Selective and Sensitive Spectrophotometric Determination of Iron(11) with 7- Nitroso-8- quino1inol-5-sodium SuIfonate M. A. Eldawy’ and S. R. Elshabouri Department of Pharmaceutical Chemistry, Faculty of Pharmacy, University of Assiut, Assiut, Egypt, A.R.
Salts of iron(I1) are commonly present in combination with other mineral salts and hormones in geriatric preparations. Various analytical procedures for iron(I1) are available. These include volumetric ( I - 3 ) , chromatographic (41, X-ray emission (5), atomic absorption (6),and colorimetric methods (7,8). Although all these methods are suitable for determination of iron(II), colorimetric methods are generally preferred, as they involve less expensive instrumentation and afford greater sensitivity when appropriate chromogenic reagents are employed. 7-Nitroso-8-quinolinol-5-sodium sulfonate (I) combines the characteristics of two of the most reactive ligands, 2nitroso-1-naphthol ( 9 ) , and 8-hydroxyquinoline. It has been recently reported that this ligand form complexes with certain metals (IO), a fact which was successfully employed in our laboratories for the determination of Cu(II), Zn(II), and Mn(I1) (11). S0,Na
I Because reports on the determination of iron(I1) in multicomponent pharmaceutical preparations are rather scarce, we decided to investigate further the iron(I1) complexing properties of this ligand, in order to develop an analytical procedure for iron(I1) without prior separation from other mineral ions, vitamins, and hormones which occur along with Fe(I1) in geritaric preparations. This investigation resulted in the development of a sensitive and selective method for the determination of Fe(II), which is not subject to interference by the aforementioned ingredients.
EXPERIMENTAL Apparatus. A Russian mode C$-10 recording spectrophotometer was used to determine the absorbance values recorded as well as to obtain the spectra presented. 7-Nitroso-8-quinolinol-5-sodium sulfonate was prepared according to a reported method (10). Materials. Ferrous sulfate, ferric sulfate anhydrous, cobalt sulfate, copper sulfate, zinc sulfate, manganese chloride, magnesium sulfate, calcium chloride, potassium chloride, hydrochloric acid, citric acid, disodium hydrogen phosphate, monopotassium dihydrogen phosphate, sodium acetate, acetic acid, and ceric ammonium sulfate were all analytical grades. Ethynyl estradiol, methyl testosterone, thiamine hydrochloride, riboflavin, pyridoxine hydrochloride, folic acid, cyanocobalamin, nicotinamide, ascorbic acid, starch, lactose, and glucose were pharmaceutical grades. One commercial ferrous sulfate tablet preparation was purchased from the local market. Reagents and Solutions. The following solutions were used. Ligand solution 0.0025M in double distilled water, acetic acid-sodiAuthor to whom all correspondence should be addressed a t his present address; Department of Pharmaceutical Chemistry, Faculty of Pharmacy, University of Tanta, Tanta, Egypt, A.R. 1844
um acetate buffer, pH range 3.42-5.89 (12); Clark and Lub’s potassium chloride-hydrochloric acid buffer, pH range 1.0-2.2 (13), McIlvaine’s citric acid-phosphate buffer, pH range 2.2-8 (13). S6rensen’s phosphate buffer, pH range 5.0-8.2 (131, 0.1N ceric ammonium sulfate, and phenanthroline ferrous complex solution. Double distilled water was used throughout. Standard Solution of Ferrous Sulfate (FeS04.7Hz0, 0.0025M). Weigh accurately 0.139 g of ferrous sulfate in a 100-ml volumetric flask, dissolve, and complete to volume with double distilled water. This is the stock solution from which appropriate dilutions were made. This solution must be prepared immediately before use. Standard Solution of Ferric Sulfate. Weigh accurately 0.1998 g of ferric sulfate in a 100-ml volumetric flask. Dissolve and complete to volume wit) double distilled water. This is the stock solution from which appropriate dilutions were made. Preparation of Assay Solutions. For Pure Ferrous Sulfate. Appropriate volumes of standard ferrous sulfate are used as the assay solution. For Synthetic Mixture. Transfer an accurately weighed amount of the powdered synthetic mixture equivalent to 100 mg of ferrous sulfate to a 100-ml volumetric flask, complete to volume with distilled water. From this stock solution, suitable dilutions were made. For Tablets Containing Ferrous Sulfate. Weigh and powder 20 tablets. Transfer an accurately weighed quantity of the powder equivalent to about 300 mg of ferrous sulfate to a 100-ml volumetric flask, dissolve, and complete to volume with double distilled water. Filter and discard the first portion of the filtrate. The clear solution obtained is the stock assay solution. Development of Color. Into separate stoppered test tubes, each containing 4 ml of McIlvaine’s citric acid-phosphate buffer, pH 5, pipet 2 ml of ligand solution, mix well, then add 2 ml of the standard solution, 2 ml of the assay solution of appropriate dilution, and 2 ml of double distilled water and shake well. Determine the absorbance of the standard and sample solutions a t 710 nm in 1-cm cells vs. the blank using a suitable spectrophotometer. Determination of the Chelate Combining Ratio. The method described by Diehl arid Lindstrom (14), was essentially employed using the amounts shown in Table I.
RESULTS AND DISCUSSION In the early stages of the work, acetic acid-sodium acetate buffer pH 5 was used to develop the color of Fe(I1) with ligand. The color develops immediately and shows maximum absorption a t 710 nm (6 = 29315) (Figure 1) [re705 nm, t = 18000 in water, ( I O ) ] . The relationported A,, ship between absorbance at 710 nm and concentration is quite linear up to 2.54 gg/ml sample solution. When interference studies were carried out in order to check the specificity of the method for the determination of Fe(II), in the presence of other metal ions likely to be compounded with Fe(I1) in geriatric formulations, the developed method faced a serious challenge. Although cobalt(II), copper(II), zinc(II), and manganese(I1) do not interfere with the developed method under the aforementioned conditions, it was found that iron(III), the main oxidation product of iron(II), does interfere. Iron(111),when present, produces an instantaneous green color showing an intense absorption maximum at 710 nm (e = 31451). This finding is in variance with the report of Aly et al. (IO); who were unable to separate the Fe(II1) chelate. Accordingly, it was decided to carry out a rigorous investigation of the conditions of chelations in a trial to find out
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
~
Table I. Continuous Variation Mixtures, volume in ml Solution numbers ~~
Components
2
1
5
4
3
0 .o 2 .o
0.8 1 0.2 0.4 0.6 2 .o 2 2 .o 2 .o 2 .o Water 1.2 1 Reagent a 2 .o 1.8 1.6 1.4 a 0.0005M Iron(I1) and reagent stock solutions were used to prepare each mixture. Iron(I1)"
7
S
9
10
11
1.2
1.4 2 .o 0.6
1.6 2 .o 0.4
1.8 2 .o 0.2
2 2 0
6
2 .o 0.8
Table 11. Effect of Acetic Acid-Sodium Acetate Buffer on the Color Product of Iron(I1) and Iron(II1) with the Ligand Iron(1I)Q
max,
PH
A
'1+n
Iron(II1) ' m a l t l'm
A
Amaxt nm
A
Amax, nm
A
710 710 710 710 710 710
0.625 0.762 0.750 0.800 0 .750 0.750
450 474
0.487 0.200
710 710 710 710 710 710
0.712 0.012 0.890 0.975 0.975 1.975
3.42 450 0.0362 4.05 450 0.287 4 -45 ... ... ..* 5 .oo 5.50 ... ,.. *.. 5.89 Iron(II), 1.65 kg/ml. Iron(III), 3.2 pg/ml.
...
...
a
Table 111. Effect of S~rensen'sPhosphate Buffer on the Chelate Color Product of Iron(I1) and Iron(II1) Iron(1I)Q PH
Amax, nm
5 6 7 8
710
... ... ...
loot
Irun(II1) A
Amax, nm
A
710 710 710 710
0.120 0.300
0.687 710 0.700 710 0.450 710 0.200 a Iron(II), 1.65 pg/ml. Iron(II1). 3.2 kg/ml.
0.125
0.100 4 3 2 2C
Iron(II)"
Iron(II1) 2 75
hax, nm
-4
hrnax,11m
A
2.2 448 0.389 710 0.487 3.0 448 0.270 710 0.300 4.0 448 0.275 710 0.500 5.0 710 0.587 6.0 ... 710 0.562 7.0 ... 710 0.525 8.0 ... 710 0.375 a Iron(I1). 1.65pg/ml. Iron(III), 3.2 kg/ml.
... ...
...
...
...
%ax,
nm
... ...
... ... ... ... ...
40
60
00 5W 20
40 60 50 6m 20 WAVELENGTH, nm
40
60 8C
7CQ 2C
4CAO
Figure 1. Absorption spectrum for iron(l1)-ligand complex in acetic acid-sodium acetate buffer, pH 5. Iron(l1) concentration is 2 pg/ml
Table IV. Effect of McIlvaine's Citric Acid-Phosphate Buffer on the Chelate Color Product of Iron(I1) and Iron(II1) pif
... ... ... ...
...
A
... ... ... .
I
I
.
... ...
...
the system which would discriminate not only between iron(I1) and other metal ions likely to be present along with it, but also between the title ion and its oxidation product, iron(II1). T o undertake such an objective, several attempts were carried out using different buffer systems, but these attempts did not result in solving this problem as shown by the data in Tables I1 and 111. Further, the use of Clark and Lub's acidic buffers (131,resulted in complete failure of the ligand to chelate either Fe(I1) or Fe(II1). The fact that citrates form complexes with Fe(II1) ( 2 5 ) , drew our attention to the use of McIlvaine's citric acidphosphate buffer as a medium for the color development. In this buffer system, Fe(II1) does not form any colored complexes with the ligand through the whole range of pH tested (2.2-8) (Table IV). Accordingly, this buffer system was used for color development. Under these conditions, Fe(I1) forms an instantaneous deep green color showing two A,, one at 710 nm, which is
0 00
4 i o 20
40
62
BC
502 23
40 60 53 630 23 WAVELENGTH n m
40
60
80
7 0 0 20
40
Figure 2. Absorption spectrum for iron(l1)-ligand complex in Mcllvaine's citric acid-phosphate buffer, pH 5. Iron(ll) concentration is 1.65 pglml
p H independent, and another less intense peak a t 480 nm, which gradually diminished in intensity when the pH of the system was raised until it completely disappears at pH 5-8. The molecular absorptivity as calculated from the absorption curve (Figure 2) is 19156. The relationship between absorbance and concentration at 710 nm was found to be quite linear up to 2.38 pg Fe(II)/ml sample solution, the laver limit for detection is 0.17 pg Fe(II)/ml. The chelate combining ratio was determined by the method of continuous variation ( 1 4 ) and was found to be 1:3 (M:L) (Figure 3). The specificity of the method for the determination of iron(I1) in the presence of other minerals, hormones, certain vitamins, and excipients likely to be present along with iron(I1) in geriatric formulations is shown in Table V. The amounts of the interferents are those of the average found in commercial pharmaceutical formulations (16). The re-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975 * 1845
Table V. Determination of Ferrous Sulfate in the Presence of Other Minerals, Vitamins, and Hormones Which Usually Occur in Combination with Ferrous Sulfate in Geriatric Formulations Fcrrous sulfatr recovereda Substanccr added (per 100 rng of fcrrouI sulfate) nig
Ethynyl e s t r a d i o l Methyl t e s t o s t e r o n e Thiamine hydrochloride Riboflavin Pyridoxine hydrochloride Folic acid Cyanocobalamin Nicotinamide A s c o r b i c acid Cobalt sulfate Zinc sulfate Copper sulfate Manganese sulfate Magnesium sulfate F e r r i c sulfate Starch Glucose Lactose a Average of 3 determinations. -
"'1
0.01 2.50 5.00 2.50 0.50 0.50
100.0 99.8 100.0 99.9 100.0 99.8
0.001
100.0
5.00 75.00 0.12 1.375 1.26 1.55 21.00 100.00 100.00 100.00 100.00
99.9 99.7 100.0 99.9 99.7 99.8 100.0 100.0
100.0 100.0 100.0
* * * *
100.0 0.3 99.8 0.1 100.0 0.1 99.9 + 0.3 100.0 0.0 99.8 t 0.1 100.0 i 0.0 99.9 i- 0.3 99.7 = 0.3 100.0 0.3 99.9 i 0.3 99.7 0.2 99.8 + 0.5 100.0 0.0 100.0 0.0 100.0 = 0.1 100.0 0.0 100.0 0.0
Std dev
Re1 std dev
0.50 0.14 0.10 0.412 0.00 0.14
0.50 0.15
0.10 0.41 0.00 0.15 0.00 0.41 0.29 0.36 0.41 0.21 0.65 0.00 0.00 0.10
0.00 0.412 0.28 0.36 0.412 0.23 0.65 0.00
* * * * * *
0.00 0.10
0.00
0.00 0.00
0.00
~~~~
Table VI. Determination of Ferrous Sulfate Tablets by the Developed Method and by the B.P. Method Dcycloped method,
Fcrrous sulfatc &lnnnfncmer
[l
found,'tablet "9
m $1
93.5 i 0.50 0.279 Std dev 0.75 Re1 std dev 0.83 0 Average of 3 experiments. Declared composition of each tablet is: ferrous sulfate 0.3 g, and vitamin Bi 2 mg.
Tablets
0.30
B . P . method,
fernon5 sulfate
clnimed/tahlet,
0.28
93.0
+
0.33 0.64
0.68
provided by the Nasr Company for Pharmaceutical Chemicals and Antibiotics, Abou Zaabal, Egypt, are gratefully acknowledged.
LITERATURE CITED
Relative Mole percent
Figure 3. Continuous variation plot for iron(l1)-ligand complex in Mcllvaine's citric acid-phosphate buffer
covery from these synthetic mixtures ranged from 99.7100%. Results obtained in the analysis of samples of ferrous sulfate tablets (Table VI), demonstrate the effectiveness of the developed method and are comparable to the British Pharmacopoeial method (2). The method developed, being sensitive and facile, possesses a further advantage that, under the conditions illustrated, Fe(III), the main oxidation product of Fe(II), does not interfere with the development of the color or the precision of the method. Accordingly, the proposed method could lend itself applicable as a stability assay for iron(I1).
ACKNOWLEDGMENT The authors thank Ali Sina Tawfik for helpful discussions during the course of this work. Technical facilities 1846
(1) N. A. Bugrim and E. L. Zatula, Farm. Zb., 25, 54 (1970); Cbem. Abstr., 28989k (1973). (2) British Pharmacopoeia, 1968. p 418. (3) "The United States Pharmacopoeia", 18th rev. ed.. Mack Publishing Co., Easton, Pa., 1970, p 251. (4) A. V. Kulebakina, Mosk. Med. inst., 61, 232 (1968); Cbem Abstr., 72, 24662 (1970). (5) H. I. Tarlin and M. Matcheider, J. Pharm. Sci., 56, 1328 (1970). (6) W. F. Beyer and K. G. Zipple, J. Pharm. Sci., 57, 653 (1968). (7) A. A. Schilt and W. C. Hoyie, Talanta, 15, 852 (1968). (8) T. Yotsuyanagi, R. Yamashita, and K. Amoura, Anal. Chem., 44, 1091 (1972). (9) Osmo Makitie and R. Niemi, Mikrocbim. Acta, 728 (1972). (10) M. M. Aly, S. Elezabi, and A. Makhyon. J. horg. Nucl. Cbem.. 35, 2727 (1973). (1 1) M. A. Eidawy. S.R. Elshabouri, and M. M. Aiy. Abstracts of papers presented at the 4th Pan-Arab Congress of Pharmaceutical Sciences, Oct. 29-Nov. 1, 1974, Cairo, Egypt. (12) A. I. Vogel. "Quantitative Inorganic Analysis", third ed., Longmans. p 35. (13) Documents Geigy "Scientific Tables", Konrad Dienn, Ed., J. R. Geigy, S. A., Basie, Switzerland, p 314. (14) H. Diehi and F. Lindstrom, Anal. Cbem., 31, 414 (1959). (15) V. N. Alexeyev. "Quantitative Analysis", Mir Publishers, Moscow, 1967, p 235. (16) Index of Specialities, The Egyptian General Organization for Pharmaceutical Chemicals and Medical Appliances, 4th ed., 1969, p 291.
RECEIVEDfor review October 17, 1974. Accepted May 27, 1975. A preliminary note on this topic was presented at the 3rd Pan-Arab Congress of Pharmaceutical Sciences, Nov. 1972, Baghdad, Iraq. Abstracted from the dissertation presented by S.It. Elshabouri, in partial fulfillment of the Ph.D. degree requirements, University of Assiut.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 11, SEPTEMBER 1975
