ANALYTICAL EDITION
May 15, 1942
44 7
(4) Baudisch, O., and Rothschild, N.,Ibid., 48,1660 (1915). (5) Knudson, H. W., Meloche, V. W., and Juday, C., IND. ENG.
Literature Cited
CHEM.,
(1) Baudisch, O.,J. Am. Chem. SOC.,63,622 (1941). (2) Baudisch, O.,Science, 92,336 (1940). (3) Baudisch, O.,and Karrzeff, N., Ber., 45,1164 (1912).
ANAL.ED.,12, 715 (1940).
PR~EENTE before D the Division of Analytical and Micro Chemistry a t the lOlst Meeting of the AMERICANCHEMICAL S O C I ~ TSt. Y , Louis, Mo.
Determination of Divalent Iron GEORG CRONHEIM’ AND WILLIAM WINK Yew York State Research Institute, Saratoga Spa, Saratoga Springs, N. Y.
I
K A previous paper (S), o-nitrosophenol was introduced as a new reagent in quantitative colorimetric analysis. It forms strongly colored inner-complex compounds with a number of metal ions, and the different metal complexes are distinguished by their color and their solubility in different solvents. The most important metals of the water-soluble group are divalent iron, copper, mercury, and nickel. The complexes of these and other metals are red or reddish violet with the exception of divalent iron, which is grass-green colored. No other metal forms a green, water-soluble complex with o-nitrosophenol. Based on this characteristic property, the authors have worked out a quantitative estimation of small amounts of divalent iron. Preliminary experiments (3) showed that the reaction between o-nitrosophenol and ferrous ions is, in every respect, suitable for a colorimetric determination of this metal. The new method makes use of a reagent dissolved in a water-immiscible organic solvent, a feature not yet common in colorimetric work. The affinity of the divalent iron for o-nitrosophenol is so strong that upon shaking the two solutions-i. e., the ferrous salt in water and the organic solventthe complex green iron salt is formed immediately and quantitatively. The use of a water-immiscible solvent for the reagent has a very important advantage. Since free o-nitrosophenol in an organic solvent is yellowish green, it can be seen directly whether or not the reagent has been used in excess, as should always be the case. If the solution of the free o-nitrosophenol becomes colorless after shaking with the ferrous salt solution, it is a t once apparent that more reagent should be added. I n using this method the solution of the ferrous salt in water is shaken with a solution of o-nitrosophenol in petroleum ether, whereupon the water solution becomes deep green, and after the two liquids have been separated the green water solution is measured in a colorimeter in the usual manner. Petroleum ether is preferred as the solvent for the o-nitrosophenol because of the characteristic solubilities of the metal salts of o-nitrosophenol ( 3 ) . These characteristic solubilities account for the fact that by this method only the divalent iron is determined. Ferric ions t o o react with o-nitrosophenol, forming a brown-colored complex compound. However, the reaction between ferric ions and o-nitrosophenol is not quantitative and the ferric complex is easily soluble in petroleum ether and thus extracted from the water solution of the green ferrous complex. The method can be used for the estimation of ferric ions after the latter have been reduced to the divalent state by means of isoascorbic acid. A description of this procedure will be published soon by Baudisch. The green ferrous complex is stable for a t least 24 hours in the absence of very strong oxidizing agents or of very strong light. 1
Present address, The G. F. Harvey Company, Saratoga Springe,
The intensity of the green color formed is always reproducible and is in proportion to the concentration of ferrous ions originally present (curve 1, Figure 1). Unfortunately, the authors did not have the facilities to determine the range in which the colored system conforms strictly to Beer’s law. 60
i’ C
-r .“ /I
2
,
/ * / . ,
-20
1 ’i/
A
0
FIGURE1.
.08 mg.
.I6 .24 Iron p e r 100 ml.
.40
.32
CURVE FOR DETERMINATIOX FERROUS IONS
CALlBRATION
OF
Absorption = deflection on the “absorption” scale of colorimeter 1, obtained with o-nitrosophenol; 2. with a,a’-bipyridine
The maintenance of the proper p H is of greatest importance in all colorimetric work with o-nitrosophenol. I n the case of its water-soluble compounds, a low p H prevents the quantitative formation of the complex, owing t o dissociation. If the pH is too high, a mixture of two compounds is formed whose general formula can be written as A’-Me-R (I) and N-Me-N (11), where N represents the o-nitrosophenol group. These two compounds differ in their color and solubility. I is soluble only in water, while I1 is soluble in certain organic solvents like ether, but insoluble in petroleum ether. For the determination of divalent iron the most suitable pH range is from 5.1 to 5.3. The preparation of o-nitrosophenol is described by Baudisch ( 1 , 2 ) . Since it can be prepared and is stable only in solution, the proper concentration of the reagent must be controlled by tests (S), which should be made a t regular intervals. The solution of o-nitrosophenol in petroleum ether is stable for 2 to 3 weeks if kept in a refrigerator.
Procedure
The solution of the ferrous salt is nearly neutralized (spot test with methyl orange or bromophenol blue) and diluted so that it contains not more than 1 microgram of ferrous iron per ml. I n a separatory funnel exactly 50 ml. of this solution are mixed with exactly 5 ml. of an acetate buffer solution of pH 5.2. After 5 ml. of o-nitrosophenol solution are added, the mixture is shaken vigorously from 15 to 20 seconds and then allowed t o separate. The petroleum ether is removed as well as possible by means of a pipet with a rubber bulb and the green water solution is shaken ? Y. i.with a second 5-ml. portion of o-nitrosophenol solution. After
448
INDUSTRIAL AND ENGINEERING CHEMISTRY
the t w o liquids have separated, the petroleum ether should still be yellowish green because of an excess of free o-nitrosophenol. The green water solution is filtered through a paper filter directly from the separatory funnel into the 30-mm. cell of the colorimeter. The filtration helps to clear the water solution completely from the last droplets of petroleum ether and prevents the formation of bubbles which might cling to the cell walls and so influence the results. Curve 1, Figure 1, show5 the value obtained with the socalled absorption scale of a photoelectric colorimeter (a modified Lange photoelectric colorimeter manufactured by Pfaltz & Bauer, Inc., S e w York, X. Y.) using a pure ferrous chloride solution. The light source was a 6-volt incandescent bulh and no light filter was used. Therefore, the values in the curve do not represent the true absorption. By means of this curve it is possible to determine the maximum error and the sensitivity of the proposed method. When using a storage battery the fluctuations of the galvanometer are not greater than one-tenth scale division, and the readings are made with an accuracy of one-quarter scale division (foi an experienced worker it is not difficult to estimate one-tenth scale division), which corresponds to 0.3 microgram of iron in 50 ml. For a n iron solution containing about 50 micrograms of iron in 50 ml. this means that the maximum error in the determination is not greater than 0.5 per cent. The sensitivity of the method can he calculated in a similar manner. The lowest limit for which a satisfactory reading can be made is two scale divisions above the zero point of the curve, corresponding to about 2 micrograms of ferrous ion in 50 ml. of solution. A11 the foregoing experiments and calculations were made with the so-called normal sensitivity of the colorimeter but by changing the resistance in the photoelectric circuit, one can increase the sensitivity of the instrument ten times ( 3 ) . The smallest amount of ferrous ion which can be determined by the described method is about 0.5 microgram in 50 m1.-i. e., 1 part in 100 million. (All measurements were made with a regular incandescent bulb. The use of monochromic light of a suitable wave length will increase the sensitivity still more.) The calculations indicate that the new method with o-nitrosophenol is one of the most sensitive for the quantitative determination of ferrous ions. An experimental proof is given in Figure 1 by curve 2 which was obtained with a,a'-bipyridine instead of o-nitrosophenol. A comparison of the two curves shows that under the present experimental conditions the new method is at least three times as sensitive as the bipyridine method. (Contrary to the literature, a,a'-bipyridine forms strongly colored red complex compounds with titanium salts. The authors made this observation when S. E. Ashley of the General Electric Company, Pittsfield, Mass., called their attention to the fact that titanium salts react in a similar manner with o-phenanthroline.) The limitations for the new method are few. The most important is the necessity of having the divalent iron present in the ionic state. This means that complex-forming compounds such as phosphoric acid, oxalic acid, and others should not be present. Trivalent iron, cobalt, and palladium do not interfere because their o-nitrosophenol salts are soluble in petroleum ether and therefore will be shaken out during the reaction, but as these metals will bind a part of the free o-nitrosophenol the amount of the latter has to be increased accordingly. In the presence of trivalent iron strong light must be excluded, because under its influence the trivalent
Vol. 14, No. 5
iron will be reduced to a certain extent' by o-nitrosophenol and thus cause too high results. Of the other heavy metals known to form water-soluble colored complex compounds with o-nitrosophenol, most important are copper, nickel, mercury, and zinc, because their color is so strong that they are detectable in concentrations as low as lo-' to lovb molar. However, the nitrosophenol complexes of these and other less important metals are all ret1 or reddish violet in color. In their presence, divalent iron can be determined by the use of suitable color filters. The theoretical considerations for the colorimetric analysis of i: two-component color system are given by Knudson, Meloche, and Juday (4). Alkali and alknliiie earth salts do not interfere with the determin:ttion of divalent iron because their o-nitrosophenol salts are formed only a t a pH above i . The described method has been applied to the determinat8ion of iron in natural Saratoga mineral waters, which arL: solutions of bicarbonates of alkalies and alkaline earths, together with an excess of free carbon dioxide. The iron ib present only in the divalent state. Because of the bicarbonate content' of these waters, the proper pH can be attained by adding only acetic acid. Owing to the high sensitivity of this method, the waters had to be diluted in some cases in the ratio of 1 t,o 25. Table I shows some of the results obtained. To demonstrate the accuracy of the new method, the iron content of Lincoln w-ttt'er was determined gravimetrically, using cupferron as precipitating agent (Table I). The differenre between the two methods does not exceed 1 per cent.
TARIAE1.
nETERMINATION OF I R O N IK S A T C R A L MINERAL
Sprina
WATERS Colorimetric MMy./l. 12.15 15.46 2.22 2.33
Iincolnfl Lincolnn Geyser Coesa Hathorn KO.2 A . 40 (1 Two samples t,aken at several days' intervai.
Gravimetric Mg./l. 12.20 15.55
...
... ...
Summary A new method for the colorimetric estimation of divalent iron is based on the reaction of divalent iron with o-nitrosophenol, yielding a green inner-complex salt. The method is one of the most sensitive colorimetric determinations for this metal, since 0.5 microgram in 50 ml. of solution can be estimated. Trivalent iron, cobalt, and palladium do not interfere. Copper, nickel, and mercury form red or reddish-violet compounds with the reagent. A comparison with a gravimetric determination of iron showed a very close agreement. Literature Cited (1) B a u d i s c h , O., J . Am. Chem. Soc.,63,622(1941). (2) Baudisch, O., Science,92, 336 (1940).
IND.ENG.CHEM..ANAL.ED..14, 445 (19491. Knudson,' H.' W., Meloche, V. W., a n d Juday, C., Ibid., 12, 715 (1940).
(3) Cronheim. G..
i4j
PRESENTED before the Divislon of Analytical and Micro Chemistry a t the loifit Meeting of the . ~ I E R I C A NCHEMICAL SOCIETY, St. Louie, Mo.
