Six Solutions

Introduction

You will receive six chemicals to study: aqueous silver nitrate; aqueous sodium chloride; aqueous sodium carbonate; aqueous nitric acid; aqueous sodium bromide; and water.

After studying the reactions of known samples, you will have an opportunity to analyze an unknown.

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Procedure
  1. Chemicals will be mixed systematically, two at a time, to discover the behavior of each combination. The chemicals may be mixed either in a 96-well plate or as puddles on an acetate sheet.
  2. To remove a sample from a pipet storage device, hold the tip of the pulled pipet inside the top of a well or above an acetate sheet, making sure not to contaminate the pipet by touching the tip to the well, sheet, or drops of other chemicals. Squeeze very gently. Individual drops will emerge.
  3. Use a toothpick to mix different chemicals.
  4. 96-well plates are labeled. Rows are designated with letters. Columns are designated with numbers. Each well's coordinates are embossed on the bottom of the plate; it is difficult to read. When using acetate sheets, it is best to make a labeled grid on a piece of white paper, then paper clip the acetate sheet to the paper and place the drops inside the appropriate boxes, just as you would with the labeled wells.
  5. One way to be systematic is to add one drop of silver nitrate solution to wells (or boxes) A1, A2, A3, A4, A5, A6; then A1, B1, C1, D1, E1, and F1. Two drops end up in A1.
  6. Once the silver is added, a sodium chloride solution may be added systematically. Add one drop of sodium chloride solution to B1, B2, B3, B4, B5, B6; then A2, B2, C2, D2, E2, and F2. Two drops of sodium chloride end up in B2. Silver ion and chloride ion are mixed in wells A2 and B1.
  7. Since order of addition doesn't matter in this experiment, and adding a chemical to itself rarely gives any useful information, a smaller number of combinations may be used, as shown in the diagram.
  8. If you are using a 96-well plate, fill it with tap water, invert, and "flick" at the sink to clean. Use a cotton swab to scrub residue from the walls and bottoms of the wells. If, instead, you are using acetate sheets, simply rinse off the sheet under running water and wipe with a paper towel.
  9. Your task comes in two parts. In the first part you must combine all of the solutions two-at-a-time to discover which give detectable reactions. You must decide how to organize the experiment, which observations are important, and how to record your data.

The second part of the experiment is to work on unknowns. First you must create the unknowns.

  1. Cover each chemical container with a 2" x 3/4" strip of electrician's tape. Place the tape so that you cannot read the label. Place all six containers in a box or bowl. Randomly select one of these. Place a small piece of masking tape over the electrician's tape, and mark it with a 1. Set it aside. Select a second container. Use the same procedure to mark it with a 2, and so on for all six containers. Although you will have numbered the containers systematically, since you do not know which was which you will also have numbered them randomly.
  2. Study the unknowns. Mix the chemicals two-at-a-time making careful observations. Based upon these observations and the knowledge you have from your earlier studies of the knowns, decide the relationship between the number that was arbitrarily assigned and the identity of the chemical. Write down an assignment for each unknown. When you are confident that you have correctly identified all of the chemicals, remove the electrician's tape and check your answers. Repeat the experiment if you have made any errors.
  3. Practice with just four chemicals -- Ag+, Cl-, CO32-, and H+ until you are confident about the procedure.

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Safety

Wear eye protection at all times.

Concentrated nitric acid is corrosive to flesh and causes severe burns. Solutions of nitric acid stain flesh and damage clothing. Solutions of silver nitrate are toxic and stain flesh and clothing. Avoid contact with silver nitrate and nitric acid. Wash immediately with water if contact occurs. Do not ingest chemicals

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TG Lab Hints

Acetate sheets may be used in place of 96-well plates. Laminates in which a paper sheet of a matrix with some light and dark areas has been laminated serves the purpose well. The long term suitability of the laminates is not proven at this time.

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TG Materials

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