1723 ANALYST, DECEMBER 1987, VOL. 112 Comparative Study of Metallochromic Indicators for Magnesium Juan Cacho,” Angel Lopez-Molinero and J. Enrique Castells Department of Analytical Chemistry, Science Faculty, Zaragoza University, Zaragoza, Spain The performances of common metallochromic indicators for the complexometric titration of Mg were evaluated by comparing the results obtained in titration with EDTA, using the following indicators: Methylthymol Blue, Eriochrome Black T, Metaiphthalein, Thymolphthalein Complexone, Pyrocatechoi Violet, Eriochrome Blue - Black B, Calmagite, Antipyryl-p-Arsenazo I and Antipyryl-o-Arsenazo I. Tristimulus colorimetry theory was applied to the chromatic transitions of the indicators. As a result, Eriochrome Black T and Antipyryl-o-Arsenazo I are proposed as being the most suitable indicators for the complexometric titration of Mg with EDTA.Keywords: Magnesium complexometric titration; tristimulus colorimetry; metallochromic indicators In recent years many new analytical reagents have been recommended as metallochromic indicators for metals in EDTA titrations. Many are now commercially available and it is the task of the analyst to choose the most suitable for each specific titration. This choice should be as objective as possible, so information on the reagents and their complexes is required.1.2 However, very few comparative studies of the performance of these indicators have been made. Work has been carried out on developing methods for evaluating the quality of chromatic transitions3-9 and on comparing them.10-12 These studies have been performed using tristimulus colorimetry, and the quality of the chromatic transitions has been evaluated qualitatively from their positions on the chromaticity diagram, the specific colour discrimination (SCD)6 or the chromatic differences (AE).7 This last method is the best for evaluating the “colour change” in a titration.13 The colour change (AE) can be evaluated by two procedures: (i) the CIE-UCS method14 and (ii) the more modern CIE-LAB method.15 The disadvantage of these methods is that the values of AE depend on the indicator concentration and, although Kotrlf and VytPasl6 established a method for calculating the optimum concentration, it does not give conclusive results. This paper presents the results obtained in evaluating the chromatic transitions of nine indicators recommended for complexometric titration of Mg with EDTA.The procedure involved studying the colour change (AE) using the CIE-UCS and CIE-LAB methods. Whereas the methods of comparison are valid for any indicator, the conclusions drawn in this work are strictly valid for the indicators available at the time the work was carried out. Different results may be obtained with different batches of indicators, but we believe that these differences would not be significant as the purities of the same indicators from different well known manufacturers are similar. In this work commercial indicators were considered as most laboratories routinely use these reagents without any previous purification. Therefore, as we worked under similar condi- tions, the results obtained can be applied generally.Experimental Apparatus A Perkin-Elmer Lambda 5 UV - visible spectrophotometer with quartz cells (1 cm optical path) was used, connected to an SBC-DUET 16 microcomputer (operating system MS-DOS) via an RS-232 C series interface. A Digital VAX 111780 computer (operating system VMS) was employed. pH was measured with a Radiometer Model H16 pH meter with a combined glass - Ag - AgCl electrode. * To whom correspondence should be addressed. Reagents The nine reagents used are listed below. Methylthymol Blue17-19 (Merck). This is the sodium salt of 3,3’-bis(dicarboxymethyl)aminomethylthymolsulphon- phthalein acid. It has six pK, values in the pH range 1.8-12.9. It is used in the complexometric titration of various metal ions, including Mg.20 When titrating at pH 10, it has a transition pM Antipyryl-o-Arsenazo I (AoA I ) .21 This is 3-(2-arsono- pheny1azo)-6- (4-antipyrylazo)-4,5-dihydroxynaphthalene- 2,7-disulphonic acid, with pK,, = 2.05, pKa2 = 3.95, pK,, = 6.90, rKa4.= 9.70 and pK,, = 12.60. Antipyrzl-p-Arsenazo I (ApA I ) .22This is 3-(4-arsonophenyl- azo)-6-( antipyrylazo)-4,5-dihydroxynaphthalene-2,7-disulph- onic acid, with pK, values of 2.15, 5 . 5 , 7.9, 10.20 and 12.45. Calmagite23-27 (Fluka). This is the sodium salt of 3-hydroxy- 4-(2-hydroxy-s-methylphenylazo)naphthalene-l-sulphonic acid, with pK,, = 8.14 and pKa2 = 12.35. It has a pM,,,,,, of 5.7. Metalphthalein2f3-31 (Sigma). This is xylenylphthaleinbis- iminodiacetic acid, with pKal = 2.2, pKa2 = 2.9, pKa3 = 7.0, pK,, = 7.8, pKas = 11.4 and pK,, = 12.0.It is used as a metallochromic indicator in titrations of alkaline earth metals. With Mg it has a PM,,,,,. of 4.7 at pH 10. Eriochrorne Blue - Black B32 (Sigma). This is 1-(1-dihy- droxy-2-naphthylazo)-2-naphthol-4-sulphonic acid, with pKal = 6.2 and pKa2 = 12.5. It has a pMtr,,,, of 4.9 at pH 10. Eriochrome Black 732-36 (Merck). This is 3-hydroxy-4-( 1- hydroxy-2-naphthylazo)-7-nitronaphthalene- 1-sulphonic acid , with pK,, = 3.85, pKa2 = 6.35 and pKa3 = 11.5. It is widely used as a metallochromic indicator in complexometry . With Mg it has a pM,,,,,, of 5.4 at pH 10. Thymolphthalein Complexone28~37-39 (Sigma). This is the sodium salt of 3,3’-bis[ di(carboxymethyl)aminomethyl] thy- molphthalein, with pKal = 7.0, pKa2 = 8.7, pKa3 = 10.8 and pK,, = 13.0.Pyrocatechol Violet4w4 (Merck) . This is 3,4-dihydroxy- 4-oxocyclohexane-2,5-dienylidene)diphenylmethane-2-sulph- onic acid, with pK,, = 7.82, pKa2 = 9.80 and pKa3 = 11.73. (PMtrans.) of 5.2. Reagent Solutions Indicator standard solutions, 2.5 X 10-4 mol 1-1. Mg standard solution, 9.00 x 10-3 mol 1-1. Prepared by weighing 0.1094 g of metal (Carlo Erba), dissolving it in a few millilitres of HCl (1 + 1) and diluting to 500 ml with distilled water in a calibrated flask. Na2EDTA solution (Panreac). Prepared by weighing 1.1167 g of Na2EDTA purified by us45 and dissolving it in, and diluting to 1 1 with, distilled water in a calibrated flask.1724 ANALYST, DECEMBER 1987, VOL. 112 Buffer solution, pH 10.Ammonium chloride (70 g) - ammonia (40 ml) diluted to 1 1 with distilled water in a calibrated flask. Photometric Titration Solutions with constant concentrations of indicator and Mg (3 X 10-5 and 1.08 x 10-3 moll-1, respectively) are prepared in 25-ml calibrated flasks. The pH is adjusted to 10 by adding 2 ml of the buffer solution. By adding various amounts of EDTA solution, the ligand to metal concentration ratio (CLICM) is varied from 0.0 to past the titration end-point. The molecular absorption spectra of these solutions are measured in the range 380-770 nm and are stored on disk in the DUET microcomputer, which is connected to the spectro- photometer. The absorbances and transmittances of spectra stored on disk are obtained at 30 different wavelengths.Evaluation of Photometric Titration Curves Using the absorbance and transmittance values of the sol- utions, the tristimulus values X, Y and 2 are calculated for each coloufi of the titration. Then the coordinates on the CIE-UCS homogeneous colour diagram are calculated using the expressions W* = 23Y1’3 - 17 U* = 13W*(U - Uo) V* = 13W*(V - Vo) where U = 4X/(X + 15Y + 32), V = 6Y/(X + 15Y + 32) and Uo and Vo are the co-ordinates of the illuminant C, with values of 0.200 and 0.3084, respectively. The chromatic co-ordinates are also calculated by the CIE-LAB method, using the expressions L* = 11.6(Y/Yn)1/3 - 1.6 a* = 500[ (X/Xn)’/3 - ( Y/Y,)”3] b* = 200[Y/Yn)1/3 - (Z/Z,)1/3] where X,, Y, and Z,, the tristimulus values of the achromatic point, are 0.98, 1.00 and 1.18, respectively.The colour change, or chromatic difference (AE), is calculated during the titration as the chromatic difference between the initial point and the point in question. This is calculated by the two methods mentioned above, using the expressions UCS: AE = [(AU*)2 + (AV*)2 + (AW*)2]1/2 LAB: AE = [(AL*)2 + (Aa*)2 + (A6*)2]1/2 The relationship between the colour change (AE) and the ratio CL/CM gives typical sigmoid curves. The equations of these curves have been found using the spline cubic interpola- tion method47 with a computer program, after first “smooth- ing” the experimental data.48 The derivative of the curve is then found mathematically, and finally the curve is drawn. Visual Titration A 10-ml volume of 0.01 M magnesium solution is transferred into an Erlenmeyer flask, buffer and indicator solution are added and, after dilution to approximately 50 ml, the solution is titrated with 5.2 X 10-3 M EDTA solution.Results and Discussion The colour of each indicator solution throughout the titration was characterised by determining its tristimulus values (X, Y, Z), dichromatism (D), real and complementary colour co- ordinates (P and Q, respectively), optical concentration ( J ) , co-ordinates in the CIE-UCS and CIE-LAB systems and the chromatic differences (AE) in the two systems. The results are I 520 0.8 0.6 z 0.4 0.2 0 0.2 0.4 0.6 0.8 X Fig. 1. Chromatic transitions of the indicators in the complexometric titration of magnesium. - , Eriochrome Black T; - - . - -, Antipyryl-o-Arsenazo I; - - - - - , Methylthymol Blue; - - - - -.Metalphthalein; oo00, Calmagite; A A A A, Eriochrome Blue - Black B; 0000, Antipyryl-p-Arsenazo I; A A A A , Thymol- phthalein Complexone; + + + , Pyrocatechol Violet 0.7 0.8 0.9 1 .o CL/ CM Fig. 2. Evolution of the colour change of indicators during the titration in the CIE-UCS system. (A) Eriochrome Black T; (B) Antipyryl-o-Arsenazo I; (C) Methylthymol Blue; (D) Metalphthalein given in Table 1. In the proximity of the end-point the results were obtained in triplicate, but only one value is given in order to avoid a very large table. Fig. 1 shows the chromatic transitions of the indicators studied, using the CIE-1931 diagram. The start of the transitions is indicated by a black point. Figs. 2 and 3 show the variation of the colour difference with respect to the CL/CM ratio [d(AlZ)ld(C~/C~)l throughout the titration, as expressed in CIE-UCS and CIE-LAB co-ordi- nates, for the four best indicators. The peak heights and widths at half-height for both systems are presented in Table 2.The peak height represents the intensity of the variation of the colour change and the peak width represents the titration range in which the greatest variation of the colour change is produced.Table 1. Chromatic parameters, CIE-UCS and CIE-LAB chromatic Indicator CLICM X Y Z J MethylthymolBlue , . . . O.OO0 0.4271 0.4387 1.0278 0.6505 0.556 0.4293 0.4414 1.0292 0.6449 0.778 0.4292 0.4411 1.0214 0.6458 0.889 0.4342 0.4443 1.0120 0.6371 0.922 0.4376 0.4475 1.0025 0.6317 0.944 0.4443 0.4530 0.9875 0.6212 0.956 0.4531 0.4598 0.9681 0.6092 0.967 0.4644 0.4708 0.9482 0.5933 0.972 0.4852 0.4901 0.9100 0.5695 0.978 0.4969 0.5032 0.8950 0.5550 0.983 0.5192 0.5259 0.8599 0.5341 0.989 0.5288 0.5356 0.8526 0.5235 0.994 0.5480 0.5564 0.8293 0.5066 1.OOO 0.5568 0.5648 0.8157 0.5015 1.011 0.5641 0.5723 0.8093 0.4952 1.022 0.5672 0.5754 0.8058 0.4930 1.056 0.5708 0.5795 0.8032 0.4896 .. O.OO0 0.3175 0.2706 0.6555 1.0327 0.556 0.3226 0.2715 0.6572 1.0261 0.778 0.3237 0.2716 0.6554 1.0257 0.889 0.3293 0.2737 0.6500 1.0186 0.922 0.3396 0.2773 0.6396 1.0072 0.944 0.3447 0.2789 0.6368 1.0015 0.956 0.3558 0.2832 0.6261 0.9910 0.967 0.3704 0.2880 0.6113 0.9814 0.978 0.3912 0.2968 0.5970 0.9648 0.983 0.4104 0.3043 0.5847 0.9532 0.989 0.4294 0.3131 0.5752 0.9402 0.994 0.4418 0.3187 0.5668 0.9344 1.OOO 0.4459 0.3205 0.5664 0.9312 1.011 0.4495 0.3219 0.5640 0.9306 1.028 0.4512 0.3229 0.5632 0.9291 .. 0.O00 0.2079 0.2225 0.5624 1.3294 0.556 0.2235 0.2218 0.5480 1.2911 0.778 0.2470 0.2253 0.5374 1.2420 0.833 0.2614 0.2287 0.5313 1.2173 0.889 0.2845 0.2358 0.5253 1.1817 0.917 0.2984 0.2393 0.5203 1.1666 0.944 0.3198 0.2477 0.5169 1.1391 0.956 0.3296 0.2514 0.5141 1.1292 0.967 0.3390 0.2542 0.5090 1.1235 0.978 0.3535 0.2603 0.5084 1.1081 0.983 0.3583 0.2613 0.5058 1.1068 0.989 0.3725 0.2678 0.5037 1.0931 0.994 0.3775 0.2694 0.5019 1.0909 1.OOO 0.3969 0.2779 0.5029 1.0716 1.028 0.3969 0.2779 0.5029 1.0716 Calmagite . . . . . . . . O.OO0 0.7643 0.6505 0.8339 0.3590 0.556 0.7526 0.6409 0.8405 0.3649 0.778 0.7410 0.6378 0.8521 0.3638 0.889 0.7222 0.6321 0.8616 0.3677 Antipyryl-o-Arsenazo I Antipyryl-p-Arsenazo I coordinates for the colour change of metallochromic indicators for Mg D P.x Py Qx Qy 0.189 0.226 0.232 0.473 0.447 0.188 0.226 0.232 0.473 0.447 0.186 0.227 0.233 0.470 0.445 0.180 0.230 0.235 0.465 0.444 0.177 0.232 0.237 0.461 0.441 0.172 0.236 0.240 0.454 0.438 0.163 0.241 0.244 0.445 0.433 0.154 0.247 0.250 0.435 0.426 0.138 0.257 0.260 0.415 0.413 0.132 0.262 0.266 0.407 0.406 0.119 0.273 0.276 0.388 0.390 0.116 0.276 0.279 0.382 0.385 0.110 0.283 0.288 0.367 0.371 0.109 0.287 0.292 0.359 0.364 0.108 0.290 0.294 0.354 0.360 0.110 0.291 0.295 0.352 0.358 0.109 0.292 0.297 0.350 0.356 0.008 0.255 0.218 0.368 0.419 0.007 0.258 0.217 0.366 0.420 0.006 0.259 0.217 0.365 0.420 0.005 0.263 0.218 0.361 0.420 0.003 0.270 0.221 0.353 0.418 0.002 0.273 0.221 0.350 0.419 0.003 0.281 0.224 0.342 0.417 0.007 0.292 0.227 0.332 0.416 0.019 0.304 0.231 0.319 0.415 0.036 0.316 0.234 0.308 0.414 0.058 0.326 0.238 0.298 0.413 0.076 0.333 0.240 0.291 0.412 0.080 0.335 0.240 0.289 0.412 0.086 0.337 0.241 0.287 0.412 0.088 0.337 0.241 0.286 0.412 0.050 0.209 0.224 0.401 0.390 0.040 0.225 0.223 0.386 0.392 0.024 0.245 0.223 0.368 0.394 0.016 0.256 0.224 0.359 0.396 0.009 0.272 0.226 0.345 0.398 0.009 0.282 0.226 0.337 0.399 0.017 0.295 0.228 0.327 0.401 0.023 0.301 0.230 0.322 0.401 0.032 0.308 0.231 0.316 0.402 0.047 0.315 0.232 0.311 0.403 0.053 0.318 0.232 0.308 0.403 0.070 0.326 0.234 0.302 0.404 0.077 0.329 0.234 0.300 0.404 0.099 0.337 0.236 0.293 0.406 0.099 0.337 0.236 0.293 0.406 0.230 0.340 0.289 0.230 0.402 0.202 0.337 0.287 0.239 0.408 0.160 0.332 0.286 0.251 0.409 0.100 0.326 0.285 0.268 0.408 u v 0.169 0.261 0.169 0.261 0.170 0.262 0.171 0.263 0.172 0.264 0.174 0.266 0.177 0.269 0.179 0.272 0.184 0.278 0.186 0.282 0.189 0.287 0.190 0.289 0.193 0.293 0.194 0.295 0.195 0.297 0.195 0.297 0.196 0.298 0.200 0.256 0.203 0.256 0.203 0.256 0.206 0.257 0.212 0.259 0.214 0.260 0.220 0.262 0.227 0.265 0.236 0.268 0.244 0.271 0.251 0.274 0.255 0.276 0.257 0.277 0.258 0.277 0.258 0.277 0.159 0.255 0.172 0.256 0.189 0.258 0.198 0.260 0.211 0.262 0.219 0.264 0.229 0.266 0.234 02.67 0.239 0.269 0.245 0.270 0.247 0.271 0.253 0.272 0.255 0.273 0.261 0.275 0.261 0.275 0.235 0.300 0.234 0.298 0.230 0.297 0.226 0.297 W* 71.172 71.352 71.332 71.545 71.757 72.120 72.563 73.272 74.489 75.358 76.664 77.166 78.441 78.919 79.342 79.515 79.744 58.056 58.139 58.148 58.341 58.670 58.815 59.203 59.631 60.404 61.050 61.795 62.262 62.411 62.527 62.609 53.315 53.242 53.609 53.962 54.689 55.042 55.876 56.237 56.508 57.091 57.186 57.796 57.944 58.724 58.724 83.544 83.047 82.885 82.587 U* - 29.238 - 29.233 -28.827 - 27.464 -26.657 - 25.028 - 22.787 -20.751 - 16.683 - 14.987 -11.844 - 10.860 -8.478 -7.001 -6.176 -5.764 -5.496 -0.513 1.345 1.935 4.097 8.205 10.128 14.360 20.302 27.459 34.174 39.998 44.013 45.141 46.383 46.807 -29.092 -19.930 -8.575 -2.166 7.069 12.993 20.412 23.907 27.822 32.360 34.500 38.782 40.667 46.165 46.165 36.758 35.291 31.792 26.830 V* - 42.981 - 42.727 - 42.209 - 41.169 -40.081 - 38.324 - 36.096 -33.267 -28.093 -25.136 - 20.049 - 18.462 - 14.268 - 12.314 -11.018 -10.419 -9.776 - 38.742 - 38,877 - 38.726 -37.994 -36.653 - 36.241 -34.759 -32.903 - 30.525 -28.545 -26.600 -25.155 - 24.91 9 - 24.547 - 24.341 -36.158 - 35.372 - 34.329 - 33.461 -32.117 -31.336 - 29.957 -29.267 - 28.452 -27.640 -27.332 - 26.289 -25.953 -25.033 - 25.033 -8.266 -9.630 - 10.587 - 11.527 AE 0.312 0.889 2.563 3,926 6.348 9.537 13.068 19.755 23.217 29.302 31.223 36.169 38.664 40,252 40.999 41.709 1.864 2.450 4.679 8.986 10.957 15.440 21.676 29.248 36.278 42.456 46.742 47.899 49.201 49.671 9.195 20.600 27.069 36.411 42.395 49.956 53.525 57.522 62.154 64.318 68.734 70.653 76.267 76.267 2.064 5.521 10.493 L* 72.142 72.322 72.302 72.515 72.727 73.089 73.533 74.241 75.458 76.326 77.632 78.134 79.408 79.886 80.309 80.482 80.711 59.030 59.113 59.122 59.315 58.644 59.789 60.177 60.605 61.377 62.023 62.768 63.235 63.384 63.500 63.582 54.291 54.217 54.585 54.938 55.665 56.018 56.851 57.212 57.482 58.066 58.160 58.770 58.919 59.699 59.699 84.509 84.013 83.851 83.553 a* -0.886 - 1.014 -0.957 -0.407 -0.328 0.051 0.659 0.793 1.283 1.092 0.939 0.895 0.629 0.762 0.743 0.755 0.649 19.956 21.427 21.779 22.928 25.097 26.220 28.298 31.266 34.580 37.720 40.194 41.803 42.343 42.878 43.009 -4.817 2.780 11.547 16.045 22.127 25.911 39.176 32.101 34.208 36.628 37.823 39.856 40.828 43.595 43.595 26.945 26.726 25.058 22.460 b* - 38.970 -38.745 - 38.296 -37.342 - 36.380 - 34.807 -32.800 -30.288 - 25.660 -23.033 - 18.482 - 17.050 -13.259 -11.459 -10.266 -9.713 -9.129 -34.992 - 34.991 - 34.825 -34.039 -32.953 -32.105 - 30.520 -28.496 - 25.903 - 23.686 -21.541 -19.966 - 19.672 - 19.252 -19.036 -34.980 - 33.763 - 32.125 - 30.932 -29.100 -28.006 - 26.237 -25.370 - 24.370 -23.306 - 22.884 -21.624 -21.188 - 19.944 - 19.944 -4.793 -6.118 -7.214 -8.393 0.315 1.737 M 2.712 4.371 M 0.696 c] 6.510 @ 9.088 5 13.887 -4 16.595 < 21.289 0 22.794 r 26.760 28.628 29.887 30.467 31.085 1.473 1.833 3.134 5.706 6.940 9.535 13.138 17.378 21.269 24.587 26.847 27.474 28.163 28.403 7.694 16.613 21.261 27.612 31.557 36.159 38.259 40.567 43.223 44.491 46.841 47.907 50.980 50.980 1.432 ~ 3.139 2 5.830Table l-continued Indicator CJCM X Y Z J D P, Py Q, Qy 0.922 0.7090 0.6241 0.8714 0.3721 0.944 0.6970 0.6209 0.8802 0.3737 0.054 0.322 0.283 0.279 0.412 0.017 0.317 0.282 0.291 0.412 0.956 0.6902 0.6200 0.8865 0.3736 -0.003 0.314 0.282 0.298 0.412 0.967 0.6770 0.6156 0.8960 0.3768 -0.027 0.309 0.281 0.311 0.413 0.978 0.6579 0.6115 0.9057 0.3826 -0.034 0.302 0.281 0.329 0.410 0.983 0.6473 0.6115 0.9196 0.3815 -0.025 0.297 0.281 0.343 0.410 0.989 0.6329 0.6085 0.9280 0.3867 -0.003 0.292 0.280 0.357 0.409 0.994 0.6290 0.6090 0.9337 0.3863 0.009 0.290 0.280 0.363 0.409 1.O00 0.6066 0.6072 0.9473 0.3946 0.049 0.281 0.281 0.386 0.404 1.011 0.5979 0.6064 0.9613 0.3956 0.066 0.276 0.280 0.399 0.406 1.033 0.5988 0.6091 0.9659 0.3923 0.068 0.275 0.280 0.402 0.407 1.056 0.5960 0.6088 0.9676 0.3936 0.072 0.274 0.280 0.404 0.406 EriochromeBlackT .. . . 0.O00 0.6187 0.4790 0.7339 0.5903 0.179 0.338 0.262 0.271 0.425 0.549 0.5962 0.4567 0.7185 0.6276 0.158 0.337 0.258 0.275 0.426 0.769 0.5993 0.4656 0.7291 0.6103 0.147 0.334 0.260 0.277 0.424 0.824 0.5953 0.4634 0.7299 0.6127 0.139 0.333 0.259 0.279 0.424 0.879 0.5891 0.4606 0.7301 0.6163 0.126 0.331 0.259 0.282 0.424 0.907 0.5896 0.4680 0.7379 0.6033 0.113 0.328 0.261 0.284 0.421 0.934 0.5816 0.4695 0.7387 0.6020 0.093 0.325 0.262 0.289 0.417 0.945 0.5793 0.4739 0.7422 0.5958 0.081 0.323 0.264 0.292 0.414 0.956 0.5650 0.4685 0.7484 0.6008 0.046 0317 0.263 0.300 0.414 0.967 0.5453 0.4614 0.7599 0.6075 0.005 0.309 0.261 0.313 0.414 0.973 0.5462 0.4721 0.7651 0.5940 -0.005 0.306 0.265 0.317 0.409 0.978 0.5332 0.4708 0.7740 0.5957 -0.021 0.300 0.265 0.327 0.407 0.984 0.5112 0.4818 0.8017 0.5846 -0.008 0.285 0.268 0.353 0.399 0.989 0.4998 0.4920 0.8190 0.5775 0.028 0.276 0.272 0.370 0.392 0.994 0.4903 0.5014 0.8334 0.5734 0.064 0.269 0.275 0.386 0.387 1.OOO 0.4853 0.5138 0.8483 0.5655 0.089 0.263 0.278 0.400 0.381 1.011 0.4811 0.5226 0.8617 0.5606 0.099 0.258 0.280 0.411 0.379 1.022 0.4803 0.5267 0.8686 0.5575 0.104 0.256 0.281 0.416 0.378 1.044 0.4777 0.5269 0.8702 0.5593 0.105 0.255 0.281 0.419 0.377 Metalphthalein .. . . . . O.OO0 0.6762 0.5361 1.1163 0.4186 0.079 0.290 0.230 0.377 0.572 0.556 0.6791 0.5409 1.1165 0.4112 0.079 0.291 0.232 0.377 0.571 0.778 0.6829 0.5471 1.1162 0.4014 0.077 0.291 0.233 0.376 0.570 0.889 0.6951 0.5667 1.1163 0.3734 0.079 0.292 0.238 0.375 0.568 0.922 0.7105 0.5911 1.1173 0.3408 0.080 0.294 0.244 0.374 0.565 0.944 0.7372 0.6322 1.1225 0.2914 0.086 0.296 0.254 0.372 0.562 0.956 0.7661 0.6765 1.1267 0.2443 0.089 0.298 0.263 0.370 0.557 0.967 0.8038 0.7362 1.1310 0.1896 0.094 0.301 0.276 0.366 0.547 0.978 0.8440 0.7986 1.1363 0.1393 0.099 0.304 0.287 0.361 0.534 0.983 0.8765 0.8495 1.1417 0.1025 0.120 0.306 0.296 0.358 0.518 0.989 0.8884 0.8681 1.1430 0.0900 0.122 0.306 0.299 0.355 0.509 0.994 0.9007 0.8866 1.1470 0.0771 0.132 0.307 0.302 0.354 0.503 1.000 0.9092 0.8999 1.1475 0.0688 0.137 0.308 0.304 0.350 0.492 1.011 0.9152 0.9100 1.1472 0.0630 0.135 0.308 0.306 0.347 0.480 1.022 0.9166 0.9123 1.1475 0.0616 0.147 0.308 0.306 0.347 0.478 1.056 0.9216 0.9211 1.1477 0.0566 0.157 0.308 0.308 0.346 0.464 EriochromeBlue-BlackB .. 0.000 0.7579 0.6802 0.9486 0.2911 0.095 0.318 0.285 0.287 0.430 0.556 0.7529 0.6777 0.9549 0.2915 0.066 0.316 0.284 0.292 0.433 0.778 0.7534 0.6829 0.9617 0.2854 0.047 0.314 0.285 0,297 0.432 0.889 0.7448 0.6804 0.9682 0.2869 0.013 0.311 0.284 0.307 0.433 u v 0.224 0.295 0.220 0.294 0.218 0.294 0.215 0.293 0.210 0.292 0.206 0.292 0.202 0.291 0.200 0.291 0.193 0.290 0.190 0.289 0.190 0.289 0.189 0.289 0.247 0.287 0.248 0.285 0.245 0.286 0.245 0.286 0.243 0.285 0.240 0.286 0.236 0.286 0.234 0.287 0.230 0.286 0.224 0.284 0.220 0.285 0.215 0.285 0.202 0.285 0.193 0.286 0.187 0.286 0.181 0.287 0.176 0.288 0.175 0.288 0.174 0.288 0.224 0.267 0.224 0.267 0.223 0.268 0.222 0.271 0.220 0.274 0.217 0.279 0.214 0.284 0.211 0.290 0.208 0.295 0.206 0.299 0.205 0.300 0.204 0.302 0.204 0.302 0.203 0.303 0.203 0.303 0.203 0.304 0.220 0.296 0.218 0.295 0.217 0.295 0.215 0.295 W* 82.165 81.995 81.947 81.713 81.493 81.493 81.332 81.359 81.262 81.218 81.364 81.348 73.793 72.361 72.938 72.796 72.615 73.092 73.189 73.470 73.125 72.667 73.355 73.272 73.969 74.607 75.187 75.940 76.468 76.712 76.724 77.266 77.547 77.906 79.02 80.385 82.592 84.866 87.778 90.659 92.899 93.695 94.476 95.031 95.449 95.543 95.904 85.051 84.926 85.186 85.061 U* 24.222 20.760 18.483 14.912 9.356 5.235 0.963 -0.701 -8.094 -11.356 -11.954 -12.854 44.556 44.645 42.146 41.334 39.949 37.231 33.794 31.347 27.045 21.637 18.381 13.489 0.665 -7.268 - 14.000 -19.851 -24.289 - 25.961 - 26.991 23.360 23.002 22.592 21.310 19.769 17.308 14.881 11.496 8.571 5.799 4.933 4.093 3.509 2.979 2.853 2.318 20.648 19.431 17.926 15.615 V* - 12.918 -13.673 - 14.085 - 15.014 -15.776 - 16.542 - 17.185 -17.446 - 18.123 - 19.035 - 19.050 - 19.143 - 19.239 - 20.658 -20.273 - 20.555 - 20.81 7 - 20.38 1 - 20.005 -19.584 - 20.498 -21.959 - 20.829 -21.395 -21.448 -21.064 -20.623 - 19.928 -19.650 - 19.605 -19.635 -40.91 1 - 40.339 - 39.569 -37.240 - 34.469 - 30.209 -25.738 - 19.917 - 14.282 -9.966 -8.405 -7.059 -5.950 - 5.064 -4.887 -4.148 - 12.941 - 13.565 - 13.445 - 13.996 AE 13.442 16.958 19.245 22.938 28.487 32.655 36.955 38.629 45.979 49.359 49.939 50.838 1.996 2.759 3.621 5.011 7.447 10.807 13.218 17.211 23.107 26.227 31.147 43.948 51.863 58.589 64.447 68.899 70.579 71.609 0.730 1.673 4.558 8.008 13.398 18.971 26.306 33.274 38.863 40.818 42.584 43.954 45.066 45.302 46.278 1.374 2.772 5.142 L* 83.131 82.961 82.913 82.679 82.459 82.459 82.298 82.325 82.228 82.185 82.330 82.314 74.762 73.331 73.907 73.766 73.584 74.062 74.158 74.439 74.094 73.636 73.324 74.241 74.938 75.576 76.155 76.909 77.436 77.680 77.692 78.234 78.514 78.874 79.994 81.352 83.558 85.831 88.743 91.622 93.862 94.658 95.438 95.993 96.410 96.505 96.865 86.016 85.891 86.151 86.026 a* 21 SO9 19.695 18.445 16.602 13.352 10.989 8.431 7.426 2.662 0.802 0.387 37.657 38.563 36.811 36.476 35.782 33.844 31.512 29.748 27.753 24.831 22.094 19.161 10.463 4.702 -0.343 -4.945 -8.362 -9.632 - 10.395 -0.205 35.588 35.010 34.283 32.080 29.494 25.542 21.609 16.949 11.907 8.132 6.871 5.730 4.864 4.135 3.976 3.309 19.161 18.689 17.670 16.459 b* -9.800 - 10.699 - 11.213 -12.264 - 13.298 - 14.230 - 15.067 - 15.397 - 16.458 -17.443 - 17.490 -17.627 - 14.157 - 15.440 - 15.275 - 15.582 -15.910 -15.692 -15.588 -15.374 - 16.444 -18.108 -17.315 -18.237 - 18.972 - 19.129 -19.161 -18.917 -18.946 - 19.005 -19.095 -33.796 - 33.324 - 32.686 -30.762 -28.478 -24.979 -21.305 -16.535 -11.878 - 8,329 -7.031 -5.916 -4.989 -4.252 -4.106 - 3.496 -10.011 - 10.638 - 10.631 -11.266 AE 7.518 9.479 10.771 12.890 16.165 18.651 21.289 22.321 27.036 29.136 29.518 30.111 2.113 1.631 2.091 2.814 4.164 6.335 8,008 10.184 13.464 15.885 18.922 27.616 33.337 38.353 42.920 46.345 47.625 48.394 0.797 1.829 4.960 8,668 14.388 20.227 27.802 34.934 40.578 42.552 44,326 45.707 46.824 47.060 48.033 0.794 1.620 2.9793 Table l-conrinued Indicator CLfCM X Y Z J D P, Py Qx Qy 0.944 0.7337 0.6795 0.9673 0.2910 -0.015 0.308 0.285 0.316 0.426 0.956 0.7308 0.6821 0.9726 0.2885 -0.028 0.306 0.286 0.323 0.425 0.967 0.7324 0.6909 0.9698 0.2840 -0.029 0.306 0.289 0.324 0.416 0.978 0.7219 0.6893 0.9804 0.2847 -0.037 0.302 0.288 0.338 0.416 0.983 0.7187 0.6936 0.9842 0.2823 -0.031 0.300 0.289 0.345 0.412 0.989 0.7156 0.7001 0.9895 0.2783 -0.015 0.298 0.291 0.355 0.407 0.994 0.7111 0.7049 0.9963 0.2756 0.011 0.295 0.292 0.366 0.403 1.000 0.7037 0.7099 1.0028 0.2744 0.048 0.291 0.294 0.380 0.397 1.011 0.6923 0.7105 1.0057 0.2788 0.082 0.287 0.295 0.393 0.391 1.028 0.6892 0.7116 1.0059 0.2799 0.091 0.286 0.296 0.397 0.388 1.056 0.6927 0.7169 1.0134 0.2734 0.096 0.286 0.296 0.401 0.389 Thymolphthalein Complexone 0.000 0.3820 0.4060 0.9170 0.7631 0.142 0.224 0.238 0.452 0.422 0.556 0.3887 0.4120 0.9160 0.7475 0.142 0.226 0.240 0.450 0.421 0.778 0.4164 0.4387 0.9267 0.6849 0.141 0.234 0.246 0.446 0.420 0.889 0.4704 0.4913 0.9405 0.5807 0.136 0.247 0.258 0.436 0.416 0.922 0.5034 0.5229 0.9516 0.5252 0.133 0.255 0.264 0.430 0.415 0.944 0.5315 0.5499 0.9552 0.4834 0.129 0.261 0.270 0.424 0.411 0.956 0.5568 0.5749 0.9648 0.4463 0.127 0.266 0.274 0.420 0.410 0.967 0.5801 0.5973 0.9679 0.4162 0.123 0.270 0.278 0.414 0.406 0.978 0.6181 0.6336 0.9702 0.3713 0.117 0.278 0.285 0.402 0.399 0.983 0.6420 0.6570 0.9773 0.3431 0.116 0.282 0.289 0.397 0.396 0.989 0.6543 0.6688 0.9778 0.3302 0.114 0.284 0.291 0.393 0.393 0.994 0.6717 0.6858 0.9762 0.3131 0.108 0.288 0.294 0.385 0.387 1.000 0.6942 0.7079 0.9803 0.2899 0.106 0.291 0.297 0.378 0.382 1.011 0.7238 0.7366 0.9860 0.2608 0.106 0.296 0.301 0.367 0.374 1.022 0.7383 0.7511 0.9865 0.2477 0.105 0.298 0.303 0.360 0.368 1.044 0.7459 0.7587 0.9891 0.2401 0.104 0.299 0.304 0.358 0.366 PyrocatecholViolet . .. . 0.000 0.3445 0.3831 0.8256 0.8258 0.121 0.222 0.247 0.439 0.399 0.556 0.3457 0.3546 0.8090 0.8510 0.108 0.229 0.235 0.422 0.411 0.778 0.3538 0.3374 0.7985 0.8658 0.106 0.237 0.226 0.410 0.420 0.889 0.3637 0.3280 0.7917 0.8738 0.109 0.245 0.221 0.401 0.427 0.922 0.3678 0.3244 0.7902 0.8776 0.112 0.247 0.219 0.398 0.430 0.944 0.3723 0.3234 0.7890 0.8778 0.115 0.251 0.218 0.395 0.432 0.967 0.3764 0.3214 0.7887 0.8804 0.119 0.253 0.216 0.393 0.434 0.978 0.3789 0.3219 0.7890 0.8784 0.121 0.254 02.16 0.392 0.434 0.989 0.3817 0.3213 0.7887 0.8786 0.123 0.256 0.215 0.391 0.435 1.000 0.3840 0.3205 0.7894 0.8794 0.125 0.257 0.215 0.390 0.437 1.011 0.3850 0.3200 0.7900 0.8807 0.129 0.258 0.214 0.390 0.437 1.028 0.3855 0.3202 0.7898 0.8805 0.130 0.258 0.214 0.390 0.437 u v 0.212 0.295 0.211 0.295 0.209 0.296 0.206 0.295 0.204 0.296 0.202 0.296 0.199 0.296 0.196 0.297 0.193 0.297 0.192 0.297 0.191 0.297 0.166 0.264 0.167 0.265 0.170 0.269 0.176 0.276 0.180 0.280 0.183 0.283 0.184 0.286 0.186 0.288 0.190 0.292 0.191 0.294 0.192 0.295 0.193 0.296 0.195 0.298 0.197 0.300 0.197 0.301 0.198 0.302 0.161 0.268 0.171 0.263 0.181 0.259 0.190 0.257 0.193 0.256 0.196 0.256 0.199 0.255 0.200 0.255 0.202 0.255 0.203 0.254 0.204 0.254 0.204 0.254 W* 85.016 85.146 85.584 85.504 85.717 86.037 86.272 86.516 86.545 86.598 86.855 68.924 69.345 71.172 74.563 76.486 78.068 79.487 80.725 82.666 83.878 84.478 85.331 86.418 87.797 88.481 88.835 67.277 65.133 63.783 63.026 62.732 62.650 62.486 62.527 62.477 62.411 62.370 62.387 U* 12.527 10.741 9.210 5.912 3.727 0.987 - 1.819 -5.501 -9.171 -10.359 - 10.874 -31.564 -30.688 -28.258 -23.664 -21.021 - 18.615 -16.997 -15.135 - 12.028 -10.545 -9.595 -8.240 -6.837 -4.976 -4.084 -3.728 - 35.041 -25.409 -16.338 -8.971 -6.062 -3.830 -1.494 -0.654 0.701 1.852 2.383 2.534 V* - 13.777 - 13.777 -12.586 - 13.273 - 12.963 - 12.508 - 12.324 -12.031 - 11.899 - 11.715 -11.713 - 38.688 -37.836 -35.229 -29.862 -27.063 -24.340 -22.349 - 20.243 -16.768 -14.997 - 13.912 - 12.203 -10.462 -8.304 -7.066 -6.587 -34.124 - 37.564 -39.887 -41.246 -41.871 -42.073 -42.515 - 42.520 - 42.678 - 42.940 - 43.106 -43.067 AE 8.164 9.942 11.456 14.746 16.934 19.691 22.508 26.206 29.874 31.069 31.598 1.308 5.286 13.119 17.420 21.381 24.305 27.375 32.419 35.023 36.584 38.918 41.403 44.568 46.234 46.923 10.450 19.881 27.358 30.340 32.539 34.912 35.714 37.064 38.243 38.798 38.934 L* 85.981 86.111 86.549 86.469 86.682 87.002 87.237 87.480 87.509 87.563 87.819 69.895 70.316 72.142 75.532 77.454 79.036 80.454 81.691 83.631 84.843 85.443 86.296 87.383 88.762 89.444 89.799 68.248 66.105 64.756 63.999 63.705 63.623 63.458 63.500 63.450 63.384 63.343 63.359 a* 14.375 13.216 11.662 9.825 8.241 6.213 4.255 1.642 -0.914 - 1.810 -2.164 -5.042 -4.734 -4.078 -3.102 -2.438 - 1,940 - 1.687 - 1.321 -0.711 -0.478 -0.310 -0.144 0.037 0.341 0.395 0.423 -10.310 -0.663 7.892 14.445 17.056 18.874 20.907 21.533 22.640 23.657 24.152 24.239 b* - 11.285 -11.402 - 10.468 - 11.285 - 11.161 - 10.947 - 10.973 -10.963 - 1 1.095 -11.015 -11.043 -35.720 -34.927 -32.492 -27.557 - 24.970 - 22.477 -20.653 -18.721 - 15.524 - 13.890 - 12.888 -11.316 -9.703 -7.690 -6.544 -6.099 -32.236 -34.734 -36.294 -37.100 -37.496 -37.549 - 37.810 -37.761 - 37.824 -37.990 -38.105 - 38.062 4.953 g 11.001 2 6.106 c-L 7.532 % 9.433 "4 13.019 r 17.606 's 20.160 21.052 21.427 14.987 w 0.949 4.049 10.108 13.397 16.387 18.702 21.023 24.806 26.848 28.026 29.808 31.757 34.214 35.538 36.103 10.193 18.973 25.584 28.235 30.022 32.071 32.665 33.764 34,793 35.301 35.377 CI 4 N 41728 ANALYST, DECEMBER 1987, VOL. 112 Evaluation of the Chromatic Results for the Indicators Methylthymol Blue In aqueous solution, at pH 10, this compound has a light blue colour with two maxima in its absorption spectrum, at 600 and 458 nm.Its complex with Mg has an intense blue colour, with an absorption maximum at 605 nm. The chromatic transition during the titration is relatively long (in true colours) and is characterised by a decolouration of the complex. The two methods (CIE-UCS and CIE-LAB) agree in the variation of the colour intensity, showing a non-unique “jump” in colour change. Further, this occurs over a relatively wide range of concentrations. The spectrum makes it difficult to locate the stoicbziometric end-point and, as is observed, this does not coincide with a stable solution colour, which appears after the equivalence point. Antipyryl-o-Arsenazo I This has a pinkish violet colour with an absorption maximum in the UV - visible spectrum, at pH 10, at 539 nm and its complex with Mg ions is blue, with a maximum at 570 nm.The colour transition during titration is shorter than that of its isomer ApA I, but it occurs almost entirely near the titration end-point. This results in a large colour change, which is easy to see, in a small titration interval. Therefore, it is a suitable indicator for Mg. Antipyryl-p-Arsenazo I This compound has a pinkish violet colour in aqueous solution at pH 10, with an absorption maximum at 595 nm, and its complex with Mg is bright blue, with an absorption maximum at 612 nm. Its colour transition during the titration is the longest studied, but it occurs with a gradual evolution, and therefore in the proximity of the end-point it shows little colour change.Calmagite This has a blue - violet colour, with an absorption maximum at 593 nm in its UV - visible spectrum. Its complex with Mg is red, with an absorption maximum at 530 nm. Its colour transition is large but it occurs progressively, with the result that a large part of the colour change occurs before reaching the region of the equivalence point, with a small colour change occurring at the end-point. Metalphthalein Its aqueous solutions at pH 10 are pink, with a maximum in its absorption spectrum at 572 nm. Its complex with Mg is red with a maximum at 566 nm. Its (true colour) chromatic transition is characterised by a shift towards the achromatic 1200 I 1 800 1 400 t 0.7 0.8 0.9 1 .o CL/CM Fig. 3. Evolution of the colour change of indicators during the titration in the CIE-LAB system. (A) Eriochrome Black T; (B) Antipyryl-o-Arsenazo I; (C) Methylthymol Blue; (D) Metalphthalein point (C).It could be said to converge with that produced by Methylthymol Blue. Only one colour “jump” is observed in the titration and it is produced in the proximity of the equivalence end-point. Eriochrome Blue - Black B At pH 10 its aqueous solutions are blue, with an absorption maximum at 629 nm, while the absorption maximum of the complex with Mg, which is reddish violet, is at 545 nm. It produces a short colour transition, with colours which are not very saturated. This makes the end-point difficult to locate, as the colour change is small, even though it is produced in a narrow interval in the titration. Eriochrome Black T Its complex with Mg in aqueous solution at pH 10 is reddish violet, with an absorption maximum at 550 nm; under the same conditions the indicator solution is bright blue, with an absorption maximum at 615 nm.The chromatic transition is located in a zone of the chromaticity diagram with contrasting colours, which makes it easy to observe. The variation in colour change is produced in a narrow titration interval, and it is fast and continuous. This gives a sharp colour “jump” which is easy to locate. Thymolphthalein Complexone Its aqueous solutions at pH 10 are light blue, with an absorption maximum at 600 nm, and its complex with Mg has a maximum at the same wavelength, but with a pronounced hyperchromic effect, i.e., an intense blue colour. The colour transition during titration is long, and can be almost super- imposed on that of Methylthymol Blue, but it occurs more gradually and the colour change takes place over a much broader colour interval.Pyrocatechol Violet Its solutions are violet at pH 10, with an absorption maximum at 595 nm, and its complex with Mg is blue - violet with an absorption maximum at 612 nm. The colour transition is short and occurs gradually throughout the titration, with the result that no colour is observed at the end-point. This indicator is not suitable for titrating Mg. Errors with and Accuracy of the Indicators In order to compare the theoretical data obtained by the CIE-LAB and CIE-UCS systems with the experimental results, a series of Mg titrations were carried out using the studied metallochromic indicators to detect the end-point.Table 3 gives these results for the different metallochromic indicators. It is observed that AoA I has the minimum titration error and the greatest accuracy. Eriochrome Black T is slightly inferior to AoA I. The other indicators give greater errors and lower accuracy than these two. Moreover, it may be Table 2. Height (h) and width (a) of the peaks in Figs. 2 and 3 CIE-UCS CIE-LAB Indicator Eriochrome Black T . . . . AoA I . . . . . . . . . . MethylthymolBlue . . . . Metalphthalein . . . . . . Calmagite . . . . . . . . Eriochrome Blue - Black B . . ApA I . . . . . . . . . . Thymolphthalein Complexone PyrocatecholViolet . . . . h a 1611 0.030 1105 0.033 780 0.042 745 0.046 681 0.034 632 0.031 549 0.063 460 0.060 h 1163 590 620 760 417 455 305 355 a 0.029 0.033 0.042 0.047 0.034 0.024 0.084 0.063ANALYST, DECEMBER 1987, VOL.112 1729 Table 3. Relative standard deviation and errors for visual titration of magnesium with different metallochromic indicators Relative standard Relative Indicator deviation, YO * error, % * EriochromeBlackT . . . . 0.11 0.02 AoA I . . . . . . . . . . 0.09 0.00 MethylthymolBlue . . . . 0.18 0.07 Metalphthalein . . . . . . 0.20 0.10 Calmagite . . . , . . . . 0.20 0.10 Eriochrome Blue - Black B . . 0.37 0.22 ApA I . . . . . . . . . . 0.41 1.20 Thymolphthalein Complexone 0.20 0.19 PyrocatecholViolet . . . . - - * Calculated from the average of five titrations. said that, except for ApA and Pyrocatechol Violet, all of them may be used in the titration of Mg with EDTA.I Conclusions In a classification of indicators based only on the height and width of the colour “jump,” different results will be obtained depending on the system used. Although the results obtained with Eriochrome Black T agree in both systems and it displays the greatest change, there is a considerable difference in the classification of the other indicators by the two systems. If the results obtained in the visual titration are compared with the theoretical data, the CIE-UCS system gives a better agreement than the CIE-LAB system. In spite of the fact that Eriochrome Black T is the indicator with the best quantitative values for the colour change, AoA I has fewer errors and greater accuracy in titrating Mg as a result of its chromatic transitions. Whereas Eriochrome Black T evolves towards blue colours, AoA I evolves towards a pinkish colour at the end-point, which is a zone with higher density on MacAdam ellipses,49 and so its visual memory effect is greater.This feature makes the end-point easy to locate, and gives great accuracy in the titration. 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