[cool32]

good morning,

please if I make mistakes in the language please forgive me because I am average in English and I hope it will not bother you.

Titration of bismuth ions
In a 250 mL beaker, put a test sample of 20.0 mL solution containing Bi 3 + and Pb 2+. Adjust the pH = 3 with a few drops of NH4OH 1/2. Check the pH with a piece of paper Congo red. Add the indicator. The test solution must take the red hue (color of the complex of Bi 3+  xylenol orange). Titrate with the E.D.T.A until the yellow color (color of the free indicator).
Titration of lead ions-At the same solution, adjust pH to 5 - 5.5 with solid salt hexamethylenetétramime until the appearance of a red hue (color complex Pb 2+ xylenol orange) and titrate up at turning from red to yellow (color of the free indicator).

Why we can not get the color of the Pb-Ind complex (witch is red) until it has alkalizing with hexamethylenetétramime ??

Please have the kindness to explain to me because I looked everywhere and I find no answers

I thank you in advance

waiting for your answers impatiently. 

[Arkcon]

I am not sure, I'm not an expert on every indicating reagent for each heavy metal. 

That said, I'd assume this protocol was written this way, because bismuth interferes.

You can certainly check for yourself.  You will probably want a pure standard of both lead and bismuth to check your procedure, before you work with an unknown.  Try and see if bismuth present interferes.

[AWK]

Both complexes of xylenol orange are stable at different pH, and EDTA forms stronger  complexes (colorless or almost colorless) than indicator at these pH.
Note, you should titrate two separate samples with adjusted pH.

[cool32]

I thank you for your answers how nice
in fact it is to titrate a mixture of Bi and Pb, Bi is titrated in first and after turning from red to yellow pH is adjusted to 5.5 to titrate Pb and on the same sample precedent.

I think and I do not know if it is correct that this  alkalizing allows the indicator to give a complex with Pb more stable (whicth is impossible at pH=1 and that's why it's still yellow) or to prevent the precipitation of Pb but I do not know if I'm on the right track.

[Arkcon]

So, like I said, you will use pure lead salts, pure bismuth salts, and prepare them separately and together, and then try it.  Right?  Then you will know, won't you?

[cool32]

in this exercise:

A solution simultaneously contains bismuth and lead. 10 ml of solution is taken and titrated in very acidic (pH = 1) 4.85 ml of a standard solution of EDTA 0.04 M. After the indicator changes, the same solution is brought to pH = 5.5 and titrated again with 6.2 ml of standard solution of EDTA 0.04 M.
1- Give all possible reactions
2. Justify the choice of the media pH
Bi: M = 209 log Ks = 27.8 alphay4- to pH = 1: 1,9.10-18
Pb: M = 207 log Ks = 18, 04 alphay4- at pH = 5: 3,7.10-7

by calculating the stability constant of the two metals for both pH I find they are stable for both pH and the difference of constant CONDITIONAL for both pH is sufficient to allow the selective dosage of each metal so I think this alkalizing returns to the stability of the indicator with the lead !!!

Please if someone could clear up me !!!

thank you