11pH ranges of :
phenophtahlein:8-10
methyl orange: 3-4
thymol blue:1.2-2.4
Q1 what are the reactions involved when phosphoric acid h3po4 is titrated using naoh standard solution?
Q2 when h3po4 is titrated using naoh solution, phenopthalien end point and methyl orange end points are observed clearly which is used for analysing the amount of h3po4 acid in any sample but the thymol blue end point is not observed in this titration which creates a problem in this method.why does this happen?how is this problem solved?
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7 Answers
11
23H3PO4 ,
ON of P = +5 so all hyd are acidic .
now if complete titration takes place den
H3PO4 + NaOH - > Na3PO4 + H2O
so pH > 7
if complete titration doesnt take place then
2 cases
[1) H3PO4 is converted to NaH2PO4 so soln is still acidic
[2) H3PO4 is converted to Na2HPO4 soln still acidic but less acidic than above case 1
11all the hydrogens are not acidic only two hydrogens are acidic...the third proton can't be removed using naoh
btw how did u know that all the hydrogens are acidic by calc. the oxidation no of P.plz explain.
11read this from 7th line
http://www.eyethinkcorp.com/download/colapage.pdf
another thing if the following reaction occurs(which doesn't happen):
H3PO4 + NaOH - > Na3PO4 + H2O
pH will not greater than 7 but much lower than 7
almost between 1-3.
11sry i wrote wrong all the hydrogens are acidic but all are not ionisable through naoh