Manuela Junghans wrote:
> we are having problems measuring the delta 34S of our sulfate samples. After
> heating the sample to 950 C, some white precipitates form in the glass tube.
> This causes problems getting the necessary vacuum and the measurements are
> We assume that our sample is responsible for that problem (sulfides work
> just fine). We precipitated the barium sulfate from the original water using
> BaCl2. The barium sulfate was heated to a temperature of 650 C, cooled and
> prepared for the measurement (mixed with V2O5).
> The original water is a highly mineralized pore water from material
> influenced by sulfide oxidation (pH=2.3, el. conductivity = 3 mS/cm, sulfate
> concentration 1 to 5 g/l, 50 to 800 mg/l).
> Does anybody have had similar problems or any idea what the precipitate
> might be?
> I appreciate any suggestions.
An usual problem is the coprecipitation of chloride, nitrate and phosphate
(among others). The chloride coprecipitation can be reduced by slow addition
of the BaCl2 solution and stirring at ph 2 -- 2.5, 70°C -- 80°C. (Give the
BaSO4 2h -- 3h at 70°C -- 80°C to grow.) Then I used to wash the BaSO4 with
small amounts of hot water chloride free and heated 1h -- 2h at 600°C.
Dr. Stephan A. Junk
TU Bergakademie Freiberg
Lehrstuhl fuer Archaeometallurgie
Tel. +49.3731.392899 FAX +49.3731.392489