Pre-lab preparation

In setting up your notebook, draw the acid-base equilibrium for BCG as your "chemical equation" (draw means drawing structural formulas for all of the species that participate in this equilibrium). Make sure you list acetic acid, sodium acetate, BCG, concentrated HCl, and concentrated NH₃ (also called concentrated NH₄OH) in your reagent, hazard, and disposal tables.

Procedure

Choose a partner and do all work in pairs. Take turns performing operations and making measurements so that both partners get experience in all phases of the experiment. Do not let one partner act as data recorder and the other partner do all of the lab work.)

An equipment list has not been provided. Compile one as you read the procedure.

Useful abbreviations: BCG = bromocresol green, Ac = CH₃C=O (this means AcOH and HOAC = acetic acid, and AcONa and NaOAc = sodium acetate).

Stock solutions

This experiment is based on a trial-and-error approach. You are going to dissolve BCG in a series of acetic acid-sodium acetate buffer solutions in the hope that some of these will contain BCG anion and dianion in significant percentages.

To facilitate this process, you will first make two "stock" solutions called Acidic BCG and Basic BCG. These solutions will be combined to make the 8 sample sample solutions.

Just to make things a little more confusing (actually a bit easier for everyone), you will make your stock solutions from three "parent" stock solutions that have been previously prepared and placed in the reagent hood: 0.50 M HOAc, 0.50 M NaOAc, and X M BCG. Note: the concentrations of the actual parent solutions may differ slightly from these values. Record the actual concentrations shown on the parent stock bottles in your notebook.

Acidic BCG. Pipet 1.00 mL of X M BCG into a clean 50 mL volumetric flask [NOTE 2]. Pipet 1.00 mL of 0.50 M HOAc into the same flask. Dilute with water to the mark [NOTE 5] and mix thoroughly. Transfer this solution to a clean, dry beaker or Erlenmeyer and label it Acidic BCG. Rinse the volumetric flask several times with de-ionized water so that it can be re-used in the next step.

Basic BCG. Pipet 1.00 mL of X M BCG into the well-rinsed 50 mL volumetric flask. Pipet 1.00 mL of 0.50 M NaOAc into the same flask. Dilute with water to the mark and mix thoroughly. Transfer this solution to a clean, dry beaker or Erlenmeyer and label it Basic BCG.

Sample solutions

Primary Data Table (PDT). Make up a table in your lab notebook with 9 rows and 5 columns. Enter the following column headings in the first row (start from left side): Sample, Vol Acidic BCG (mL), Vol Basic BCG (mL), pH, Abs. Enter the following values in the first column under Sample (start with second row): 1, 2, 3, ..., 8.

Test tubes. You will need 8 large clean and dry test tubes. Label them 1, 2, 3, ..., 8.

Samples - general. Each sample is made by combining Acidic BCG and Basic BCG in a pre-determined proportion. The total volume of each sample will nominally be 10 mL, but it is not necessary to hit this value dead on. It is positively vital, however, to record exactly how much of each solution you use.

The following table shows how much of each stock solution you should aim for when making up each sample. Do not enter these volumes in your PDT. Leave blanks in your PDT and record the volumes you actually use.

Samples 2-7. Pour Acidic BCG and Basic BCG into two burets [NOTE 6]. Make up samples 2-7 by filling your test tubes with the target volumes listed in the above table.

Sample 1 (A_a). Dispense 10 mL of Acidic BCG into one test tube. Add 1 mini-drop of concentrated HCl (concentration ~ 12 M) to the solution [NOTE 3]. Mix thoroughly to spread the HCl throughout the sample. The absorbance of this sample corresponds to A_a (see Background).

Sample 8 (A_b). Dispense 10 mL of Basic BCG into one test tube. Add 1 mini-drop of concentrated NH₃ (concentration ~ 15 M) to the solution [NOTE 3]. Mix thoroughly. The absorbance of this sample corresponds to A_b (see Background).

Spectrophotometic measurements

Measure the absorbance of all 8 sample solutions at 620 nm and enter these values in your PDT.

Before you fill the cell with sample solution, rinse the cell once or twice with a small amount of the sample solution. After you measure the absorbance, transfer the absorbance sample back to its original sample container and set this solution aside for the pH measurement [NOTE 7].

After you "empty" the cell, rinse it well with de-ionized water before inserting a new sample. You may also need to wipe off the outside of your cell if you splash any liquid on it.

pH measurements

Measure the pH of all 8 sample solutions to +/- 0.01 pH unit and enter these values in your PDT [NOTE 4]. Wait for each reading to stop drifting before recording your measurement. The pH electrode should be rinsed with de-ionized water and adhering droplets should be shaken off between measurements.

Use of the pH meter will be explained at the beginning of the lab session.

Disposal and storage

All of our sample solutions contain acetic acid, acetate, and various forms of BCG, dissolved in water. The samples have pH sufficiently close to 5 that they can be discharged directly into the sink. The amount of BCG released this way is extremely minor.

Molecular modeling (week 2)

Instructions for studying electron delocalization in BCG and other molecules will be distributed in lab. A copy of the instructions can also be downloaded.