Engaging questions:

  • What substances are present in the tap water?
  • Which of those substances are desirable and why?
  • What are the differences in composition of the mineral water and the tap water?
  • Can the mineral water be used in the water supply?
  • The presence of which substances in water may have a negative impact on the human body?
  • Who/what institution controls the composition of the tap water?
  • Is the rainwater safe for drinking?

Look for answers in following articles:





The amount of iron(III) [1] in water may be estimated with the use of so-called thiocyanate method. The Fe3+ ions, form with the thiocyanate ions, a complex ion of an intense red colour.

Fe3+ + SCN- = Fe(SCN)2+

Ions of iron(III) can form with the thiocyanate several different coordination compounds of similar colour: Fe(SCN)2+, Fe(SCN)2+ to Fe(SCN)63-. In the solutions where the Fe3+ cation concentrations are on microgram level, the Fe(SCN)2+ dominates. In addition, this coordination ion may undergo hydrolysis in solutions with a pH>3. The thiocyanate complex is unstable, and degrade with time.

Imagine that to the laboratory where you work, a water sample with unknown amount of Fe3+ ions was delivered. Plan an Activity  in which you determine the molar concentration of Fe3+ by the colorimetric method. Answer the question whether the water is safe for drinking. Compare the result obtained for the sample with the results for the tap water.

You have:

  • 0,1% HCl solution
  • 20% KSCN solution (CAS: 333-20-0)
  • Fe3+ standard solution of 1 mg/mL concentration

Information for the teacher: a maximum acceptable iron concentration in water is 0.2 mg/L (0.3 mg/L in some countries), recommended iron concentration in the sample: 0.05 – 0.5 mg/L.


  • Is the delivered water safe for drinking as regards the iron(III) ions concentration?
  • What is the influence of the exceeding amount of iron in water on the human body?
  • What ways of treatment/purification of water do you know?
  • How can the Activity  be modified to the concentration of the iron(II) ions be taken into account in the obtained result?

[1] After earlier oxidation of iron(II) ions, the  total content of iron in the sample may be determined with the use of the thiocyanate method.