Analytical Chem istry - DePauw University

320 Analytical Chemistry 2.0Table 7.6 Selected Inorganic and Organic Masking Agents for Metal IonsMasking Agent Elements Whose Ions Can Be MaskedCN –Ag, Au, Cd, Co, Cu, Fe, Hg, Mn, Ni, Pd, Pt, ZnSCN –Ag, Cd, Co, Cu, Fe, Ni, Pd, Pt, ZnNH 3Ag, Co, Ni, Cu, ZnF –Al, Co, Cr, Mg, Mn, Sn, Zn2–S 2 O 3 Au, Ce, Co, Cu, Fe, Hg, Mn, Pb, Pd, Pt, Sb, Sn, ZntartrateAl, Ba, Bi, Ca, Ce, Co, Cr, Cu, Fe, Hg, Mn, Pb, Pd, Pt, Sb, Sn, ZnoxalateAl, Fe, Mg, Mnthioglycolic acid Cu, Fe, SnSource: Meites, L. Handbook of Analytical Chemistry, McGraw-Hill: New York, 1963.because it binds both Al and Fe. Thioglycolic acid, on the other hand, isa selective masking agent for Fe in the presence of Al. Other acceptablemasking agents are cyanide (CN – ) thiocyanate (SCN – ), and thiosulfate(S 2 O 3 2– ).Practice Exercise 7.6Using Table 7.6, suggest a masking agent for the analysis of Fe in thepresence of Al.Click here to review your answer to this exercise.As shown in Example 7.13, we can judge a masking agent’s effectivenessby considering the relevant equilibrium constants.Example 7.13Show that CN – is an appropriate masking agent for Ni 2+ in a methodwhere nickel’s complexation with EDTA is an interference.You will find the formation constants forthese reactions in Appendix 12.So l u t i o nThe relevant reactions and formation constants are+ − −Ni ( aq) + Y ( aq) NiY ( aq) K = 42 . × 1012 4 2 182Ni + ( aq) + 4CN − ( aq) Ni(CN) − ( aq) β = 1.7×10442 30where Y 4– is an abbreviation for EDTA. Cyanide is an appropriate maskingagent because the formation constant for the Ni(CN) 4 2– is greater thanthat for the Ni–EDTA complex. In fact, the equilibrium constant for thereaction in which EDTA displaces the masking agent2− 4− 2+−Ni(CN) ( aq) + Y ( aq) NiY ( aq) + 4CN( aq)4