Anal. Chem. 1984, 56, 1417-1422
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Simultaneous Kinetic Determination of Iron and Chromium at the Nanogram Level Soledad Rubio, Agustina Gijmez-Hens, a n d Miguel ValcBrcel* Department of Analytical Chemistry, Faculty of Sciences, University of Cdrdoba, Cdrdoba, Spain
The oxldatlon of pyridoxal 2-pyrldylhydrarone by hydrogen peroxlde Is Induced by Fe(I1). The reaction Is monitored fluorimetrlcaliy and is preceded by an inductlon period, the length of whlch Is inversely proportlonal to the Fe(I1) concentration. This effect allows the quantitative determlnatlon of thls ion. The traditlonal kinetic methods (tangent, fixed the, and variable time) have also been applied and between 5 and 60 ng/mL of Fe( 11) can be determlned. The transient Inhibitory effect of Cr(V1) on the first stage of this oxldatlon Is reported. A noncatalytlc cycle appears to be assoclated wlth them. The conversion of the rate-modifying Ion to an Inactive form by a redox reactlon seems to account for the lack of a catalytic cycle. A judicious choice of reactlon condltlons permits the determination of 50-1000 ng/mL of Cr(V1). The determlnatlon of the Fe( 11)-Cr(V1) mixture (10-70 ng/mL Fe(I1) and 50-350 ng/mL Cr(V1)) is reported on the bask of this transient inhlbltory effect through inltiai rate and length of inductlon period measurements.
This transient inhibitory effect provides a new, simple method for the determination of ion mixtures (catalyst or inductor and inhibitor) by means of the combined measurements of initial rate and length of induction period. The initial rate is not dependent on the Cr(V1) concentration since the effect of this ion only produces an increase in the length of the induction period. Therefore, the slope of the reaction curves is directly related to the Fe(I1) concentration. If the length of the induction period is an additive property for the Fe(I1) and Cr(V1) concentrations, the determination of the mixture is possible. Determinations of Fe(I1) and Cr(V1) mixtures are reported in this paper based on this principle. The main interest of this paper lies in the simultaneous determination of two species by two different kinetic effects. Rodriguez and Pardue (9) have shown a precedent in the determination of two species by the combined use of catalytic and inhibitory effects. They have determined Os(VII1) and I- by measuring rates in the presence and absence of Hg(II), which acts as an inhibitor of I-. They have, however, used more than a single kinetic run for the determinations. Furthermore, this inhibitory effect is not transient as occurs in our system.
The oxidation of organic compounds with hydrogen peroxide in the presence of iron(I1) is known as the Fenton reaction and it constitutes a traditional example of induced chain reaction (1-4). In this paper, the kinetics of the iron(I1)-induced hydrogen peroxide-pyridoxal 2-pyridylhydrazone (PPH) reaction is studied. It is found that the PPH is transformed into a highly fluorescent product by the action of the Fenton reagent (iron(I1)-HzQ2). The appearance of the fluorescence is preceded by an induction period, the length of which is proportional to the iron(I1) concentration. The determination of this cation can be made by initial rate and induction period methods (5). The latter improves the selectivity since the measurements are made before the fluorescent product is formed, so that the action of possible quenching is avoided. The reactions, which are characterized by the presence of induction periods, are grouped as “Landolt reactions” (6)and are generally autocatalytic reactions or free radical reactions. The Cr(V1) ion is capable of inhibiting the early stages of the above oxidation. This ion increases the induction period length of the fluorescence-time curves of the Fe(I1)-PPHHzOzsystem, but it does not affect their slopes, which only depend on the amount of Fe(I1) present. The evaluation of this rate-modifying effect leads to the development of a simple method for the determination of Cr(VI). The transient nature of the rate-modifying effect indicates that Cr(V1) acts as a modifier and is concurrently either destroyed or rendered inactive. When the ion is inactivated the overall rate tends to be the rate of the indicator reaction in the absence of the rate modifier. Some analytical applications of reaction rate-promoting effects have been reported by Dutt and Mottola (7). Only one reaction rate procedure based on a transient inhibitory effect has been described (8),but in this method the inhibitor affects the slopes of the signal-time curves, contrary to what occurs in our method.
EXPERIMENTAL SECTION Apparatus. All fluorimetric measurements were taken on a Perkin-Elmer fluorescencespectrophotometer,Model MPF-43A, fitted with a device for kinetic measurementsand with 1-cm quartz cells. The cell compartment of the spectrofluorimeter was thermostated by circulating water. All measurements were recorded by use of excitation and emission slits of 5 nm spectral band-pass. A set of fluorescencepolymer samples was used daily to adjust the spectrofluorimeter to compensate for changes in source intensity. A Perkin-Elmer 575 spectrophotometer with 1-cm glass cells was also used. Reagents. Pyridoxal 2-pyridylhydrazonewas synthesized by the condensation of pyridoxal hydrochloride with 2-pyridylhydrazine (10). A 6.7 X lo4 M solution of the reagent in ethanol was used. This solution is stable for at least 1 month. A standard iron(I1) solution was prepared by dissolving 4.023 g of (NH4)2Fe(S04)2.6H20 in 1L of 0.1 M sulfuric acid. A standard chromium(VI)solution was made by dissolving 2.830 g of K2Cr207 in 1L of distilled water. All diluted solutions were prepared just prior to use. The buffers used were as follows: acetate buffers, pH 4.00,1.75 M in total acetate; pH 4.20, 0.45 M in total acetate; phosphate buffer, pH 4.20, 0.45 M in total phosphate; tartrate buffer, pH 4.20,0.45 M in total tartrate; phthalate buffer, pH 4.20, 0.45 M in total phthalate. All chemicals used were Analytical Reagent Grade. Procedures. Determination of Fe(1Z). One milliliter of 6.7 X lo4 M PPH solution,2 mL of 1.75 M acetic acid-sodium acetate buffer (pH 4.00). 1mL of 0.9 M hydrogen peroxide, 1 mL of 2.5 M sodium chloride, and the volume of sample needed to give a final iron(I1) concentration between 5 and 60 ng/mL were mixed in this order in a 10-mL standard flask and made up to volume with distilled water. A portion of the reaction mixture was immediately transferred to a 1-cm cell thermostated by a water bath at 64 f 0.1 “C and the intensity of the emitted fluorescence was recorded as a function of time (Aex 325, A,, 390 nm). The measurements were begun exactly 3 min after the addition of the
0003-2700/84/0356-1417$01.50/0
0 1984 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984
"I
Flgure 1. Inductory effect of iron(I1)on the oxidation on PPH by
hydrogen peroxide in acetic acid-sodium acetate buffer. Excitatlon and emission spectra: l,l', PPH 2,2', PPH + H,02; 3,3', PPH H20, + Fe(I1);[PPH] = 9.7 X lom5M; [H202]= 2.6 X M; [Fe(II)] = 100 ng/mL; [H'] = M; [buffer] = 0.1 M; temperature, 25 "C. Graphs were recorded after a reaction time of 1 h.
+
sample. A blank rate with no iron(I1)present was recorded. The reaction rate was calculated from the fluorescence-time curves by using one of the standard procedures later indicated. Determinationof &(VI). A solution waa prepared containing 1.5 mL of 6.7 X lo-* M PPH solution, 2 mL of 0.45 M acetic acid-sodium acetate buffer (pH 4.20), 2 mL of 0.9 M hydrogen peroxide, 25 ng/mL of iron(II),and the volume of sample needed to give a final chromium concentrationof between 50 and 1000 ng/mL. The solution was then diluted to the mark in a 10-mL volumetric flask with distilled water. The intensity of the emitted fluorescence was recorded as a function of time, starting the recording 3 min after the addition of the sample and working at 40 "C (A, 325, A, 390 nm). A blank rate with no chromium(V1) present was recorded. The At was calculated from the fluorescence-time curves as is later indicated.
RESULTS AND DISCUSSION Study of the Iron(I1)-Induced Hydrogen PeroxidePPH Reaction. Diluted aqueous and ethanolic solutions of PPH at acid pH values cause a greenish yellow fluorescence (Aex 420, A, 485 nm). In the presence of hydrogen peroxide, these maxima of fluorescence show a hypsochromic change (Aex 325, A,, 390 nm) which is accelerated by iron(I1) traces. This effect is shown in Figure 1. The mechanism of the oxidation of organic compounds by iron(I1)-Hz02 has been investigated by many authors (1-4). The Fenton reagent is an oxidant that acts through an induced chain mechanism involving hydroxyl radicals. The PPH-H202 reaction in the presence on iron(I1) is an induced reaction (11, 12) where hydrogen peroxide is the actor, PPH is the acceptor, and iron(I1) is the inductor (13). This reaction shows an induction period during which no appearance of fluorescence is observed. The H202-Fe(II)reaction is the primary reaction and H2O2-PPH is the induced reaction. Although iron(I1) is reproduced during the reaction because the H202and PPH concentrations are very large in comparison to the iron(I1) concentration, iron(I1) acts as an inductor and not as a catalyst, since, a t the end of the reaction, the iron(I1) is oxidized to iron(II1). The possibility that iron(I1) may be oxidized to iron(II1) and iron(II1) reduced back to iron(I1) by H202 (14) or PPH is ruled out for the following reasons: (1)the Fe(1II)-H202 and Fe(II1)-PPH reactions are slower than the Fe(II)-H202reaction; (2) iron(II1) accelerates the H202-PPH reaction but not as strongly as iron(I1). The same observations
have been made by Hadjiioannou and Lazarou (15) in the iron(I1)-induced perbromate-iodide reaction. Iron(II1) accelerates the H202-PPH reaction because of the iron(I1) produced in the iron(II1)-Hz02reaction which is much faster than the PPH-H20, reaction. Effect of the Reaction Variables. The system was optimized by altering each variable in turn, the others remaining unchanged. The optimum reagent concentrations taken for the induction period method were those that did not affect the length of the induction period. For the other methods (initial rate, fixed time, variable time) the optimum concentrations taken were those giving a minimal relative standard deviation for the initial rate measurement, under conditions in which the reaction order was zero, or as close to it as possible, with respect to the variable concerned. The length of the induction period was evaluated from the intercept (on the x axis) by the extrapolation of the linear section of the reaction curve. The initial reaction rate was also determined from the tangent. The effect of temperature on the reaction rate was studied in the 20-70 "C range. The rate of the reaction increases when increasing the temperature between 20 and 60 O C . Temperatures higher than 70 "C have not been tested because they are near to the boiling point of the ethanol. A temperature of 64 "C was selected for further studies. The activation energy was calculated to be 21.5 f 0.3 kcal/mol from Arrhenius plots. The influence of ionic strength on the initial rate and induction period was studied and a different behavior, according to the electrolytes used to control it (sodium perchlorate, sodium chloride, and potassium nitrate), was observed. An analogous effect was shown on Fe(II)-H202redox reaction and is due to the equilibria between various forms of iron(I1) present in solution (16) or to the reaction of the electrolytes with the active intermediate (17). An increase in the ethanol concentration decreases the initial rate and augments the length of the induction period. This behavior cannot be only interpreted as a function of the dielectric constant of the medium because it has been shown (17)that the primary reaction between iron(I1) and hydrogen peroxide induces the reaction between hydrogen peroxide and ethanol, with the formation of acetaldehyde. This fact was verified in the oxidation reaction of PPH by means of transformation of the aldehyde in furfural (18). The extent of this induced reaction increases with increasing concentrations of ethanol, when the initial concentrations of iron(I1) and hydrogen peroxide are kept constant. The effect of the reagent concentration on the system was studied in the 1.5 X to 1.2 X M range. When the PPH concentrationincreases, the initial rate and the induction M, respectively. period go up to 6 X M and 9 X The increase in the hydrogen peroxide concentrationcauses an increase in the initial rate and a decrease in the induction period up to a 0.1 M concentration. Greater concentrations do not affect either parameter. There is a drastic reduction of the PPH oxidation rate in the absence of the oxidant which is concordant with the mechanism of induced chain oxidation since the radicals and ions originated by the hydrogen peroxide through endothermic reactions are the real oxidants. The only function af the Fe(1I) ions in this process is to reduce the nature of this reaction. The optimum interval of pH is different for measurements of initial rate (3.3 5 pH 5 4.6) and induction period (3.8 I pH I4.2). A pH of 4.0 was chosen. Various buffers (acetate, phosphate, and tartrate) were tested. The acetic acid-sodium acetate buffer was chosen because it gave the greatest initial rate and the smallest induction period. When the pH is regulated with HC1 and NaOH the reaction occurs very slowly.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984
The tested buffers accelerate the oxidation of P P H because the iron(I1)-iron(II1) electron exchange is accelerated by many anions (19). The different behavior of the reaction rate as a function of the buffer can be explained by the different stability constants of compounds that iron(II1) forms with the above ions (20). The initial slopes indicate a first-order reaction with respect to iron. In the case of our working conditions the kinetic equation (I) is proposed d[PPH],,/dt
= k[Fe2+]
(1)
Characteristics of the Analytical Methods. The fluorescence-time curves, for different amounts of iron(II), obtained with optimum conditions, were analyzed by the four kinetic methods: tangent (10-60 ng of Fe(II)/mL), variable time (5-50 ng of Fe(II)/mL), fixed time (30-60 ng of Fe(II)/mL), and induction period (10-60 ng of Fe(II)/mL). For the variable-time method, the inverse of time necessary to obtain a relative fluorescence intensity of 30% was plotted against the Fe(I1) concentration. For the fixed-time method, measurements were made after 5 min. For the induction period method, the inverse of the length of this period was plotted against the inductor concentration. The precision is better in the tangent (% RSD = *0.28) and induction period (% RSD = f0.28)methods than in the variable-time (% RSD = f0.56) and fixed-time (% RSD = A0.68) methods. The selectivity of methods was determined by studying the effect of foreign ions on the initial rate and induction period. The tolerance limits are summarized in Table I. In the induction period method, only four ions interfere at an equal concentration of Fe(I1): Co(I1) causes a positive interference and Cu(II), Mn(II), and Cr(V1) cause a negative interference. In the initial rate method, Co(I1) and I- interfere positively and Cu(I1) and Mn(II), negatively. It is seen that the induction period is that which presents the highest selectivity. The interferences are produced by ions whose effect is added to that caused by iron(I1) (i.e., Co(II)),ions of high oxidant strength which modify the redox cycle (i.e., Cr(VI)), and ions that form complexes with the reagent (i.e., Cu(I1)). The induction period method is the best of the four methods, giving the highest precision and least number of interferences, and it is therefore recommended.
Study of Transient Inhibitory Effect of Cr(V1) on the Fe(I1)-InducedH202-PPHReaction. Figure 2 shows that the increase in the concentration of chromium(V1)lengthens the induction period of the Fe(I1)-induced H,O,-PPH reaction but does not affect its initial rate. It is possible to determine iron(I1) in the presence of a 100-fold Cr(V1) concentration by the slopes of the fluorescence-time curves. The inhibitory effect of Cr(V1) ion is exercised only temporarily. Once this effect ends, all overall rates become equal to the rate in the absence of this ion. In an attempt to understand this behavior, the chemical reactivity of Cr(V1) ion toward the species in the solution was examined. By use of the experimental procedure described for Cr(VI), the fluorescence-time curves for the following solutions were recorded (1)PPH + Cr(V1); (2) PPH + HzOz + Cr(V1); (3) P P H + HzO2 + Fe(I1) + Cr(V1). The results were compared with the curves obtained for blank solutions with no Cr(V1) present. No measurable reaction between Cr(V1) and PPH took place, at least in the region of analytical interest. When hydrogen peroxide is present, two effects can be observed. For Cr(V1) concentrations less than 350 ng/mL, there is no difference between solution 2 and the blank solution. For greater concentrations of Cr(VI), an inhibition of the blank reaction is observed. These facts indicate that the Cr(V1) ion reacts with HzOz and, therefore, with less oxidant, the oxidation of PPH is slower. The results obtained for solution 3 show that inhibition of the blank reaction is
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984
Table 11. Effect of Various Ions on the Determination of 135 ng/mL of Chromium(V1) tolerance ratio of ion to Cr(V1) Ion added 100
AsOd3-,Br-, NO;, F', BrO;, ClO,', ClO;, C03*-,tartrate, SO,", Na(I), K(I), Ca(II), Sr(II), Ba(I1) Aa
Cd(I1)
75 50 25 10 5 3 1
Bb
Mg(II), SO,*Bi(II1)
Th(IV), IO;, IO;, C,0,2-
Al(III), AsO;, Be(I1) Ce(IV), V(V), Ag(I) Tl(I), Sb(II1) Sn(II),Zn(I1) La(II1) Hg(II),Mo(VI), W(VI), Ni(II), Pd(I1) a A, ions which cause positive interference when the foreign ion/Cr(VI) ratio is higher than those indicated. which cause negative interference when the foreign ion/Cr(VI) ratio is higher than those indicated.
B, ions
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Figure 2. Effect of the Cr(V1) concentration on the fluorescence-time curves of the Fe(1I)-induced H202-PPH reaction. Solutions were prepared according to the procedure described for FHII) determination. [F@II)l = 3o ng/mL. [cr(vI)l: (curve O, (2) 2oo, (319 (4) 8oo, (5) 1000, (6) 2000 ng/mL.
produced in all regions of analytical interest. These facts indicate that the role of Cr(V1) in the reaction is associated with the Fe(I1) ion and HzOz. The reactions of Cr(V1) with Fe(I1) and H20, have been also studied spectrophobmetricdy at 350 nm, in the presence of acetate buffer, and the same results have been obtained. Therefore, only the reaction between Cr(V1)and Fe(I1) is detected for concentrations of Cr(V1) lower than 350 ng/mL under the conditions described in the recommended procedure. For greater concentrations, HzOzcan react with Cr(V1) also, resulting in a diminishing of the inhibitory effect of Cr(V1). Thus, the calibration graph has a lower slope than that for Cr(V1) concentrations lower than 350 ng/mL. Effect of the Reaction Variables. The method for the determination of the Cr(V1) ion involves the measurement of At which is the difference in the length of the induction period under a particular set of conditions (e.g., temperature, acidity, hydrogen peroxide concentration,etc.) in the presence and absence of chromium. The length of the induction period is evaluated in the same way as the method for the Fe(I1) determination. Under controlled experimental conditions, At is directly proportional to the concentration of the inhibitor.
The reaction conditions were chosen so that they did not affect the length of the induction period. The value of At for one constant concentration of Cr(VI), comprising between 50 and 300 ng/mL, is not dependent on Fe(I1) concentration. For greater concentrations of Cr(VI), At decreases when the concentration of the inductor increases. The effect of temperature was studied in the 30-65 "C range. At remains constant between 35 and 45 "C, while an increase in the temperature decreases At in the 45-65 "C interval. A temperature of 40 "C is recommended. The variation of the ionic strength has no influence on the At value. The behavior of the inhibited reaction against the ethanol concentration is analogous at the induced reaction. The value of At increases when the PPH concentrationrises to 1.45 X M and it descends for larger concentrations. A 1.45 X M reagent was selected for further study. The increase in the hydrogen peroxide concentration does not affect the At value up to a 0.2 M concentration of this oxidant. Greater concentrations decrease the At value. The inhibited reaction is highly dependent on the pH of the medium except for the 4.00-4.25 range. Various buffers (phthalate, acetate, tartrate, and phosphate) were tested. The sodium acetate-acetic acid buffer was chosen because the sensitivity of the inhibited reaction is greater in this medium. Characteristics of the Analytical Method. A linear calibration graph is obtained when At (min) is plotted against Cr(V1) concentrations between 50 and 1000 ng/mL. The slopes of the calibration graph change for concentrations of &(VI) greater than 350 ng/mL, resulting in a slope of 0.22 for between 50 and 350 ng/mL and of 0.10 for between 350 and 1000 ng/mL of Cr(V1). This behavior is explained by the considerations previously shown. For two series of 11 measurements made on 135 and 625 ng/mL of Cr(VI), relative standard deviations of 0.8 and 0.7 were obtained. The selectivity of the method was determined by studying the effect of foreign ions on the induction period. The results are summarized in Table 11. The most serious interferences are from Cu(II), Sz-, NOz-, I-, and Co(II), which perturb at an equal concentration with chromium(V1). The first ion causes positive interference and the others cause negative interference. The positive interferences are produced by oxidant ions which act like the Cr(V1) ion or ions which form complexes with the reagent. The negative interferences are caused by reductor ions that react with Cr(V1) or catalyze the decomposition of hydrogen peroxide or ions that induce or catalyze the oxidation reaction of PPH. The proposed method for Cr(V1) shows an extensive range of application overcoming in this way what is the principal disadvantage in the determination of inhibitors. Only two methods are available in the literature for the fluorimetric determination of this ion. The procedure described in this paper is more sensitive than that of the enzymatic inhibition
ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984
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Figure 3. Influence of Fe(1I) concentration on the fluorescence-tlme curves of the H,O,-PPH reaction, induced by Fe(I1) and inhibited by Cr(V1). Solutions were prepared according to the procedure described for Fe(l1)determlnatlon. [Cr(Vl)] = 225 nglmL. [Fe(II)]: (Curve 1) 50, (2) 40, (3) 30, (4) 20, (5) 10, (6) 5 kg/mL.
method (21). It has a wider range of application and is less cumbersome than the ion pair formation method (22).
Simultaneous Determination of Fe(I1) and Cr(V1). The method for an "in situ" analysis of the Fe(I1)-Cr(V1) mixture is based on: (1) the measurement of the slopes of the fluorescenceintensity-time curves, which are directly related to the Fe(I1) concentration; (2) the measurement of the length of the induction period of said curves, to which both concentrations of Fe(I1) and Cr(V1) contribute. Through a previous calibration ( l / t vs. Fe(I1) concentration), the contribution of the Fe(I1) to this induction period is shown. The difference between the total length and the length attributed to the action of the Fe(I1) is directly related to the Cr(V1) concentration. In order to carry out this simultaneous determination, two conditions must be fulfilled (1) the Cr(VI) concentrationmust not have any influence on the slopes of the fluorescence intensity-time curves; (2) the length of the induction period must be an additive property corresponding to the action of both ions. The accomplishmentof the fist requisite is shown in Figure 2, since the slopes do not change for samples containing increasing amounts of Cr(V1). Figure 3 shows the influence of increasing amounts of Fe(I1) in samples containing a fixed concentration of Cr(V1). From these curves it can be easily inferred that the length of the induction period depends on the Fe(I1) and Cr(V1) concentrations. Two different methodologiesare proposed for the "in situ" analysis of the Fe(I1) and Cr(VI) mixture, according to whether the experimental conditions (buffer concentration, temperature, ionic strength, etc.) are optimum for the determination of Fe(I1) alone (method A) or for the determination of Cr(V1) alone (method B). The interval of concentrations that can be determined is restricted by the loss of additivity in the lengths of the induction periods. The analyses of numerous samples containing variable concentrations of both ions prove that the determination of the mixtures is possible in the 10-30 ng of Fe(II)/mL range
Table 111. Resolution of Empirical Mixtures of Fe(I1) and Cr(V1) Fe(I1): Fe(I1) Fe(I1) Cr(V1) taken, found,a taken, Cr(V1) ratio ng/mL ng/mL ng/mL Method A 25 24.9 50 1:2 60 30 30.2 70 35 34.9 75 15 14.8 1:5 100 20 19.9 150 30 30.1 1:lO 100 10 9.8 20.3 200 20 29.9 300 30 150 1:15 10 10.2 200 10 1:20 10.3 Method B 50 1:l 50 50.3 60 60.2 60 70 70 69.7 60 30 30.3 1:2 40 80 39.8 120 60 59.9 150 30 29.7 1:5 300 60 59.8 350 70.7 70 30 240 30.1 1:8 320 40 39.6 3 50 35.1 1:lO 35 a Average of four separate determinations.
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Cr(VI) found,a ng/mL 50 60 69 75 100
149 100 201 301 151 200 49.5 61.2 69.6 61.3 81.2 119.2 149.0 301.2 349.3 241.0 319.5 348.7
(% RSD = f0.26)and the 50-350 ng of Cr(VI)/mL range (% RSD = f0.93)using method A and in the 30-70 ng of Fe(II)/mL range (70RSD = f0.35) and 50-350 ng of Cr(VI)/mL range (% RSD = f0.80) using method B.
The following observations can be made: The interval of Cr(V1) concentrations determined by both methods is the same. For greater concentrations, At decreases when the concentration of the inductor increases for a fixed amount of Cr (VI). Method A is more sensitive for the determination of Fe(I1) than method B, but its range of determination is lower. Table I11 shows the values obtained for different empirical mixtures of Fe(I1) and Cr(V1). The favorable determination of all mixtures indicated can be observed. The selectivity of the simultaneous methods is analogous to that specified for the Cr(V1) and Fe(I1) determinations. The analysis of mixtures of ions based on a combination of a Landolt effect and an initial rate method has the advantage of low detection limits (they are approximately equal to the limits for the separate determination of the species) and has an easy numerical resolution. Registry No. Fe, 7439-89-6;Cr, 7440-47-3;PPH, 87877-49-4.
LITERATURE CITED (1) Haber, F.; Weiss, J. f r o c . R . SOC.London, Ser. A 1934, 147, 332. (2) Barb, W. G.; Baxendale, J. H.; George, P.; Hargrave, K. R. Trans. Faraday SOC. 1951, 4 7 , 462. (3) Barb, W. G.; Baxendale, J. H.; George, P.; Hargrave, K. R. Trans. Faraday Soc. 1955, 51, 935. (4) Medalia, A. I.; Kolthoff, I. M. J . Polym. S d . 1949, 4 , 377. (5) Yatslmirskii, K. E. "Kinetlc Methods of Analysis", 1st ed.; Pergamon: Oxford, 1966; Chapter 3. (6) Svehla, G. Analyst (London) 1969, 9 4 , 513. (7) Eswara Dutt, V. V. S.; Mottola, H. A. Anal. Chem. 1974, 4 6 , 1090. (8) Eswara Dun, V. V. S.; Monola, H. A. Anal. Chem. 1974, 4 6 , 1777. (9) Rodriguez, P. A.; Pardue, H. L. Anal. Chem. 1989, 4 1 , 1376. (IO) Rubio, S.; Gbmez-Hens, A.; ValcBrcel, M. An. Ouim. 1983, 7 9 , 72. (1 1) Koithoff, 1, M.; Stenger, V. A. "Volumetric Analysis"; Interscience: New York, 1942; Vol. 1. (12) Medalia, A. I. Anal. Chem. 1955, 2 7 , 1678. (13) Kolthoff, I . M.; Elving, P. J. "Treatise on Analytical Chemlstry", 2nd ed.; Wiley: New York, 1979; Part I , Vol. 2, p 701. (14) Walling, C.; Goosen, A. J . Am. Chem. SOC. 1973, 95, 2987.
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(15) Lazarou, L. A.; Hadjlioannou, T. P. Anal. Chem. 1979, 5 1 , 790. (16) Mark, H. B.; Rechnitz, G. A. "Kinetic in Analytical Chemlstry"; Interscience: New York, 1968. (17) Kolthoff, I. M.; Medalia, A. I. J . Am. Chem. SOC.1947, 7 1 , 3777. (18) Feial. F. "SDOt Tests in Oraanic Analvsis": Elsevier: New York. 1956.
(21) Guilbault, 0.G.; Brignac, P.; Zinner, M. Anal. Chem. 1968, 4 0 , 190. (22) Pilipenko, A. T.; Shevhenko, T. L.; Volkova, A. I . Zh. Anal. Khim. 1977, 32, 731.
Color Changes in Screened Indicators Elisabeth Bosch, Enric Casassas, Alvaro Izquierdo,* and Marti Roses Departament de Qdmica Analhica, Universitat de Barcelona, Barcelona, Spain
Several eff lcient screened lndlcators are prepared by use of complementary trlstlmulus data of different acld-base Indlcators and screenlng dyes. The results show that color changes from one pure acld-base lndlcator and dlfferent dyes, when represented In the complementary chromatlclty dlagram, are on the same chromatic straight ilne. For a varlety of neutrallratlon indlcators the equatlon deflnlng thls line from the color parameters of the indlcator is developed theoretlcally and compared with the experlmentai equatlon. An expression Is developed defining the best screened lndlcator that can be prepared from a given pure acid-base indlcator and dyes in order to obtaln the optlmum color change. Thls optimum color change always occurs between two compiementary colors with the same relative grayness.
The chromaticity system CIE (1,2),which specifies each color by means of three coordinates X , Y, and 2 (R) was developed for characterization of additive colors and it does not allow the treatment, in a simple way, of subtractive colors such as the ones encountered in indicator solution. Reilley et al. ( 3 , 4 )proposed use of absorbance instead of transmittance in the computation of the chromaticity coordinates, designated X,, Y,, and 2, (R,) in this case, thus introducing the complementary tristimulus colorimetry which renders the subtractive color coordinates additive. It is difficult to plot a color point in a X-Y-2 or Xc-Yc-Zc three-dimensional space; therefore, x , y, z (1) or Q,, Qy, Q, (QJ coordinates are usually used ( I - 3 , 5 ) . These coordinates represent the color vector direction and they can be plotted in a x-y or Q,-Q, bidimensional chromatic diagram. The Q, coordinates are constants for each colored species and are independent of the concentration and path length. Since R, coordinates are additive for subtractive color systems, it is possible to calculate easily the coordinates of a mixture of two colored species, like both forms of an acidbase indicator, at any pH value within the color change range (8) and thus the pK, of this indicator ( 4 , 9 ) . To specify more accurately a color, Reilley and co-workers (3) introduced color parameters such as the color concentration, J , which depends on the experimental conditions concentration and path length, and the relative grayness, g, which measures the dirtiness of a color. Complementary tristimulus colorimetry finds an important application in the screening of indicators, since it allows calculation of the concentration ratio that the pure indicator and the screening dye have to hold in order that the screened indicator shows a gray color a t a certain pH value (3,9). The present work studies different screened indicators and it shows 0003-2700/84/0356-1422$01.50/0
that all the screened indicators which may be prepared from the same pure indicator and different screening dyes show color changes that, when plotted in the complementary chromaticity diagram, are on the same straight line. Parameters for this line can be calculated from the color parameters of the pure indicator alone. As a consequence the color changes of all the possible screened indicators to be prepared from this pure indicator can be calculated easily. In this paper equations are proposed to calculate the optimum concentration ratio for a screened indicator prepared from a given pure indicator and different screening dyes in order to obtain the best color change, occurring between two complementary colors with the same relative grayness. For this study the indicators chosen are several semicarbazones and thiosemicarbazones from 1,2-naphthoquinone derivatives, i.e., 1,2-naphthoquinone-2-semicarbazone (NQS), 1,2-naphthoquinone-2-semicarbazone-4-sulfonic acid (NQS4S), 1,2-naphthoquinone-2-thiosemicarbazone-4-sulfonic acid (NQT4S) (studied and described as indicators in a previous paper (IO)), and 1,2-naphthoquinone-2-thiosemicarbazone (NQT), whose characteristics are given here. These compounds have been selected for the following reasons; (a) their color transitions (from yellow shades to reds) take place between noncomplementary colors and this fact makes them specially suited for preparation of screened indicators with dyes; (b) their color change pH ranges are in basic media where only a few two-color indicators are described (11).
EXPERIMENTAL SECTION Apparatus. For complementary chromaticity coordinated determination an Acta M-VI1Beckman spectrophotometer with 10-mmcells, to record spectra, and a Rockwell AIM-65 20 K RAM microcomputer were used. The pH of solutions was measured with a Radiometer pH meter (Model PHM64) with a glass/ calomel combined electrode, GK 2401 B. Chemicals. NQS, NQSIS, and NQT4S were synthesized and purified as reported in a previous paper (IO);NQT was prepared according to Luque et al. (12). The characteristics of NQT are the following: mp, 182 OC; IR (KBr) 3420,3270, and 3150 cm-l (NH), 1505,1425,1310,and 965 cm-l (NCS) agree with literature data (13-15). Anal. Calcd for CllN3H90S: 57.1, C, 18.1 N, 3.9 H, 13.8 S. Found: 57.0 C, 16.3 H, 3.9 H, 13.8 S. NQS, NQS4S, and NQT4S were used in aqueous solutions and NQT was used in ethanol/water 1:4 (v/v) solution. The commercial indicators studied were Methyl Orange, Bromocresol Green, Methyl Red, and Phenol Red from Eastman-Kodak (ACS); and the dyes, reported in Table 11, where supplied by Sandoz, except picric acid, Doesder (ACS), and Methylene Blue and Indigo Carmine, Scharlau. Buffer solutions used covered the pH range from 3.3 to 11.8, and they were prepared at the constant ionic strength I = 1 M (16). 0 1984 American Chemical Society
