can anyone tell me the error i think i've made in mixing the ingredients to make silver thiosulphate?

Question

i am doing a botany experiment involving silver thiosulphate solution. i have made the solution as per advised with 2.5 grams of silver nitrate. made up separately and 5 grams of sodiumthiosulphate. made separately. both were disolved into 2 x 500 mills of distilled water. once disolved they were mixed together while stirring briskly. when the solution had cleared there was a fair amount of black sediment suspended in the bottom of the solution. is this normal and has it mixed correctly or have i erred with something. the solution is supposed to stain skin brown.permanently. it does not do so..as you can see, i'm no chemist. just an keen amateur botanist. one last point, both the ingredients were both pure as . thank you.

Answer 1

There seem two possibilities. One is that your sodium thiosulfate, Na2S2O3, was "old," and it had hydrolyzed to a certain amount of sodium sulfide, Na2S. Then,

2AgNO3(aq) + Na2S(aq) ===> Ag2S(s)(black) + 2NaNO3

The second possibility is that there was something in the solution that reduced silver ions to silver metal, which is also black in its finely divided state:

AgNO3(aq) + e- ===> Ag(s) + NO3-

You might try filtering the black sediment from the solution and going ahead. If this is a qualitative and not an exact quantitative botany experiment, you may get good results anyway. Else, get a different source of Na2S2O3.