July, 1924
TNDUSTRIAL A N D ENGINEERING CHEMISTRY
709
A Mixed Indicator for Carbonate-Bicarbonate Titrations‘ By Stephen G. Simpson MASSACHUSETTS INSTITUTE OB TECHNOLOGY, CAMBRIDGE, MASS.
N T I l R A T I N G alkali carbonate-bicarbonate and carbon- resulting in the disappearance of the blue is followed by the ate-hydroxide mixtures to two end points using a single change from pink to orange-yellow, which shade should be sample, the most commonly used indicator for the first taken as the end point. The solutions should be titrated very end poini, which should correspond to the change from slowly, especially from the point of the disappearance of the carbonate to bicarbonate, is phenolphthalein. Changes in blue. Titrations of pure sodium carbonate using such a the concentration of this indicator have considerable effect mixture of cresol red and thymol blue gave the results listed on the end point,2but an equally great objection to phenol- in Table I. phthalein for titrations of this sort is the fact that the color TABLE I-TITRATION OF PURE NaxCOs USING CRESOI. RED + THYMOL change is so very gradual that considerable uncertainty arises BLUEAS INDICATOR FOR HALFNEUTRALIZATION NazCOa NazCOa in recognizing the exact end point. This is especially true Taken Calcd. Normality Ratio when titrating with 0.1 N acid. Scales3has suggested thymol A B Acid B/A 0.2025 0.2022 0,1053 0.999 blue (thymolsulfonephthalein) as a substitute for phenol0.2800 0.2806 1.002 phthalein, but similar difficulty is found with this indicator in 0.2123 0.2124 1.000 0.2849 0.2861 1.004 that the Hue color fades so gradually that there is a con0,2234 0.2230 0.998 0.3919 0.3916 0,999 siderable range of an indefinite tint before a distinct yellow is 0.1686 0.1690 1.002 seen and it is difficult to decide on the exact end point. 0.1641 1.000 0.1640 1.004 0.2070 0.2078 In choosing a substitute indicator for phenolphthalein or 0,1893 0,1887 0.997 0.1989 0,999 0.1987 thymol blue, one is not restricted to a single indicator, for 0.3316 0.3318 1,000 Cohen4 has shown the advantages in certain cases of using 0.8953 0.8958 0.5302 1,001 1.001 1.2670 1.2680 mixtures of indicators the hydrogen-ion exponent values of 0.9278 0.999 0.9268 0,9889 0,999 0.9879 which lie dose together. A mixture of indicators of slightly 1.2071 1.2030 0.997 different constants has a wider color change interval than a 0,9230 0.9228 1.000 1,004 1.1814 1.1863 single indicator, but the advantage of mixing indicators is 0.8731 0.8716 0,998 that a very marked change in tint may sometimes be secured Average 1.0001 thereby.6 Following the use of the indicator, described above, it was For the first end point in alkali carbonate titrations, the most promising of the common indicators having a color found possible to recognize the second end point correspondchange in the vicinity of pH 8.3 are phenolphthalein, thymol ing to complete neutralization using methyl orange in the blue, and cresol red (0-cresolsulfonephthalein), In each case same solution, but a sharper color change was obtained with the change in tint is to a light shade of color permitting the sub- bromphenol blue (tetrabromphenolsulfonephthalein) which sequent use of another indicator for the second end point. changes from blue to green when the solution is completely Phenolphthalein changes from pink to colorless over a p H neutralized. Using this indicator in the presence of cresol range of 10.0 to 8.3; thymol blue changes from blue to yellow red and thymol blue, the average of thirteen results expressed over a pH range of 9.6 to 8.0; cresol red changes from red to in terms of a 1-gram sample of sodium carbonate was found to be 1.0006, with an average deviation of *0.0012. yellow over p H range of 8.8 to 7.2. Titrations of known mixtures of sodium carbonate and soSamples of pure sodium carbonate were dissolved in 125 ml. of water and titrated both with 0.1 N and 0.5 N sulfuric dium bicarbonate using a cresol red-thymol blue mixture for acid (standardized gravimetrically), Phenolphthalein and the first end point and bromphenol blue for the second thymol blue gave indefinite end points and had a tendency end point gave results listed in Table 11. to give low results if the first indication of change was taken TABLE11-TITRATION OF NaiCOa-NaHCOa MIXTURESUSING CRBSOI. RED + THYMOL BLUEAND BROMPHENOL B&UE as the end point. This was especially true with thymol NaHCOa NazCOs NaHCOa blue. Cresol red, on the other hand, gave comparatively NazCOa Taken Taken Calcd. Calcd. Normality Ratio Ratio A B C D Acid C/A D,/ -B sharp end points, but the results were always too high, 0.1195 0.1183 0.1198 0.1183 0.1053 1.003 1.000 in that the color change took place when the carbonate had 0.1145 0.1144 0.1381 0.1387 0,999 0.995 0.0614 0.2817 0.2808 0.998 1.003 been more than half neutralized. Expressed in terms of a 0.0615 0.1335 0.1342 0.1139 0.1142 1.003 0.995 1-gram sample of sodium carbonate, the average of eleven 0.1839 0.1842 0.0467 0.0474 0.986 1,002 0,6247 0.4147 0.5302 0.4145 0.000 0.999 titrations using cresol red gave a calculated weight of 1.026 0.6253 0.4723 0.4303 0.4275 0.4738 0.992 1.003 0,4364 0.9020 0.4381 grams of sodium carbonate, with an average deviation from 0,9006 0,996 1.002 0.5586 0,5574 0.8028 0.7998 0.998 0.997 the mean of *0.003. 0.7966 0,7980 0.4177 0.4204 1,002 1.007 0.4457 0.4462 0.4995 0.5005 1.001 1,002 The use of cresol red mixed with thymol blue, however, Average 1.0002 0.9983 gave a change in color of the cresol red corresponding very closely to the half neutralization point, with no sacrifice to The indicators used in the titrations were prepared as sharpness of color change. This was equally true with 0.5 N follows : and 0.1 N acid. It was found to be important to mix the two Thymol blue 0.03 gram + 1 42 ml 0 05 N NaOH diluted to 25 ml. indicators in the proportion by weight of 1 part of cresol 0.01 gram + 0:58 ml: 0:05 N NaOH: diluted t o 25 ml. Cresol red Bromphenol blue 0.02 gram + 0.66 ml. 0.05 N NaOH, diluted to 25 ml. red to 6 parts of thymol blue, a greater quantity of the former giving high results. The color change of the thymol blue The cresol red and thymol blue were then mixed in the pro1 Received March 3, 1924. portion by volume of 1 to 2 and three drops were used in each Prideaux, “Theory and Use of Indicators,” 1917, p. 299. titration. Four drops of the bromphenol blue were used for a . h e n c e , 61, 214 (1920). the second end point. J . A m . Chem. SOL, 44, 1851 (1922).
I
4
6
Prideaux, “Theory and Use of Indicators,” 1917, p. 165.
8
Clark, “The Determination of Hydrogen Ions,” 1920, p . 66.
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
710
(n
L
1
-li[ 4
1
Vol. 16, hTo. 7
