Revised ms # 1999-0728



Lab Documentation

1.DIRECTIONS FOR THE EXPERIMENTAL PROCEDURE FOR INSTRUCTORS AND STUDENTS

1a) Preparation of the stock solution of bromocresol green (BCG)

Bromocresol green [76-60-8], ACS reagent, was used as received (Merck, Darmstadt, Germany).

The 1.8 x10-4 mol L-1 stock solution of BCG is prepared using no more than 12 mg of indicator, because it has low solubility in water, weighed within 0.0001 g. The solid is transferred quantitatively to a 100 mL volumetric flask and 90 mL of distilled water is added. In order to accelerate the dissolution of the indicator, the solution should be sonnicated during 5 to 10 min. After complete dissolution, the solution is diluted to the mark.

1b) Dilution of the stock solution

Students should take aliquots of 0.50, 1.00, 1.50, 2.50, 4.50, 5.10, 6.00, 6.30 and 7.50 mL of stock solution with an adjustable pipette. These aliquots should be diluted in separate 10 mL volumetric flasks. This is a good range of concentration for this indicator to observe the color changes, as shown in Figure 1.

Figure 1 shows six cuvets containing the diluted aqueous solutions of BCG in decreasing order of concentration. The color change can be clearly observed from the initial reddish color, which changes to blue for the most diluted solution.

[pic]

Figure 1: Aqueous BCG solutions at different concentrations: (1) 6.00, (2) 1.80, (3) 0.60, (4) 0.30, (5) 0.18, (6) 0.059 x 10-4 mol L-1.

For each solution, students should calculate the total analytical concentration of the indicator ([HInd]0) taking into account the volume and the initial concentration of the stock solution used in this section. These [HInd]0 values are the data for the first column of Table I

1c) Measuring the pH values

In this step, the solutions prepared in Section 1b, are placed in 10 mL beakers and the pH is determined through a calibrated pH meter. The electrode of the pH meter should be washed and dried after each measurement so that it does not dilute the solutions, because these same solutions will be used to measure the electronic spectra in section 1d. This procedure can be adopted in order to save time and minimize indicator waste during the experiment. The pH values should be placed in the second column of Table I.

1d) Obtaining the electronic spectra

The electronic spectra are recorded in the wavelength range of 250 to 750 nm. In Figure 2, the electronic spectra are shown and two bands are detected, centered at 440 nm and 628 nm respectively, which are associated to the HInd (yellow color) and Ind- (blue color) species in solution, respectively. The absorbance at 440 nm should be registered in the third column of Table 1

[pic]

Figure 2: Electronic spectra of aqueous solutions of BCG.

The instructor should ask his students to look at the absorbance ratio at 628 nm and 440 nm (Abs628/Abs440) for each diluted solution. Firstly, students might discuss why the absorbance ratio should not vary if only a dilution was carried out. But from what can be observed, the ratio (Abs 628/Abs 440) changes to a higher value, which is not expected for a simple dilution.

2a) Determination of the ( value for the HInd species

Students should take aliquots of 1.00, 1.50, 2.50, 4.50, 5.10, 6.00, 6.30 mL of stock solution of BCG with an adjustable pipette. These aliquots are diluted in separated 10 mL volumetric flasks, using a 0.10 mol L-1 aqueous HCl solution as solvent. The electronic spectra are measured in the 250 to 750 nm range. In this case, only a band at 440 nm is observed, as can be seen in Figure 3.

[pic]

Figure 3: Electronic spectra of each diluted solution of BCG in acid aqueous medium.

In the acid medium, the ( value is very small and we can assume that [HInd]0 = [HInd]. The absorbance of each diluted solution should be measured at 440 nm. Plotting [HInd] and the absorbance values of each aqueous acid indicator solution, students can calculate the ( value for the HInd species from the slope of the Beer’s Law plot, as shown in Figure 4.

[pic]

Figure 4: Beer’s Law plot for the HInd species

According to the graph in Figure 4, the ( value obtained for the HInd species was 16,850 L mol-1 cm-1. Knowing the ( value it is possible to calculate the [HInd] for each diluted solution of BCG of section 1b, using the absorbance measured at 440 nm. In this manner, [Ind-] can be estimated from the difference between [HInd]0 and [HInd]. The values calculated for [HInd] and [Ind-] are recorded in columns 4 and 5 respectively of Table 1.

2b) Experimental data treatment

The instructor should ask his students to organize their results in a table, as shown below, in order to facilitate the calculation of the pK for the BCG indicator.

Table 1: Values of concentration, pH and the absorbances measured at 440 nm for diluted BCG solutions. The calculated [HInd] and [Ind-] species, and the ionization degree (() are also shown.

|[HInd]0 /10-4 mol L-1|pH |Abs |[HInd] / 10-4 mol |[Ind-] / 10-4 mol |[Ind-] / [HInd] |log([Ind]/ [HInd]|( |

| | |(440 nm) |L- |L- | |) | |

| 0.182 | 5.28 | 0.18 | 0.090 | 0.092 | 1.013 | 0.006 |0.51 |

| 0.273 | 5.04 | 0.26 | 0.141 | 0.132 | 0.936 | -0.029 |0.48 |

| 0.710 | 4.28 | 1.04 | 0.605 | 0.105 | 0.174 | -0.761 |0.15 |

| 0.819 | 4.26 | 1.19 | 0.693 | 0.126 | 0.182 | -0.740 |0.15 |

| 0.928 | 4.15 | 1.37 | 0.800 | 0.128 | 0.160 | -0.796 |0.14 |

| 1.092 | 4.13 | 1.66 | 0.970 | 0.120 | 0.123 | -0.908 |0.11 |

| 1.365 | 3.96 | 2.30 | 1.345 | 0.020 | 0.015 | -1.828 |0.02 |

The [HInd]0 values in the first column are the analytical concentrations of the diluted solutions used in Section 1b. The second column contains the pH measured as described in Section 1c. The third column has the absorbance values at 440nm for diluted solutions of BCG, obtained in Section 1d. In the fourth and fifth column, the [HInd] concentration, determined from the calibration curve, previously described in Section 2a, and the [Ind-] value, obtained from the difference between the [HInd]0 (1st column) and [HInd] (2nd column) are shown. The degree of ionization can be calculated from the definition:

( = ([Ind-]/ [HInd]0) (1)

As can be observed in Table 1, the ( values increase for the more diluted solutions in accordance to Le Chatelier’s principle on the chemical equilibrium between non-dissociated (HInd) and dissociated (Ind-) species.

The students should then calculate log([Ind-]/[HInd]) in order to determine pKInd according to the Henderson-Hasselbalch equation.

A typical plot of pH of the solutions as a function of log ([Ind-]/[HInd]), obtained from students, is shown in Figure 4. The pKInd, obtained from the angular coefficient of the Henderson-Hasselbalch equation, is 5, which is in good agreement with the literature value.

[pic]

Figure 4: Plot of pH vs log [Ind-] / [HInd] for bromocresol green

3. Color changes in other indicator solutions

It is possible to calculate the ( values for other indicators in water according to equation 4, obtained from equation 3 (Lab summary).

(2 [HInd]0 + (KInd – KInd = 0 (4)

Using equation 4, ( values were calculated for a list of some acid base indicators with different KInd values are shown in Table 2. Color changes are only observed when the analytical concentration is in the same order of magnitude as the KInd of the indicator.

Table 2: ( values for some indicators as a function of their concentration in water.

| [HInd]0/mol L-1 |( value for thymol blue (pKInd =1.7)|( value for methyl orange (pKInd= |( value for bromocresol green |( value for bromothymol blue |( value for alizalin yellow |

| | |3.5) |(pKInd=4.7) |(pKind=7.1) |(pKind=11.0) |

|100 |0.1 (red) |0.0 (red) |0.0 (yellow) |0.0 (yellow) |0.0 (yellow) |

|10-1 |0.4 (orange) |0.1 (red) |0.0 (yellow) |0.0 (yellow) |0.0 (yellow) |

|10-2 |0.7 (yellow) |0.2 (red) |0.0 (yellow) |0.0 (yellow) |0.0 (yellow) |

|10-3 |1.0 (yellow) |0.4 (orange) |0.1 (yellow) |0.0 (yellow) |0.0(yellow) |

|10-4 |1.0 (yellow) |0.8 (yellow) |0.4 (green) |0.0 (yellow) |0.0 (yellow) |

|10-5 |1.0 (yellow) |1.0 (yellow) |0.7 (blue) |0.1 (yellow) |0.0 (yellow) |

• ( was calculated by using the equation: (2 [HInd]0 + (KInd – KInd = 0; obtained from the expression of the equilibrium constant

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