Spectrophotometric cerimetric titration of iron(II) with ferroin as indicator

MICl1OCHEMICAL

VOL.

JOURNAL

Spectrophotometric Iron(I1)

with

Cerimetric Ferroin

V, PAGES

56%572

Titration

(1961)

of

as Indicator*

I. M. KOLTHOFF and R. WOODS, School of Chemistry, University of Minnesota, Minn.eapolis, Minnesota

ln a rcccnt paper* the amperomctric titration of very dilute solutions of ferrous iron in the presence of large concentrations of ferric iron was described. Ceric cerium was used as titrant, ferrous phenanthroline as amperometric indicator, and a rotated platinum wire as indicator electrode. In the present paper it is shown that the titration can also be carried out spectrophotometrically. The main purpose of this paper is to account for a mysterious blank in the ampcrometric titration’ corresponding to an apparent presence of some ferrous iron and to present’ conditions under which this blank is eliminated. In the spectrophotometric method, ferrous phenanthroline is added to an acidified solution of ferrous iron which may contain ferric iron and titrated with standard ceric ccrium solution in a cell which can be placed in the carriage of a Beckman D.U. spectrophotometer. The titration is carried out at 500 rnp and at a temperature of 04°C. The absorbancy (corrected for volume change during the titrat’ion) remains constant and decreases linearly after the endpoint.

The Indicator Blank Ferroin slowly dissociates in the acid solution used in the cerimetric titration, to yield ferrous iron: FePh, + 3H+

-

Fez+ + 3PhH +

The ferrous iron produced by this dissociation appears as an indicator blank, the value of which depends on the time between the * This investigation was carried out under a grant from the National Foundation. 569

Science

570

I. M. KOLTHOFF

AND R. WOODS

addition of ferroin to the acidified solution and the end of the titration and particularly on the temperature of the solution. The blank is observed in both the amperometric and spectrophotometric cerimetric titrations of ferrous iron at normal room temperature. The dissociation is a first-order reaction2 and therefore for small times the blank titre will be approximately equal to a molar concentration in ferrous iron of C&t (where Co is the original molar concentration of the ferroin, k is the rate constant, and t is the reaction time). The rate constant at 25°C. is 0.0045 min.-Iv2 and therefore for a lo-*M ferroin solution the blank will be 4.5 X lo-‘t&I Fe2+. The results given in Table I substantiate this interpretation. TABLE I Rate of Increase of Blank at 25°C. in 1N Sulfuric Acid Time between addition of ferroin and the endpoint

Blank Fez+ formed M ’

4.5 x 10-t

0 10 20 37

0 4.4 x 10” 8.0 x 10-C 16.9 x 10-G

0 4.5 x 10-e 9.0 x 10-t 16.7 X 10-G

The rate of dissociation of ferroin is temperature dependent and becomes very small when the temperature is reduced to 0-5°C. The rate constant at this temperature is of the order of 7 X 10B5 min. -l, and the dissociation during the time taken for a titration is negligible. In order to have a negligible blank in both the amperometric and spectrophotometric methods, the titration should be carried out at 0-5°C. No blank correction is then necessary upon graphical location of the endpoint.

Spectrophotometric Titration of Ferrous Iron 50 ml. of ferrous iron solution which is 1N in sulfuric acid is placed in a titration vessel consisting of a stoppered bottle with a l.OOO-cm. cell attached. The solution is cooled in an ice bath to 0-5°C. 1 ml. of 2.5 X 10-3M ferroin, also cooled to 0-5°C. is added, and the solution titrated immediately with standard ceric cerium solution added from a microburet. The absorbance is measured at ,500 rnp MICROCHEMICAL

JOURNAL,

VOL.

V, ISSUE

4

TITRATION

0.6

-

2 0.4 :: ?I 5 0.3

-

0.2

-

0.1

-

OF IRON( II)

571

-

0.50

0.0

1.5

I .o ml Cerium

Fig. 1. Titration

of 50 ml. 1.038 X lO+M Fe*+ in NHzSOa with 0.005N ceric cerium. Concentration of ferroin 5 X 10-6M.

and corrected for volume change by multiplication by (V + v)/V (where V is the original volume and 0 the volume of reagent added). The solution is kept at O-5%. by returning the titration vessel to the ice bath between additions of standard ceric solution. The corrected absorbance remains constant up to the endpoint and then decreases linearly. The endpoint is found graphically (Fig. 1). In a humid atmosphere, moisture condensation on the walls of the cell can be eliminated by passing a slow stream of dry air over the surface of the cell in the chamber. TABLE II Spectrophotometric Titration of Ferrous Iron Concentration of Fez+, moles/liter 4.15 2.075 1.038 6.23 2.08 9.82

x x x X x x

10-d 1O-4 1O-4 10-h 10-s 10-e

Fez+ taken, meq. 2.075 1.038 5.19 3.12 1.04 4.91

x X X X x X

10-Z 10-Z 10-a 10-a 10-Z 10-d

Fez+ found, meq. 2.075 1.036 5.17 3.13 1.03 4.95

X x x X X x

1O-2 1O-z 10-a 10-s 10-a lo-’

Error, y0 0.0 -0.2 -0.4 +0.3 -1.0 +0.8

572

I. M. KOLTHOFF

AND

R. WOODS

The results in Table II show that the titration is accurate and precise to tvit8hin lye for iron conccnt’rations greater than 10-541. Summary

Using ferroin as indicator, concentrations of ferrous iron greater than 10P5,!lil can be determined rapidly by amperometricl or spectrophotometric titration with ceric cerium at a temperature of 0-5’C. with an accuracy and precision better t.han 1%. The indicator blank is zero at this low temperature. References 1. Kolthoff, I. M., and B. B. Bhatia, Micro&em. J., 4, 451 (1960). 2. Lee, T. S., I. M. Kolthoff, and D. L. Leussing, J. Am. Chem. Sot., 70, 3596 (1948).

Received May 8,196l

ERRATA Amperometric

Cerimetric

Titration as Indicator

I. M. KOLTHOFF (Microchemical

of Iron(I1)

with

Ferroin

VOL.

V, ISSUE

and B. B. BHATIA

Journal,

4, 451-457 (1960))

The legends for Figures 2 and 3 should be interchanged.

MICROCHEMICAL

JOURNAL,

4