PRE LAB 8 - PRE LAB PDF

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Raina Patel Elementary Analytical Chemistry Section 902 Pre-Lab Report 8 Lab 8: Determination of pKa of Bromocresol Green, an Acid-Base Indicator

Introduction: The purpose of this lab is to help be able to determine the pKa of the Bromocresol Green through the use of Spectrophotometry. Within this experiment, there is the need to use spectrophotometry which is used measure the pKa of the bromocresol green, which is an acid base indicator. The indicator is able to be dissociated through the equation. HIn + H2O >H3O + InTo solve for the pH there is the rearrangement of the equilibrium constant expression, which is −¿¿ able to give us the equation of ¿ log ¿ = pH - pKHIn When graphing the equation of the straight line, slope-intercept form is used, where the y in the equation is used which stands for log [In-]/[HIn], while the slope is equal to 1, and then the b stands for the -pKhin. So, in the situation that the y is plotted against the pH, the slope is equivalent to 1. Since the y intercept is equal to the -pKhin, the line is able to cross at the pH axis since the positive pKhin is equal to ph. The ratio of the [ln]/[HIn] is determined with the use of spectrophotometry. The first step in the experiment is completed through the bromocresol green to be prepared in the acidic solution, where the indicator is still in the HIn form. After the completion of this, the absorbance values are read over the different wavelengths, reading from the graph it is possible to see the absorbance vs the wavelength values. The relationship that was

present between the pH and the indicator and the concentration all were described by the acid dissociation constant Ka. Within the acidic solutions, the maximum absorbance will occur at the lower wavelengths, compared to the basic solutions where the maximum absorbance will occur at higher wavelengths. The maximum wavelength is the one that is preferred for the acidic solution, as the HIn forms stronger but the In- does not as well (Department of Chemistry USF, 2020).

Materials: 1. 0.20g Bromocresol Green 2. 500mL 95% Ethanol 3. 1M NaOH 4. 1L 0.1M Acetic Acid 5. 1L 0.2M Sodium Acetate 6. 500mL 0.1M HCl 7. 100mL 0.1M NaOH 8. pH 4 buffer 9. pH 10 buffer 10. Kim wipes 11. Parafilm 12. Spectrophotometer

Safety:

Chemical 95 % ethanol

Hazards Has the ability of

Personal Protection Goggles, Gloves, and

Handling Must be stored in a

being oxidized,

Lab Coats must be

cool area

irritant to the lungs,

worn at all times

can cause a fire due

NaOH

HCl

to combustible dust Can cause severe

Goggles, Gloves, and

Must be able to

damage to eyes and

Lab Coats must be

provide ventilation

skin if it is repeatedly

worn at all times

for the exhaust

used in contact Can be irritant to the

Goggles, Gloves, and

Must be used in areas

skin, as well as

Lab Coats must be

that are highly

danger to the mucosal worn at all times

ventilated

membranes and can

Sodium Acetate

Bromocresol ¿

cause eye damage Not regarded as being Goggles, Gloves, and

Must be used with

a danger to the

Lab Coats must be

good handling

physical body

worn at all times

techniques, as well as

Not regarded as being Goggles, Gloves, and

good hygiene Must be used with

a danger to the

Lab Coats must be

good handling

physical body

worn at all times

techniques, as well as good hygiene

Procedures:

While the TA prepares the solution of the Bromocresol Green, through using .2g of the bromocresol green to be diluted in 200mL of the 95% ethanol, and then it is further diluted down with the NaOH. There will be a series of solutions that will be prepared and they must all be labeled to help prevent the mixup. The first solution is labeled as the BCG, 0.1M acetic acid, 0.2M sodium acetate, 0.1M HCl, and 0.1M NaOH. The measurements for the different flasks is listed in the table below. Flask

Instruction

Estimated pH

#1

Mix 48.0 mL of 0.1 M HCl with 2.0 mL BCG

1.02

#2

Mix 48.0 mL of 0.1 M HAc with 2.0 mL BCG

2.89

#3

Mix 46.0 mL of 0.1 M HAc and 2.0 mL 0.2 M NaAc with 2.0 mL BCG

3.70

#4

Mix 44.1 mL of 0.1 M HAc and 3.9 mL 0.2 M NaAc with 2.0 mL BCG

4.00

#5

Mix 31.5 mL of 0.1 M HAc and 16.5 mL of 0.1 M NaOH with 2.0 mL BCG

4.80

#6

Mix 13.0 mL of 0.1 M HCl and 35.0 mL of 0.2 M NaAc with 2.0 mL BCG

5.40

#7

Mix 7.4 mL of 0.1 M HAc and 40.6 mL of 0.2 M NaAc with 2.0 mL BCG

5.80

#8

Mix 48.0 mL of 0.2 M NaAc with 2.0 mL BCG

9.02

There is a need to measure pH using the PASCO pH meter, every time there is the switch of the samples you must first rinse the electrode with the DI water, and then read the pH. To read the absorption values of the HIn you must use the Ocean View to indicate the different absorption values, of the bromocresol green, at the low pH. As well as the absorption values at both the low pH, and the high pH.

References:

Chang, Yu, Determination of the Equilibrium Constant for Bromocresol Green. (201)

University of South Florida. Laboratory Manual for Elementary Analytical Chemistry 3120C; Fall 2020; Department of Chemistry University of South Florida: 2020...