C_30J_Topic_2_Contamination_and_interferences_part_2

C_30J_Topic_2_Contamination_and_interferences_part_2 -...

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Topic 2.2 Removal of Interferences SWHC 8 th edition Chapter 30 a) Selective precipitation : eg: the determination of total base and OH - content of Bayer (bauxite to alumina) process liquors. Bauxite (soil with aluminum and iron oxyhydroxide minerals) digested in caustic soda (~2.5 M NaOH) to dissolve the alumina minerals. The insoluble fraction is separated from the alumina saturated liquor and Al(OH) 3 is precipitated. The remaining liquor is recycled to digest more bauxite. The liquor becomes very complex containing, amongst other things, OH - , CO 3 2- , Al(OH) 4 - and Na + ions. Titrate a diluted sample of the liquor with standard acid to determine total base (OH - + CO 3 2- ). (see C 10 lab manual) after masking Al(OH) 4 - . Determine OH - after precipitation of CO 3 2- with excess BaCl 2 (K sp BaCO 3 = 5 x 10 -9 ), filter and then titrate the remaining OH - to the pH 7 equivalence point (mask Al(OH) 4 - ). K sp = a(Ba 2+ ) x a(CO 3 2- ) = γ (Ba 2+ )[Ba 2+ ] x γ (CO 3 2- )[CO 3 2- ]. μ α μ γ X X Z X 3 . 3 1 2 51 . 0 log + = ...) 2 ] [ 2 ] ([ 2 1 + + = B Z B A Z A μ μ – ionic strength Precipitation can also be used to pre-concentrate analytes. If μ > 0.1 use conditional solubility products
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b) Masking – Use a reagent to “speciate” the interferant so that it will no longer participate in the analytical reaction.
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C_30J_Topic_2_Contamination_and_interferences_part_2 -...

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