Spectrophotometric Determination of Iron in Stron SIR: Numerous methods for the spectrophotometric determination of iron have been described. N o s t of these involve the complexation of iron in acidic or neutral solution. A few methods for the determination of iron in strongly basic solutions have also been described. These include procedures utilizing 4,7-dihydroxy- 1 1O-phenanthroline (S), picolinethioamide ( I ) , 2pyridinealdoxime ( 2 ) and phenyl-2-pyridyl ketoxime (4) as complexing agents for the iron(I1) ion. I n this work a method is described for the determination of iron in strong alkali using 2,6-pyridine diamidoxime as the color forming reagent. The method is rapid, selective, and sensitive and off ers several advantages over available methods for this specific application. EXPERIMENTAL
All absorbance measurements were made with a Cary hlodel 14 recording spectrophotonieter. hfeasurenients were made a t room temperature in matched silica cells. A Becknian Zeromatic pH meter, equipped with a high alkalinity glass electrode (Beckman 40495 Type E-2) and a saturated calomel reference electrode, was used for all p H measurements. Reagents. 2,6 - Pyridinediamidoxime was synthesized by the addition of hydroxylamine hydrochloride to 2,6-dicyanopyridine. Fifty milliApparatus.
liters of solution containing 2.8 grams (0.04 moles) of hydroxylamine hydrochloride and 11.54 grams (0.11 moles) of sodium carbonate were added to 2.6 grams (Q.02 moles) of 2.6 dicyanopyridine in 20 nil. of hot ethanol. The solution was heated at 70' C. for 4 hours and then at reffux temperature for an additional 4 hours. The desired product crystallized from solution upon cooling and was recrystallized from water. The calculated values for CiH9N502 are 43.07% C. 4.657, H, and 35.89% N. Found mere 42.607, C, 4.47% H, and 35.50% S. A O.lY0 solution of 2,6-pyridinediamidoxime was prepared by dissolving 0.25 gram of reagent in 250 ml. of 0.1X hydrochloric acid., A standard solution of iron was prepared by dissolving 0.1197 gram of standard iron wire in a minimum amount of hydrochloric acid and diluting the resultant solution to 1 liter with distilled water. The solution was standardized by titration with standard sulfate cerate solution. A 57, solution of sodium dithionate was prepared fresh daily by dissolving 5 grams of the salt in 100 nil. of distilled water. All other reagents used were of reagent grade quality. Recommended Procedure. Dissolve a weighed sample of alkali in 100 ml. of distilled water. The sample should be of sufficient size t o giye a 6 to 8 X solution. Transfer a 25-ml. aliquot of the sample solution to a 50-ml. volumetric
WAVELENGTH (mu1
Figure 1 . Visible absorption spectrum of bis-(2,6-pyridinediamidoxime)iron(l1) at pH 13.0
flask. Add 5 ml. each of a 0.1% solution of 2.6-pyridinediamidoxinie, of a freshly prepared 5% solution of sodium dithionite, and of 1X sodium hydroxide. Place in a boiling water bath for 5-10 minutes. Remove from the bath, cool to room temperature, and dilute t o volume with water. Prepare a reference solution in an identical manner using 25 ml. of water instead of the sample aliquot. Neasure the absorbance of the sample solution us. the reference solution a t 523 mp. Calculate the amount of iron present from a previously prepared calibration curve. DISCUSSION
Table I.
Sample LiOH
KOH
NaOH
a
Determination of Iron in Selected Alkali Samples Sample w-t., grams Iron,a p.p.m. 2,016 5.6 5.7 5.9 6.4 Av. 5 . 9 i 0 . 5 1.4(2) 5.471 1 . 6 (2) 1 . 7 (2) Av. 1 . 6 i 0 . 2 10.140 1.5 1.6( 3 ) 1 . 7 ( 3 ) Av. 1 . 6 =k 0 . 1 a . 090 2.6 (5) 2.7 2 . 9 ( 2 ) Av. 2 . 7 & 0 . 2
The figure in parenthesis is the number of determinations of identical value.
Table II.
wt. of
ISaOH, grams 5.158 6.038
1950
@
Iron in Sodium Hydroxide by Different Methods Phenyl-2-pyridyl 2,6-Pyridine
ketoxime ... ... ... 2.8 2.7 2.6
ANALYTICAL CHEMISTRY
1,lO-Phenanthroline
diamidoxime
3.8 3.8 3.8
3.2 3.2 3.2 2.9 2.9 2.9
...
... ...
The reagent, 2,6-pyridinedianiidoxime, is slightly soluble in water but readily soluble in acidic solution. It forms colored complexes with cobalt, copper, iron, and nickel ions under controlled pH conditions. The complexes of copper and nickel are insoluble in basic solution of pH 10, whereas the cobalt and iron species are soluble. In solutions of higher pH, the cobalt species is insoluble. I n the determination of iron, the three diverse ions precipitate out of solution and can be separated by filtration. In basic solution, the iron(I1) ion fornis an intensely red complex with the reagent. The formation of this species is constant over a hydroxyl ion concentration range of 0.01X to 5 X . Mole ratio studies showed that the single absorbing species is a bis-2,6-pyridine dianiidoximato iron(I1) complex. Ion exchange studies showed that the colored chelate is anionic in nature. It is assumed that the reagent exists in the form of a divalent anion in basic solution and functions as a tridentate ligand in reactions with iron.
The iron(I1) complex exhibits a wavelength of maxinium absorption a t 523 niQ with a molar absorptivity of 16,800 (Figure 1). Solutions of the complex within the optimum pH range conform to Beer‘s law over the concentration range studied, 6.88 X 10;12-6 to 1.24 X 10-4X (0.4 to 7.0 p p n ) iron(I1) ion. The rate of complex formation between iron(I1) and the reagent is relatively slow a t room temperature. ;\faximum color is developed within a few minutes upon heating at 80’ to 90” C. Solutions treated in this manner retain their maximum color for a period of 6 hours. The presence of a reductant in the reaction mixture facilitates the reduction of any iron(II1) to the iron(I1) oxidation state. Sodium dithionite was the most satisfactory reducing agent
for this purpose. The addition of a few milliliters of a freshly prepared solution of the reducing agent is recommended in the analytical procedure.
(inherent in the ketoxime method), it has the added advantage of simplicity. LITERATURE CITED
(1) Hartkamp, H., Naturwissenschaften 45,
RESULTS O N SELECTED SAMPLES
Results of iron determinations on selected alkali samples are summarized in Table I. Yalues obtained from different methods for iron in sodium hydroxide are presented in Table 11. The data in Table I indicate that the method yields reproducible results. The data in Table I1 show that the method gives results which are comparable to those obtained by standard methods. Because the recommended procedure does not involve neutralization and pH control (vital in the phenanthroline method) nor extraction
211 (1953).
(2) Hartkamp, H,, Z. Anal. Chem. 170,
309 (1959). (3) Schilt, A. A, Smith, G. F., Heimbuch, $., ANAL.CHEX28, 809 (1956). (4)Trussell, F., Diehl, H., Ibid., 31, 1973 (1959). MILANW,WEHKING~ RON.ALDT. PFLAUV E. SCOTT TUCKER I11 Department of Chemistry University of Iowa Iowa City, Iowa 1 Present address, Department of Chemistrv, Wisconsin State University, River Falis, Wis. WORK supported by National Science Foundation Grant G 15738.
Direct Determination of Sodium und Potassium in the Presence of Ammonium with Glass Electrodes Automatic Continuous An a1ysis of Urine S ~ R There : is a growing literature on the use of glass electrodes for monitoring sodium and potassium. The singular characteristic of the electrode method, compared to classical procedures such as flame photometry or gravimetric analysis, is that activity is measured in contradistinction to concentration values given by the other techniques. ,ictivity values are, of course, more basic than concentration values, because thermodynamic events (e.g. transport phenomena, complexation, precipitation) are a direct function of activity. Kevertheless. in many instances concentration values have been desired and undoubtedly will continue to be. The simplified technique afforded by the electrode is attractive. Also, the possibility of measuring both activity and concentration in a sample exists. It has been recognized that when employing the electrode, variations in the activity coefficient must be taken into account ( 6 ) . Often such variations have been anticipated by the maintenance of a constant ionic strength (3, 7 ) . However, this technique has not been explicitly utilized to enable a direct measure of concentration. Some previous techniques for obtaining concentration values from electrode nieasuremente necessitated employing empirical activity coefficients (4,or dependence on standards that very closely approximated the unknown in composition (6) It is the purpose of this report t o illustrate a simple and
direct technique for obtaining concentration values when employing the electrodes, and t o apply the technique to the automatic (continuous) analysis of urine, a substance of niuch variability. Furthermore, a means is illustrated that allows the determination of potassium in samples containing ammonium ions with a cationic electrode which has almost equal sensitivity to both. EXPERIMENTAL
The Beckman sodium ion glass electrode (KO. 39046) and cationic glass electrode (KO. 39047) were employed. The electrodes were fitted into two Beckman (No. 14846) sample chambers (ground glass connections) employing a single reference electrode, (,4g, AgCl internal, S o . 39070) that was mounted remotely and fitted with a irY’r to effect connection with both sample chambers. A Becknian electrode switch box (No. 97200) was used to alternately read one or the other electrode. Because it was millivolts that were recorded, the bucking (standardizing) potential of the switch box was defeated by removing the mercury batteries. The Beckman research pH meter, with a readability of 0.05 mv., was employed. For recording, the output of the pH meter was taken to a Sargent SR recorder arranged to give a 100-m~.full scale, thereby preserving a few hundredths of a millivolt readability. Standardization of the electrodes followed, in general, the procedure of Friedman et al. (1). Two sodium concentrations were used to establish the
slope for each electrode and one niixed sodium potassium solution was used to establish the selectivity constant for the cationic electrode. When analyzing human urine, standard sodium chloride solutions of 0.2N and 0.1N were employed and the mixed solution contained 0.1N NaCl and 0.05N KC1. For dog urine, the standards employed were 0.021V NaC1, 0.05N NaCl, and a mixture of 0.0ZX KaC1 and 0.02N KCl. The sodium values were obtained from a calibration curve. The potassium values were obtained by solving the following equation:
AE = S l o g
[(Ka-)
+
1
k~,rc(Ii+) (Xa+)std.
where AE is the difference in millivolts between a sample and a standard containing only sodium, S is the slope, k N a K is the selectivity constant of the cationic electrode for potassium relative to sodium, which is first determined by using standard solutions. At a pH of 8.15 k.\7v,X was about 11 -I 1; a t a pH of 11.55, ICN~Kwas about 7 i 1. L4 precise value was always determined before, during, and after a series of measurements and the value was constant to i 2%. Titrations were performed in beakers using a magnetic stirrer. For continuous automatic analysis, a Technicon pump, sampling table, tubings, and fittings were employed. Considerable electrical noise was encountered traceable to the pump. The noise was eliminated by inserting a short length of platinum wire into the stream just past the de-bubbler YOL. 38, NO. 13, DECEMBER 1966
195’1
