Complexometric titrations masking agents

What are masking and demasking agents with reference to complexometric titrations Give specific examples to justify your statements. 

Oxyacids, like citric or tartaric acids, and polyols, like saccharose are also used, mainly as masking agents, in qualitative analysis. The action of some specific reagents, like oc-a -bipyridyl for iron(II) and dimethylglyoxime for nickel(II), is also based on the formation of chelate complexes. In quantitative analysis the formation of chelates is frequently utilized (complexometric titrations). ... 

Efficient chelating agent for heavy metals (Hg, As, Sb, Au, Cd). Antidote to As poisoning. Used as 0.0IM aq. soln. as a masking agent in complexometric titration of Ca, Mg photometric detn. of Mo (A 335 nm, e 7200) extraction-photometric detn. of As. 

Acid and salts are powerful chelating agents for metal ions. Used as pharmaceutical aids in cases of poisoning etc. Used as titrant in complexometric titrations of many metal ions photometric detn. of Cr(//7), Ni, Co masking agent. Volumetric standard for detn. of Mg,... 

The complexometric method for determination of Ca(II) and Mg(II) is based on two titrations with EDTA in alkaline solution, one where both ions are determined together and the second after one of them has been masked with a specific complexing agent. The effect of interfering heavy metals such as Cu, Fe, Mn or Zn can be avoided by adding cyanide. The AOAC Official Method 964.01 for determination of acid-soluble... 

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