This test was performed: Fluoride test Place one drop of SOLUTION in a tube and

Question

This test was performed: Fluoride test Place one drop of SOLUTION in a tube and add two drops of deionized water and one drop of 0.005 M Zr/alizarin. Mix thoroughly. The conversion of the red Zr/alizarin "lake" to the yellow alizarin dye confirms the presence of the F– ion.

Explain why the test for fluoride would be unsuccessful if the Zr/alizarin reagent is quite a bit more concentrated than it is supposed to be (which actually happened one year). Include a specific observation and a specific potentially incorrect conclusion

Explanation / Answer

The Alizarin dye reacts with Zr4+ ion in HCl/ H2SO4 mixture to give red coloured Zr-Alizarin lake. The Zr-Alizarin lake is bleached to lighter colour when F- ions react with Zr4+ ion to form colourless and stable ZrF62- complex. The bleaching action is dependent on the concentration of the F- ion and it is very slow for F- ions. Hence, after mixing the Zr-Alizarin lake and sample one has to wait for at least one hour to notice the colour change. If the concentration of Zr-Alizarin lake is higher, the wait time will be longer. With very low concentration of F- ions, the colour change will not be prominent. Note, that Cl- and Br- ions can interfere with the F- ions detection thus being a source of error.

If these measurements are done using spectrophotometer, it obeys inverse Beer-Lambert’s law. The absorbance of the bleached solution is inversely proportional to the concentration of the F- ion. If the concentration of Zr-Alizarin lake is high, then in the solution there will be unreacted Zr-Alizarin lake, alizarin dye and colourless ZrF62- complex. The absorption of this sample will be higher due to absorption by Zr-Alizarin lake and alizarin dye thus giving faulty results.

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