Clearing Water with Alum

Introduction

Where does drinking water come from and how is it purified? In this demonstration, a commercial purification procedure for removing suspended solids from drinking water will be introduced.

Concepts

• Precipitation
• Water purification

Materials

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The liquid may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b. The sand may be placed in the trash according to Flinn Suggested Disposal Method #26a.

Prelab Preparation

Before beginning the demonstration, ask the students where their drinking water comes from. The initial discussion may revolve around your area’s waterways and the condition of these waterways. Ask the students where the water goes when they flush the toilet. Many students will be surprised that it may be to the same source as their drinking water.

To lead directly into the demonstration, ask the students how dirt and other solid particles may be removed from drinking water. Many students are under the impression that some form of mechanical filtration such as a paper or metal strainer is used. Stress that these methods may be impractical for filtering large volumes of water due to the propensity of the system getting clogged and restricting water flow.

Procedure

1. Fill a 600-mL beaker with about 500 mL of water.
2. Add about ¼ teaspoon (~1.5 g) of alum to the beaker and stir until dissolved.
3. Add a tablespoon (23 g) of sand to the beaker. Stir to make the water cloudy.
4. Add 1 teaspoon (5 mL) of ammonia, stir very gently once or twice. Once the ammonia is added, a white fluffy solid will begin to form around the sand particles. The sand particles will then be pulled down through the solution and settle to the bottom.

Teacher Tips

• This kit contains enough materials to perform the demonstration seven times.

• While you are waiting for the sand to settle, this may be a good opportunity to explain the many uses of alum other than as a means to purify water.
  a. As an astringent. A solution of alum on a wound will constrict the blood vessels and stop bleeding.
  b. As an emetic to induce vomiting.
  c. As a pickling agent
  d. As a mordant (binder) to fix dye to cotton and other fabrics, rendering the dye insoluble.
  
• As an extension to this activity, you may wish to further test the water for bacterial contamination. Set up two beakers of the water samples that have been treated with alum. Add a few drops (the normal dosage used by water purification plants is 8 drops per gallon) of ordinary household bleach (5.25% sodium hypochlorite) solution to the first beaker. Leave the second beaker untreated as a control. Allow the treated water to sit for half an hour. Use sterile loops (and other appropriate materials) and place samples of each solution on sterile nutrient agar plates or Petrifilm™. Allow the samples to sit for a few days to a week and compare the results. More quantitative results may be seen if sample water is tested using Flinn Scientific’s Bacterial Pollution (Coliform) Kit (Catalog No. AB1152).

Discussion

Many of the suspended particles in water are so small that their removal by filtration is just not feasible. Most of these small particles are negatively charged. The negatively charged particles are naturally repelled and remain suspended in water. A process known as coagulation is used to help “break up” the suspended particles. Coagulation is a chemical technique that destabilizes the particle suspension in water. In this demonstration, coagulation is put into motion by using a chemical known as alum (aluminum potassium sulfate). The alum helps neutralize the negatively charged particles and allows for attraction of the particles.

In order for coagulation to occur using alum, a certain degree of alkalinity is required. When ammonia is added to the water the optimum pH is obtained. If the pH is correct, alum reacts with ions naturally found in water to produce insoluble precipitates such as aluminum hydroxide, Al(OH)₃, and calcium sulfate, Ca(SO₄). See Equation 1. The insoluble, neutrally charged particles will then fall downwards out of solution (see Figure 1). In this demonstration, the entire process should occur in less than a minute.

Coagulation is generally followed by a technique known as flocculation. Flocculation is a slow mixing technique that further promotes the aggregation of the coagulated particles. After coagulation and flocculation, a water treatment facility would then have to siphon off the top layer of clean water and treat it with chlorine (commonly in the form of bleach) to kill bacteria.

References

Flinn Scientific would like to thank Dale Moss, Carrick High School, Pittsburgh, PA, for the idea for this demonstration.