Charles 8. Leonard, Jr. School of Dentistry University of Morvland Baltimore, 21201
II
A Laboratory k p e t h e n f "sing Indicators
A laboratory experinlent utilizing indicators has been designed to teach several physical-chemical principles, namely optical density measurements, the change of optical density as a functiou of wavelength, the preparation of a standard curve and its subsequent use, and countercurrent distribution. .4 pair of indicators is chosen for the experiment depending on their preferential solubilities for the countercurrent distribution solventr, as shown in Table 1. Using a Spectronic-20 spectrophotometer, the student is directed to determine the wavelength of maximum optical density of a 0.000025 M solution of indicator in 0.1 M NaOH between 320 and 700 mp as well as a standard curve prepared in the appropriate equilibrated phase. Table I.
Preferential Solubilities of Indicators
n-Bot,anol Phase
Aoueous NanCO. Phase
Methyl Red Nentral Red Methyl Orange Lahlotte Yellow Brornohenol Blue ~rom&esolGreen Brnmcresol Pnrple
Pheuol Red Meta Cresol Purple LsMatte Sulfo Orange
.I countercurrent distribution procedure utilizing six separatory funnels, n-butanol, and 0.05 A l aqueous Na,CO, has been described previously.' When optical density measurements are determined following the separation a problem arises because the sample from the lower phase will occasionally become turbid when removed from the separatory funnel. If the sample is kept rold until the optical density is determined, this difficulty is surmounted. An experimental countercurreut distribution of met,hyl red aud meta cresol purple, recorded in optical density units, is given in Table 2. The results can be plotted eit,her in opt,ical density units or in milligrams of indicator present. When the Table 2.
Funnel Ziumber
Countercurrent Distribution of Mixture Methyl red, 430 rn& Upper Lower Phase Phase
on
Indicator
Meta cresol purple, 580 w Upper Lower Phase Phase
Optical Density Units
results of the separation are plotted the values for the upper and lower phases in a given funnel should be added together. However, occasionally a small optical density value is recorded due to the absorption of light by the other indicator at that particular wavelength. I n Table 2 the optical density values for methyl red, as recorded in the lower phase of funnels 3, 4, and 5, are apparently due to meta cresol purple light absorption at 430 mp. Inspection of the optical density values for meta cresol purple in this same area shows that these are the fumlels in which the major amount of this indicator is found. Thus, it has been the practice to use only the optical density values or milligrams of indicator present of that phase in which the major amount of indicator appears. Such a graph is shown in the figure, where the optical density values of the upper phase are used for methyl red and those of the lower phase for meta cresol purple.
Funnel Numbel
Theorelicol and experiment01 curves far methyl red (lines broken with dots) and met. cresol purple (lines broken with boxed. Methyl red: N = 0.385, K = 0.0834. Meto crerol purple: N = 3.94, K = 3.73.
To calculate the theoretical distribution curve and to determine the funnel in which the maximum concentration of indicator will be present, the partition coefficient K, which is defined as the ratio of the concentration of substance in the moving phase to that of the stationary phase, can be calculated in terms of optical density units for the respective phases. The theoretical results for methyl red and meta cresol purple are show11in the figure.
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AKKEGUIN, B., PADILLA, J., EDUC., 39,539 (1962).
AND
HERRAN,J., J. CHEM.
Volume 44, Number 6, June 1967
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