[jkorn]

I recently  done  a conducumetric titration to investigate the interaction between metal ions and pyrophosphate once i plotted my graphs for  all  four  metal salts theres a clear trend,  a decrease until a certain point and then infinite increase. but what i cant wrap my head around at the moment is the decrease? i know im going to kick myself here, little help please?

[jkorn]

no ideas?

[Arkcon]

I have no specific knowledge, but I'm intrigued enough by the topic.  What else can you tell us about your experiments.  For starters, conducumetric, is that a typo?  Because Googleing for it only gives me this forum.  What metal ions, and what pyrophosphate salt do you use?  And the brand and model of instrument?

[Borek]

Decrease means ions 'dissapear' from the solution. To do so they must react - complexation, neutralization, precipitation are best bets.

[jkorn]

well ive been thinking the pyrophosphate (sodium pyrophosphate) forms complexes with ions hence decreases the conductance and once it is saturated the conductance will obviously rise with the metal ion being added.

[Borek]

Sounds logical, although depends on the metal ions. Some will precipitate, some will be complexed. Note that conductance afterwards wil not grow to infinity, there is an obvious limit given by the concentration of titrant.

[jkorn]

well yes, just a bad choice of words there,the metal ions used were Mg, Ca , Cu, and Mn and the same trend was seen  in all. Something that is bothering me though is why the pyrophosphate solution had a conductance of say 230 at an added volume of  0ml but then decreased once the first couple of 0.2 ml additions were made? why have a higher conductance with no metal ions added than when some are added and complexed?

[Borek]

At least two reasons I can think of. First - complexes can be either chargeless or have smaller charge then original ions, thus the charge that can be used for transfer is smaller. Second - complexes are bulkier and as such less mobile. Both things should end with lesser conductivity.

Could you describe your experiment in more details? You started with what (pyrophosphate?), you added what (listed cations?), you observed what (V shaped curve - first a decrease, then an increase in conductance)?

[jkorn]

u have pretty much  described the whole experiment  really, i will post my graph and then actually look at my notes and write the experiment exactly

[Yggdrasil]

Mg and Mn bind pyrophosphate quite well.  These metal ions are commonly found associated with pyrophosphate groups in crystal structures of proteins containing ATP (which has a triphosphate moiety).