Testing for Ions in Water

In this lab, we use qualitative tests to check for the presence of certain ions in school tap water, and students' tap water. What ions do you think are present in a sample of school tap water, water from your house? A positive test a change in colour or the formation of an insoluble precipitate indicates that an ion is present. A negative test no colour or precipitate however, does not mean that the ion is absent. (See Table 1) The ion may be present in small amounts that it does not produce enough colour or solid precipitate to be seen.

Table 1: Confirming Tests

Ion Expected Product

Fe(III)

red with SCN - (aq)
Ca(II) white precipitate with C2O4 2- (aq)

Materials

 

Procedure

  1. Make an observation table.
  2. Rinse a microtray with distilled water, and shake it dry.
  3. Using a microdropper, add one drop of distilled water to three wells in the first row of the microtray.
  4. Using another microdropper, add one drop of school tap water to three wells in the second row of the microtray.
  5. Using another microdropper, add one drop of your tap water to three wells in the third row of the microtray.
  6. Using another microdropper, add one drop of a friend's tap water to three wells in the fourth row of the microtray.
  7. Add one drop of iron(III) nitrate (reference solution a source of iron ions) to three wells in the fifth row of the microtray.
  8. To test for iron ions, add one drop of potassium thiocyanate (testing solution) to each of the 15 wells. Record your observations in your table. Note the appearance and colour of the starting solutions and any product.
  9. Using a clean and dry microtray, repeats steps 3-6.
  10. Add one drop of calcium chloride (reference solution a source of calcium ions) to three wells in the fifth row of the microtray.
  11. To test for calcium ions, add one drop of sodium oxalate (testing solution) to each of the 15 wells. Record your observations in your table. Note the appearance and colour of the starting solutions and any product.

 

Discussion

  1. What ions are present in the samples of drinking water?
  2. Why are three samples tested for each ion?
  3. What is the purpose of a control? Which solution was the control in these tests?
  4. Write balanced chemical equations to represent each of the reactions between the ions and the test chemicals?
  5. Suggest any necessary improvements.
  6. How might your observations have changed if you had not used a clean microdropper for each solution?
  7. Suggest why the test you performed does not absolutely confirm the absence of an ion.

 

 

Image

water droplet - http://www.moondance.org/2004/water04/sns/images/sns_water_04.jpg

 

Resource

Davies, L., Giuseppe M.D., Gibb T., Sanader M., Vavitsas, A. (2004). Chemistry 12: College Preparation - 4.4 Investigation: Testing for Ions in Water. Toronto : Nelson.