[thasan]

hi all,
we have made a solution of:
0.4 M of sodium citrate
0.1 M of nickel sulphate
0.6 M of sodium tungstate

in 100 ml water at room temperature (18 degree C).

the initial ph was around 10.1 and we tried to lower the ph by adding about 25 ml of 5% sulphuric acid solution.
and now, we got a lot of precipitation. we cant get rid of this even if we heat it upto 80 degree C and stir it with a magnetic stirrer for 6 hours +.

can anybody please help in finding what is going on? and what this precipitation might be?

thanks.
TH

[Borek]

Call it educated guess at best:

Nickel is complexed by citrate anion. Lowering pH you are protonating citrate converting it to citric acid, thus increasing concentration of free Ni²⁺ ions. I doubt nickel tungstate is soluble.

Try mixing tungstate with nickel salt without citric acid addition - precipitate will confirm my guess.

[thasan]

hi borek,
thanks.
i will do that.
i will explain what we did a bit further:

we used 5% sulphuric acid with the initial solution to change the ph...and there were precipitation...but we stirred it for 6 hours at 80 degree c and it was gone.
everything was fine even after we cooled it down.

then, we changed the ph by putting more sulphuric acid. this time, the volume of the solution content increased by about 25% (by the water from h2so4 solution).
so, we added same M concentrations of salt (1/4th of previous since we have 25 mm of water more making it 125 ml of solution. i hope u remember that we had 100 ml of water atfirst)
then we got this precipitation which we cant get rid of. we have stirred it so quite a long time (48 + hours) at 80 degree C, and some of it has gone, but around 5/6 cc is still remaining ...

my question is, why it is happening this time only? any guess?

regards,
tawfique

[Borek]

my question is, why it is happening this time only? any guess?

For exactly the reasons described before. You are lowering concentration of dissociated citric acid.

[thasan]

hmm....but ni tungstate looks yellowish, the precipitation we are getting is completely white.
at the moment, i am continuing a 6 hour heating and stirring and it looks a lot better now after i increased the temperature to around 85 degree C. i will also try to see what happens by doing what u suggested before.
thanks and regards,
tawfique

[thasan]

just for an update:
clearly, it was not ni tungstate. the precipitation dissolved completely when heated and stirred at 95 degree C for an hour. the solution is at 18 degree C now, and till now, it is clear without any kind of precipitation...
any idea what it might was?

thanks

[Borek]

Perhaps some polyacids creation... but that's terra incognita for me.

[thasan]

he he he might be...
thanks a lot borek.
i really appreciate your help.

tawfique