-glass crucible, No. 5 porosity, into a 100-ml graduated flask, and the crucible washed with a few millilitres of distilled water. Remove the flask, add 50ml of 20 per cent. sodium chloride solution, followed by 5 ml of 0.5 M sulphuric acid, and mix. Finally, add 5 ml of iron(I1) - iron(II1) reagent, mix, make up to the mark and mix again. Measure the optical density at 700 mp. Carry out a blank determination (omitting the sample), subtract from the sample reading and calculate the [Fe(CN)J- concentration from the calibration graph.NOTES- 1. A simple, rapid method of determining the approximate amount of ferrocyanide present is to mix 100 g of sample with 100 ml of distilled water in a 250-ml conical flask, and add to the slurry lOml of 0-5 M sulphuric acid, 6 ml of the iron(I1) - iron(II1) reagent and 35 ml of a solution containing 2.6 per cent. of M sulphuric acid pZus 7 per cent. of potassium dihydrogen orthophosphate, mixing well after each addition. The blue colour of the supernatant liquor is compared visually with similarly treated standards of [Fe(CN),]d- in the 100 to 800-pg range. (If any solution is greener than the others, add, dropwise, additional amounts of the potassium dihydrogen orthophosphate solution until the colours are similar.) 2.The tedious manual filtration of large volumes can be avoided by using as a container far the sample solution, a separating funnel, the tap of which has been cut off and replaced by a piece of tubing closed with a screw-clip. By placing the separating funnel so that the open end of the tubing projects about one third into the crucible, inserting the stopper in the funnel and opening the screw- clip, the flow of solution will automatically control the level in the crucible and the filtration will require no further attention.240 ROBERTS AND WILSON : DETERMINATION OF FERROCYANIDE AND [Analyst, Vol. 93 PROCEDURE FOR RANGE 2-5 TO 20pg OF [Fe(CN)J4-- Dissolve 200 g of sample in about 900 ml of distilled water, add 5 ml of 0.5 M sulphuric acid and 50 ml of 20 per cent.copper sulphate solution. Allow to stand overnight and extract on kieselguhr, as described above, but develop the final colour in a 25-ml graduated flask. For this, the reagents should be reduced to 5 ml of 0-05 M potassium hydroxide, 12 ml of 20 per cent. sodium chloride solution, 1 ml of 0-5 M sulphuric acid and 2 ml of the iron(I1) - iron( 111) reagent. METHOD FOR DETERMINATION OF AQUOPENTACYANOFERRATE PREPARATION OF AQUOPENTACYANOFERRATE FOR CONSTRUCTION OF CALIBRATION GRAPH- The method of H~fmann,~ modified by Asperger, Murati and PavlovicS was further modified as follows, giving a product of purity greater than 97 per cent. All of the solutions are kept at 0" to 1" C during the preparation. Dissolve 20 g of sodium nitroprusside in 60 ml of distilled water and add 300 ml of methanol.Follow with 20 ml of 40 per cent. sodium hydroxide and then 7 g of hydroxylammonium chloride, pre- viously dissolved in 20 ml of distilled water. Allow to stand for 3 days at 0" to 1" C. Filter the precipitate and purify it by dissolving in distilled water and re-precipitating with 300 ml of methanol. Repeat the re-precipitation twice and dry over concentrated sulphuric acid in a vacuum desiccator. The prepared aquopentacyanoferrate can be assayed by determination of the complex of cyanogen and iron, allowing for any ferrocyanide present by oxidation of a solution of the product with acidified hydrogen peroxide, and determination of ferricyanide after chromato- graphic separation with an activated alumina column.Standard aquopentacyano ferrate stock solution-Dissolve 0-334 g of sodium aquopenta- cyanoferrate, Na,Fe(CN),H,O, in water, add 5 ml of dilute potassium hydroxide solution, and make up to 1 litre. Standard dilute aquopentacyano ferrate solution-Dilute 100 ml of standard aquopenta- cyanoferrate stock solution to 1 litre. 1 ml of solution = 25 pg of [Fe(CN),H,0I3-. Solutions should be freshly made before use. Construct a calibration graph and carry out the determination exactly as described for When ferrocyanide and aquopentacyanoferrate are present together, the corrected optical ferrocyanide, but take a further optical density measurement at 860 mp. density (O.D.) for ferrocyanide is calculated as follows. O.D. at 700mp of ferrocyanide = (total O.D.at 700 mp x 1-5) - total O.D. at 860 mp 0.93 This is derived from the simultaneous equations- F at 700 mp + A at 700 mp = total O.D. at 700 mp (F at 700mp x &) + ( A at 700mp x Ra) = total O.D. at 86Omp A is the O.D. of aquopentacyanoferrate at 700 mp, Rf is the ratio, and R , is the ratio, and where F is the O.D. of ferrocyanide at 700 mp, (Calculated from cali- bration graphs at any selected level of O.D. of aquopentacyanoferrate at 860 mp O.D. of aquopentacyanoferrate at 700 mp 1 concentration). O.D. of ferrocyanide at 860 mp O.D. of ferrocyanide at 700 mp Similarly, the corrected optical density of aquopentacyanoferrate can be calculated from the relevant considerations- O.D. at 860mp of aquopentacyanoferrate = (total O.D. at 860mp x 1-76) - total O.D.at 700mp 1 -08April, 19681 RELATED COMPOUNDS IN COMMERCIAL SODIUM CHLORIDE 241 When carbonylpentacyanoferrate is present, a third term, the optical density at 550 mp, and relevant ratios at 550, 700 and 950mp axe introduced to give- (total O.D. at 700 mp x 2433) - [(total O.D. at 950 mp x 14391) + total O.D. at 550 mp] 1 *095 and O.D. at 700mp of ferrocyanide = O.D. at 950mp of aquopentacyanoferrate = [(total O.D. at 950 mp x 30.33) + total O.D. at 550 mp] - (total O.D. at 700 mp x 10.48) 30.86 METHOD FOR DETERMINATION OF CARBONYLPENTACYANOFERRATE PURIFICATION OF CARBONYLPENTACYANOFERRATE FOR CALIBRATION GRAPH- To 100 ml of solution containing about 0.25 g of commercial sodium carbonylpenta- cyanoferrate, Na,Fe(CN),CO, add 10 ml of M sodium hydroxide, followed by 3 ml of hydrogen peroxide (100 volume). Boil for 5 minutes and filter.Acidify with acetic acid (glacial), add 5 ml of 8 per cent. w/v lead nitrate solution, stir, and allow to stand for 15 minutes. Filter the solution, then add sufficient sulphuric acid to remove the excess of lead and filter again. Precipitate the sodium carbonylpentacyanoferrate with 5 ml of 6 per cent. w/v iron(II1) chloride solution. Leave the mixture to stand, decant off the supernatant liquor, centrifuge and wash the precipitate. Decompose it with 10 ml of M sodium hydroxide, filter and make up to 1 litre. Assay by determining the complex of cyanogen and iron. Store the solution in the dark. REAGENTS- Citric acid, 40 per cent. w/v. Potassium nitrate solution, 20 per cent. w/v.Nitric acid, 10 per cent. v / v . Hydrogen peroxide, 3 per cent. w / v . Iro.n(I11) nitrate solution-Dissolve 30 g of iron(II1) nitrate, Fe(N03),.9H,0, in 60 ml of distilled water to which has been added 1Oml of concentrated nitric acid. Make up to 100 ml with distilled water and filter. Standard carbonyZPentacyano ferrate soZution-DiIute the purified sodium carbonylpenta- cyanoferrate solution so that 1 ml =- 50 pg of [Fe(CN),C0I3-. PREPARATION OF CALIBRATION GRAPH- Add to a series of 25-ml graduated flasks, amounts of the standard carbonylpentacyano- ferrate solution to cover the range 0 to 500pg of [Fe(CN),C0I3-. Make each volume up to about 10 ml with distilled water. Add to each flask, mixing after each addition, 5 ml of 0.05 M potassium hydroxide, 1 ml of 10 per cent.nitric acid, 1 ml of 3 per cent. hydrogen peroxide and 2 ml of 30 per cent. iron(II1) nitrate solution. Make up to volume and measure the optical density at 530mp, with 4-cm cells and distilled water as reference solution. Subtract the blank from each measurement and construct the calibration graph. PROCEDURE- Dissolve 100g of sample in about 450ml of distilled water and add, in turn, mixing after each addition, 5 ml of 40 per cent. citric acid and 25 ml of 20 per cent. copper sulphate reagent. Allow to stand for about 15 minutes, then filter through a kieselguhr pad and wash with two 15-ml portions of 20 per cent. potassium nitrate solution. Decompose the iron complex with 5ml of 0.05 M potassium hydroxide, wash the crucible twice, with 5ml of distilled water each time, filter, add 1 ml of 3 per cent.hydrogen peroxide, and boil for 3 minutes. Cool and transfer into a 25-ml graduated flask. Add 1 ml of 10 per cent. nitric acid, 1 ml of 3 per cent. hydrogen peroxide and 2 ml of 30 per cent. iron(II1) nitrate solution, mixing after each addition. Make up to the mark and determine the amount in micrograms of [Fe(CN) ,cOl3- present from the optical density at 530 mp, as described in the preparation of the calibration graph.242 ROBERTS AND WILSON : DETERMINATION OF FERROCYANIDE AND [AIzdySt, VOl. 93 TABLE I RECOVERY OF FERROCYANIDE ADDED TO COMMERCIAL SODIUM CHLORIDE Ferrocyanide, p.p.m. NO. Added Found No. Added Found No. Added Fouid 1 4.75 4.70 5 1.42 1.42 9 0.23 0.23 2 2.83 2.76 6 1.13 1.13 10 0.13 0.13 3 2.60 2.60 7 0.85 0.84 11 0.029 0.029 4 1-54 1.58 8 0.29 0-29 12 0.017 0.023 TABLE I1 RECOVERY OF FERROCYANIDE AND AQUOPENTACYANOFERRATE ADDED TO COMMERCIAL SODIUM CHLORIDE Ferrocyanide , Aquopentacyanoferrate , p.p.m.p.p.m. - - No. Added Found Added Found 1 2.79 2-79 0.75 0.74 2 1-53 1-60 1.87 1-76 3 0-36 0.37 1-17 0.92 4 0.31 0.23 0.37 0.36 DISCUSSION Typical results for ferrocyanide alone are given in Table I. Table I1 shows results for ferrocyanide and aquopentacyanoferrate, when present together. Table I11 gives results for ferrocyanide and aquopentacyanoferrate, when in the presence of carbonylpentacyano- ferrate. Table IV shows typical results for carbonylpentacyanoferrate, determined by the iron(II1) nitrate reagent. TABLE I11 RECOVERY OF FERROCYANIDE AND AQUOPENTACYANOFERRATE IN THE PRESENCE OF CARBONYLPENTACYANOFERRATE ADDED TO COMMERCIAL SODIUM CHLORIDE Ferroc yanide , Aquopentacyanoferrate, Carbonylpentacyanoferrate, & & p.p.m.p.p.m. p.p.m. NO. Added Found Added Found 1 3.60 3.59 1.87 1.95 2 1.74 1.63 Nil Nil 3 1.09 1.03 0.37 0.33 4 0.54 0.52 0.37 0.23 Added 2.80 3.36 0.56 3.80 TABLE IV RECOVERY OF CARBONYLPENTACYANOFERRATE ADDED TO COMMERCIAL SODIUM CHLORIDE AND DETERMINATION WITH IRON (111) NITRATE REAGENT Carbonylpentacyanoferrate , p.p. m. 1 NO. Added Found 1 4.78 2 4.16 3 0-70 4 0.70 4.88 4.16 0.73 0.76 When the iron(I1) - iron(II1) reagent is used for concentration of the ferrocyanide, there is no interference from the normal impurities or usual additives in commercial salts, e.g., calcium sulphate, sodium sulphate , magnesium chloride , sodium iodide, sodium bromide, basic magnesium carbonate, silica, silicates or other free-flow additives.There is also no interference from comparable amounts of metals, cyanide, thiocyanate or nitroprusside. Pentacyanoaminoferrate, [Fe(CN) ,NH3I3-, will interfere, but its presence is unlikely, because it decomposes fairly rapidly in solution to form ferrocyanide (a salt solution with added pentacyanoaminoferrate was tested after 24 hours, and 70 per cent. had become converted into ferrocyanide) .April, 19681 RELATED COMPOUNDS IN COMMERCIAL SODIUM CHLORIDE 243 Aquopentacyanoferrate also decomposes to ferrocyanide, but more slowly. The require- ment of a ratio of 1 : 2 of [Fe(CN),]*- to [Fe(CN),H,O]S- to ensure that the determination of aquopentacyanoferrate is quantitative has not, therefore, presented any problem; there is invariably a relatively large excess of ferrocyanide present. The very occasional appearance of aquopentacyanoferrate may be as an intermediate decomposition product of other com- plexes, or by formation from ferrocyanide in solution by the action of light.’ Any ferricyanide present will be included with the ferrocyanide.When copper sulphate solution is used to concentrate the complexes, any small amounts of cyanide present would interfere with the determination. This could be prevented by pre-treating the sample solution with a dilute solution of hydrogen peroxide before adding the copper sulphate reagent. The determination of carbonylpentacyanoferrate with the iron( 111) nitrate reagent is not affected by the usual impurities in commercial sodium chloride or other cyanogen com- plexes ; the method ensures that pentacyanoferrates are converted into ferrocyanide by boiling with alkaline hydrogen peroxide,* the ferrocyanide being then oxidised to ferricyanide by acidifying the solution with nitric acid and adding further pero~ide.~ Iron(II1) fern- cyanide has a negligible absorption at 530mp. Although designed for samples of salt, the methods have since been used on samples of water and efiuents. Acknowledgment is made to the directors of I.C.I. Mond Division for permission to publish this paper. 1. 2. 3. 4. 5. 6. 7. 8. 9. REFERENCES British Patent 752,582, 1956; British Patent 818,386, 1959. British Patent 667,101, 1962. Marier, J. P., and Clark, D. S., Analyst, 1960, 85, 574. Williams, H. E., “Cyanogen Compounds, ” Second Edition, Edward Arnold & Co., London, 1948, Hofmann, A., Justus Liebigs Annln Chem., 1900, 312, 18. Asperger, S., Murati, I., and Pavlovic, D., J . Chem. Soc., 1960, 730. Williams, H. E., op. cit., p. 172. American Cyanamid Co., “The Chemistry of the Ferrocyanides,” Volume 111, The Beacon Press Received September 1 lth, 1967 p. 197. -, oP. cit., p. 386. Inc., New York, N.Y., 1963, p. 34.