blogfast25 - 7-11-2010 at 12:50
Using the galvanic cell (Cu/Cu2+/Pt) described here:
http://www.sciencemadness.org/talk/viewthread.php?tid=14764
… I’ve been running the first of a series of redox titrations using potentiometric endpoint determination. For this a 0.1 N solution (nominal, not
standardised) of Mohr’s Salt (Fe2+) was prepared and titrated with KMnO4 and K2Cr2O7 (both nominally 0.1 N).
With KMnO4 the titration curve (cell potential U v titrant volume added) was as smooth as a baby’s bottom and with a very sharp endpoint of about
ΔU ≈ 400 mV, coinciding with the visual endpoint (KMnO4 excess turns titrated solution slightly pink at endpoint). Several have now been
carried out with excellent reproducibility. The 10 ml to be titrated were in all cases acidified with the same amount of 1 M H2SO4.
K2Cr2O7 too gave a smooth curve with short cell stabilisation times but a much smaller potential jump, ΔU ≈ 100 mV, still strong enough for
accurate endpoint determination. The 10 ml to be titrated were acidified with the same amount of H2SO4 1 M.
Then some ‘real’ titrations on a sample of pewter stock solution, estimated [Sn2+] ≈ 0.1 N.
First up, ferric alum 0.1 N as titrant solution. That didn’t work at all: no endpoint. I then tried the titration with visual endpoint (KSCN for
Fe3+) and found that the stock solution contains Fe3+ to start with! This iron comes from my HCl but should strictly be in (II) state…
The same stock solution was then titrated with KMnO4 which gave a strong potentiometric endpoint but slightly in the wrong place: it suggested that
the pewter contained only 75 % tin, hardly likely…
With K2Cr2O7 I obtained no discernable endpoint on the pewter stock solution…
I suspect the pewter stock solution is at fault: either not enough aluminium was used to reduce the Sn IV to Sn II or residual nitrate ions (from the
dissolution of the pewter with aqua regia) are playing up… A new sample will be prepared, this time reducing the freshly obtained solution of the
pewter to almost nothing, twice, thereby eliminating residual HNO3…
[Edited on 8-11-2010 by blogfast25]
blogfast25 - 8-11-2010 at 12:58
A new pewter digestion/reduction procedure was tried: about 1.5 g pewter, accurately weighed, digested first with 20 ml of 22 % HCl, then digested the
residue with 5 ml 38 % HNO3. Boiled that down to almost nothing, then added 100 ml 1 M H2SO4 and boiled down again.
Another 100 ml of 1 M H2SO4 and 1 g of aluminium was added to carry out the reduction from Sn IV to Sn II. The reaction of Al with the H2SO4 was quite
slow. After cooling the solution was quantitatively transferred into a 250 ml calibrated flask and diluted to the mark. This is the stock solution of
pewter, approx. 0.1 N in Sn2+.
Three titrations of 10 ml stock with 0.1 N KMnO4 gave very close results and very strong potential jumps of about 500 mV. The average volume titrated
indicated a tin content in the pewter of about 95 w%, more or less in line with expectations.
There is unfortunately some measuring error coming from the Fe3+ that contaminates my HCl: on reduction it too reduces to Fe2+ which is titrated with
the Sn2+ by the KMnO4. For that purpose I’d like to get rid of the HCl and use H2SO4 instead (H2SO4 is actually cheaper on an equivalent/equivalent
basis).
So I tried dissolving the pewter in 6 N H2SO4 but to no avail: it’s completely impervious to it…
blogfast25 - 10-11-2010 at 13:14
Yesterday I replaced the platinum wire active electrode with a titanium (>99 %) rod, about 0.5 cm diameter, cleaned thoroughy and titrated an Sn2+
solution with KMnO4.
Surprisingly the starting potential was positive (+) by about 100 mV. The titration proceeded uneventfully but with an endpoint potential jump of
about 100 mV, as opposed to the usual 400 – 500 mV with platinum. Readings weren’t much more stable either (there is considerable drift also with
the Pt electrode). I wonder how cell potential could be made more stable…
[Edited on 10-11-2010 by blogfast25]