pH Adjustment in Colorimetric iron Determinations NELLIE
F.
DAVIS, CLYDE
E.
OSBORNE, Jr., and HAROLD A. NASH
Pitman-Moore Co., Indianapolis, Ind.
b
With the 2,2'-bipyridine and the ophenanthroline methods for iron determination, low values are frequently obtained when pH adjustment is made before addition of the color-forming reagent. Circumstantial evidence was obtained which indicates that this i s due to the precipitation of ferrous and ferric hydroxides. If the color-formIng agent is added before pH adjustment, difficulties are not encountered.
I
using the 2,2'-bipyridine method described by Moss and 1Iellon (9) for the colorimetric determination of iron, some rather erratic and nonreproducible results were obtained. These difficulties were finally traced t o the technique of p H adjustment of the solutions and it is now evident that this point of technique requires more attention than is indicated in the literature. Moss and Nellon suggest a final pH of 3 to 9 and describe 20% ammonium acetate buffer and 6 5 hydrochloric acid, or 6 S ammonium hydroxide a s suitable reagents for making p H adjustments. Koenig and Johnson ( I ) in their study of the method, report that ammonium hydroxide, potassium hydroxide, or sodium hydroxide can be used interchangeably for p H adjustment. I n the present studies the p H range of 3 t o 9 was a satisfactory range for color development, b u t only if the p H adjustment was made after the reducing reagent and bipyridine had been added to the iron solution (Table I). The data presented in items 1 to 8 of Table I were obtained using a solution a t a n initial p H of 0.7, which was prepared by dissolving iron wire in sulfuric acid. Ferrous rather than ferric iron was used in this part of the study in order t o eliminate the reduction step as a possible variable. The color intensity mas low whenever base mas added to the iron solution in the absence of bipyridine. The final p H was not the governing factor, as shomn by the normal color development even in two solutions of p H close to 9.0. I n one of these, the iron solution was added last and in the other the p H adjustment with strong base was made N
Table I.
Effect of Order of Addition of Reagents on Color Development Order of Addition of Reagents Final pH Transmittance,a %
Ferrous Iron Iron soln., S H A c buffer, hydroxylamine.HC1, bipyridine 2. Iron soln., NH4Ac buffer, hydroxylamine.HC1, bipyridine, 0.5 ml. 2N KaOH 3. Iron soln., N H J w buffer, hydrosylamine.HC1, 0.5 ml. 22V NaOH, bipyridine 4. NHaAc buffer, hydroxylamine.HC1, bipyridine, 0.8 ml. ZiV NaOH, iron soln. 5. Iron soln., NH4Ac buffer, hydroxylamine.HC1, bipyridine, 0.8 ml. 2N NaOH 6. Iron soln., 0.35 ml. Z N NaOH, NH4-4c buffer, hydroxylamine.HC1, bipyridine 7 . NH44c buffer, hydroxylamine.HC1, bipyridine, 0.5 nil. 22%- KaOH, iron soln. 8. Iron soln., hydrosylamine.HC1, bipyridine, SH4.k buffer, 0.5 ml. 21\- NaOH 1.
4.95
60.0, 60.1, 60.2, 60.3
6.95
60.0, 60.1, 60.2
6.95
71.3, 72.1, 72.3, 72.5
8.92
60.0, 60.2, 60.4, 60.7
8.92
60.0, 60.1, 60.1
5.81
82.4, 86.6
6.95
60.0, 60.0, 60.1
6.95
60.0, 60.1, 60.1
Ferric Iron 9. Iron soln., hydroxylamine.HC1, bipyridine, NH4Ac buffer 5.87 10. Iron s o h , ISHailc buffer, hydroxylaniine.HC1, bipyridine 5.87 11. Iron soln., bipyridine, XH4Ac buffer, hydroxylamine.HC1 5.90 12. Bipyridine, hydroxylamine.HC1, NH4Ac buffer, iron soln. 5.88 a Each entry represents a determination on a separate aliquot of
after the bipyridine had been added t o the iron solution. Erratic results are thought to be due t o the addition of base to the iron solution with subsequent precipitation of ferrous hydroxide by local excesses of base. When bipyridine is present, the ferrous ion activity is so reduced that precipitation of the hydroxide does not occur. Consistent with this behavior is the fact that similar results were obtained with ophenanthroline. An even greater sensitivity t o p H adjustment was found when ferric iron was used. A few results with the ferric ion are shown in items 9 to 12 of Table I. A ferric sulfate solution having the same concentration as the ferrous iron solution and a n initial pH of 2.01 was used in studies on ferric ion. Whenever the p H of ferric iron solutions was brought into the neighborhood of 3.5 or above, low results were obtained. This greater sensitivity of the ferric ion might be expected from the lesser solu-
60.0 65.5 65.4 60.1, 60.1 the iron solution.
bility of ferric hydroxide. Low results could be avoided by adding reducing agent and bipyridine before attempting a n y pH adjustment, even with buffer. T h a t the low results n ere not a function of incomplete reduction of the ferric ion a t higher p H values is shown by item 12 of Table I. The iron TTas added last, so reduction has to take place a t a p H near 5.9. From these results it appears that the order of reagent addition to an iron solution should be: reducing agent, colorforming agent, buffer, and finally base, if hecessary. LITERATURE CITED
(1) Koenig, R. A , , Johnson, C. R., J . Bioi. Chem. 143, 159 (1942). ( 2 ) MOES. RI. L.. hlellon. M. G.. IND. ' ENG. CHEM., ANAL. ED.'14, 862 (1942). RECEIVED for review February 17, 1958. Accepted August 1, 1958.
VOL. 30, NO. 12, DECEMBER 1958
2035
