Pretend you are a teacher preparing a quiz for the text I will provide. 
            Each quiz question should follow the format below. 
            Ensure that you carefully recheck the questions and answers for accuracy. 
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                    "question": "WRITE THE QUESTION HERE",
                    "options": ["A) OPTION_1", "B) OPTION_2", "C) OPTION_3", "D) OPTION_4"],
                    "answer": INDEX_OF_CORRECT_ANSWER_IN_OPTIONS_LIST,
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            ]

            Text:
            
• Mg EDTA and Zn EDTA are frequently used weak EDTA complex.
• The weak metal EDTA complex is replaced with strong metal
EDTA complex.
• The equivalent Amount of M2 freed from the weaker complex 
can be titrated with standard EDTA solution.
• This method is useful for the determination of Ca+2 ion.
Ca2+ + MgY2- →  Ca Y2- + Mg2+
Mg2+    +  EDTA → [ Mg-EDTA] Complex 
Cadmium, lead and mercury can also be determined by this 
titration .
4 .Alkalimetric titration:
• When a solution of disodium EDTA , Na2H2Y, is added to a
solution containing metallic ions, complexes are formed with the
liberation of two equivalents of hydrogen ions:
• M2+ + H2Y2- → MY2- + 2H+
• The free hydrogen ions can be titrated with a standard
solution of sodium hydroxide using an acid base indicator.
The solution of the metal to be determined must be
accurately neutralized before titration.
Methods of increasing selectivity of EDTA
Methods
pH control 
Masking
Demasking
Chemical 
seperation
1. pH Control: 
• In this technique the selectivity increase by controlling pH of
the solution, that is depend on hydrogen ion concentration.
• E,g Ca+2 can be determined in presence of Mg+2 in strongly
alkaline solution (ph ˂ 10).
• First group:  trivalent and tetravalent cations e.g.(Bi+3,Fe+3,Th+4) and Hg+2
titrated (from stable complex) at pH 1-3 using conc HNO3.
• Second group: divalent metal 
• e.g (Co+2,Ni+2,Cu+2,Zn+2 ,pb+2 and Cd+2) titrated (from stable complex) at ph
4-6 using acetate buffer.
• Third group: alkaline earth metal
• e.g (Ba+2,Sr+2,Ca+2) and Mg+2 titrated (from stable complex) at pH =10
using amonia buffer or 8%NaOH.
2. Masking agents: 
• Masking: Means a process in which a substance, without physical
separation, is transformed that it does not enter into a particular
reaction.
• By the use of masking agents, some of the cations in a mixture can often
be masked, so they can no longer react with EDTA or with the indicator.
• Example of masking agent is the cyanide ion; which forms
very stable cyanide complex with Cd2+ , Zn2+ , Hg 2+, Cu2+ , Co2+
, Ni2+ , Ag+, Pt,
• whereas, manganese and lead form weak complex, while Ca2+
, Mg2+ do not form complex at all with cyanide.
• M2+ + CN- →  [M (CN)4]2-
• It’s possible to determine cations such as Ca2+ , Mg2+ ,
Pb2+ , and Mn2+ in presence of the mentioned metals by
masking with an excess of potassium or sodium cyanide.
3. Selective demasking:
• Demasking :It is a process in which the masked substance
regains its ability to enter a particular reaction.
• The cyanide complexes of cadmium and and zinc may be
demasked with formaldehyde-acetic acid solution, which will
give Zn2+ & Cd2+free again.
• [Zn(CN)4]2- + 4H+ + 4HCHO →Zn2+ + HO.CH2.CN
• The use of masking and selective demasking agents permits
the successive titration of many metals.
4. Chemical seperation:
• In this technique the selectivity increase by seperation of the 
species from other components from the sample solution.
• The seperated species is then dissolved in sui

            Quiz: