Expt 010 -- Dead Stop Titration

Barium hydroxide is used to titrate hydrochloric acid and sulfuric acid with a pH indicator. The conductivity of the solution is measured during the titration. Qualitative comparisons of these titrations are made and interpreted in terms of strong and weak electrolytes.

Background

• Barium sulfate is very insoluble. Even though barium compounds are quite toxic, barium sulfate is used in medical procedures because the amount of barium available to a patient from this insoluble salt is so small.
• In the titration of HCl with Ba(OH)₂, both Ba²⁺ and Cl^- ions remain to conduct current. Significant amounts of strong electrolyte always are present in the solution. The HCl is replaced with BaCl₂ as the conductor.
• When H₂SO₄ is titrated with Ba(OH)₂, all ions are being removed:

  H⁺ + OH_^- --> H₂O

  Ba²⁺ + SO₄^2- --> BaSO₄

  the concentration of sulfuric acid declines steadily through the titration. At the end point, the conductivity is very low. As the end point is passed, added Ba(OH)₂, a strong electrolyte, becomes the principal conductor. As more Ba(OH)₂ is added past the end point, the conductivity increases.

Safety

Barium compounds are very toxic. Barium hydroxide, sulfuric acid, and hydrochloric acid are corrosive. Do not ingest chemicals. Wear goggles and apron. Wash spills with a large amount of water. Wash hands after the experiment.

Procedure

1. Dim the lights or cup your hand over the flashing LED to observe it better.
2. Place 20 drops 0.2 M HCl in a well of a 24-well plate. Add 1 drop 0.1% bromthymol blue. Use the conductivity tester to test the conductivity of this solution. Note and record the conductivity, color, and any color changes.
3. Add 1 drop 0.1 M Ba(OH)₂. Use the conductivity tester to test the conductivity of this solution. Note and record the conductivity, color, and any color changes.
4. Continue adding 0.1 M Ba(OH)₂ dropwise with stirring. Note and record color changes and conductivity with the addition of each drop. Be sure to record the conductivity when the color first changes.

   !!!Click here to See Movie.

5. Add 0.1 M Ba(OH)₂ dropwise for 5 drops past a permanent color change.
6. Place 20 drops 0.1 M H₂SO₄ in an adjacent well of a 24-well plate. Add 1 drop 0.1% bromthymol blue. Use the conductivity tester to test the conductivity of this solution. Note and record the conductivity, color, and any color changes.
7. Add 1 drop 0.1 M Ba(OH)₂. Use the conductivity tester to test the conductivity of this solution. Note and record the conductivity, color, and any color changes.
8. Continue adding 0.1 M Ba(OH)₂ dropwise with stirring. Note and record color changes and conductivity with the addition of each drop. Be sure to record the conductivity when the color first changes.

   !!!Click here to See Picture.

9. Add 0.1 M Ba(OH)₂ dropwise for 5 drops past a permanent color change.

   !!!Click here to See Movie.

10. Compare HCl and H₂SO₄ mixtures after titration. Record your observations.

    !!!Click here to See Picture. HCl is on the left

11. Use a plastic transfer pipet to remove the contents of the well and transfer them to a disposal jar. Rinse the well with water, and transfer the first rinse to the disposal jar.
12. Wash the 24-well plate, and shake it dry. Use a cotton swab the clean the walls of the wells if they seem cloudy, and then rinse again.
13. Wash hands.

Questions

1. Describe the conditions that lead to high conductivity in a solution.
2. Predict the conductivity of a barium chloride solution.
3. Barium sulfate, BaSO₄, is a "sparingly soluble" salt. What does dissolve does dissociate, but not very much dissolves. Predict the conductivity of a saturated BaSO₄ solution.
4. Account for the observations in this experiment.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 010 Dead Stop Titration

Watch the movies.

Record color changes and flashing changes observed for each titration:

HCl titration

H₂SO₄ titration

Answer the questions:

1. Describe the conditions that lead to high conductivity in a solution.
2. Predict the conductivity of a barium chloride solution.
3. Barium sulfate, BaSO₄, is a "sparingly soluble" salt. What does dissolve does dissociate, but not very much dissolves. Predict the conductivity of a saturated BaSO₄ solution.
4. Account for the observations in this experiment.

Curriculum-

This experiment fits when electrolytes are discussed. It is best if pH indicators and acid-base reactions already have been discussed.

Safety-

Barium compounds are very toxic. Barium hydroxide, sulfuric acid, and hydrochloric acid are corrosive. Do not ingest chemicals. Wear goggles and apron. Wash spills with a large amount of water.

Time-

Teacher Preparation: 20 minutes if apparatus is being reused.

Class Time: 30 minutes

Materials-

• 3 mL of 0.1 M Ba(OH)₂ -- (Dissolve 3.155 g barium hydroxide octahydrate (Ba(OH)₂¥8H₂O) in enough water to make 100 mL solution.)
• 1.5 mL of 0.1 M H₂SO₄ -- (Add 3.3 mL 3 M sulfuric acid (H₂SO₄) to enough water to make 100 mL solution.)
• 1.5 mL of 0.2 M HCl -- (Add 6.7 mL 3 M hydrogen chloride (HCl) to enough water to make 100 mL solution.)
• 0.2 mL of 0.1% Bromthymol blue -- (Dissolve 0.1 g bromthymol blue in a mixture of 20 mL 95% ethanol and 50 mL distilled water. Add enough distilled water to bring the final volume to 100 mL.)
• conductivity tester (see Lab Hints below)
• 24-well plate
• toothpicks
• cotton swabs
• plastic transfer pipets
• (disposal jar; Na₂SO₄)

Disposal-

Neutralize the contents of the disposal jar with vinegar. Add several spatulas of Na₂SO₄ to the disposal jar, and stir. Wait several days. Filter. Discard the solid with ordinary solid trash. Pour the filtrate into the sink with large amounts of running water.

Lab Hints-

• Phenolphthalein may be substituted, but it changes color well past the equivalence point.

Build conductivity apparatus:

• Disconnect the battery during storage to minimize shorting-out problems.
• After many years of trying different devices, we have reached the conclusion that the simplest device works best. When tested over excellent conductors, the diode may burn out. This has not happened often in classroom experience, however. Construct an apparatus using a 9-volt battery and a blinking LED (light emitting diode). (See a schematic diagram below.)

  

• To build the electrical conductivity apparatus suitable for use by students (Radio Shack^® catalog numbers are provided):
  • Blinking LED 276-030 (or 276-036)
  • 4 cm length black 7-mm i.d. rubber tubing
  • 9-volt battery clip 270-325
  • 9-volt battery
  • 40-cm length of two-conductor, 24-gauge, ribbon wire 278-755
  • (soldering iron, solder, razor or sharp knife, wire cutters, wire strippers)
  1. Construct the electrodes from a 10-cm length of ribbon wire. Use a razor or sharp knife to remove about 10 mm of insulation from both sides of the end the length of ribbon wire.

     !!!Click here to See Movie.

  2. The insulation is removed to expose the wires so that they can come into direct contact with the solutions. Insulation is left between the wires to keep them from touching one another thereby preventing a "short circuit." For reproducible measurements the distance and area of the exposed wires must be constant.

     !!!Click here to See Picture.

  3. Solder the red wire from the battery clip to the long lead of the LED.

     !!!Click here to See Movie.

  4. Solder the short lead of the LED to one wire of the electrode ribbon. Solder the black wire from the battery clip to the other wire in the ribbon/electrode assembly.

     !!!Click here to See Picture.

  5. Place a short length of black rubber tubing over the LED to improve visibility when faint flashes are emitted.
  6. Note the increased visibility in a light room with the tubing in place.

     !!!Click here to See Movie.

Answers-

Q1. Describe the conditions that lead to high conductivity in a solution.

A2. High concentrations of ions, charge carriers, lead to high conductivity.

Q2. Predict the conductivity of a barium chloride solution.

A2. BaCl₂, an ionic substance, is a strong electrolyte, so its solutions are conductors.

Q3. Barium sulfate, BaSO₄, is a "sparingly soluble" salt. What does dissolve does dissociate, but not very much dissolves. Predict the conductivity of a saturated BaSO₄ solution.

A3. Although BaSO₄ is a strong electrolyte, the solubility is so low that saturated solutions are not good conductors.

Q4. Account for the observations in this experiment.

A4. When HCl is titrated, one source of ions (HCl) is replaced by another source of ions (BaCl₂). When H₂SO₄ is titrated, the formation of water and BaSO₄ removes ions continuously, and the only source of ions is a declining concentration of H₂SO₄. The conductivity of the solution is minimal at the end point.

Key Words 1-

electrolyte, strong electrolyte, weak electrolyte, acid, base, neutralization, conductivity, pH indicator, end point, precipitation, solubility

Elements-

Ba S Cl