[aeacfm]

in our lab we measure sulfides in water spectrophotometry or by the method known as methylene blue method , but due to the highly interfering ions present in the water we intended to use the other method iodometry titration.

after prepering the sample and performing titrations the volume of thiosulfate used was very high so that according to calculations the concentration of sulfides was in minus sign ....lol!!!!

i dont know what can i do ?!!!!1
i supposed the following :

- may thiosulfate react with other component in the sample together with iodine.
- may be interfering ions still present (so i washed the sample many times - still the confusion present)

i dont know what is the problem can any body help?

[el13]

you can try potentiometric method, using AgNO3 solution.

[aeacfm]

we exactly compare between many methods to finally use precise and fast method .

but still want to know what happened there ?

[Pradeep]

Sulfide is at its minimum oxidation  number (-2). Therefore liberation of iodine is not possible according to me. Did you got a blue color with starch ? But at higher acidity thiosulphate can be reacted by depositing sulfur. 

[aeacfm]

Sulfide is at its minimum oxidation  number (-2). Therefore liberation of iodine is not possible according to me. Did you got a blue color with starch ? But at higher acidity thiosulphate can be reacted by depositing sulfur.  

i cant understand you
could you please explain more ?

[AWK]

Are you sure you did calculations correctly. Show them!

[aeacfm]

Are you sure you did calculations correctly. Show them!

I2 = 0.0187 N
Na2S2O3 = 0.0246 N

100 ml sample  + 25 ml I2 soln took  55ml Na2S2O3....

so equivalents of S-- in sample would be = (0.0187 N *25 ml ) - (0.0246 N *55 ml )
which give us minus no of equivalents = -0.8855 eqs

[AWK]

Error in concentration of iodide is the most probable, eg 0.187

[aeacfm]

Error in concentration of iodide is the most probable, eg 0.187

 i dont think , because i was going to prepare 0.025 N but mis-weighting make it 0.0187
any way thanks alot

[AWK]

Check your all calculation when preparing solutions

[aeacfm]

i can understand that , but why you exclude other factors?

[AWK]

If  your sample, in fact, contains sulfides, an  error in preparing solutions is the only possibility.

[aeacfm]

even if sulfides traces ?
no possibility for interferene ?