F. R. Meeks and L. L. Meal University of Cincinnati Cincinnati, Ohio 45221
Analysis of Ba"(aq) in the Presence of BasO.(s)
In a study of BaSOn precipitation reactions in agar gel, it became necessary to develop a method for determining separately the amount of barium as Bai+ in a solution in contact with solid BaSOain the same system. A rapid and accurate method was devised by modifying the well-known titrimetric determination of barium with I3DTA.I The main advantages of this procedure over othersZare the sharpness of the endpoints and the speed with which the dctermination can be carried out. Experimental
An EDTA solution was prepared and standardized with a standard CaC03 solution. The standardized EDTA solution was then used to standardize a BaCh solution. Approximately 40 ml of doubly distilled water was added to each sample and a methylthymol blue indicator tablet (Metab, manufactured by Fisher Scientific) added to each. Some samples contained barium only in the form of an accurately weighed amount of reagent grade BaS04 powdcr, while others consisted of a measured aliquot of standardized BaCL solution in addition to an accurately weighed amount of BaS04. .In place of the usual NH3-NH&I buffer, diethylamine was used and was found to buffer the solution quite efhciently during these titrations. Preliminary work, which involved the measurement of the pH of the solution with a pH meter and a low sodium glass electrode a t various intervals during the titration, revealed that the sharpest endpoints mere obtained when the solutions being titrated were buffered to an apparent pH lying between the values of about 11.2 and 12.0. When the pH was adjusted to values much higher than 12, there was no color change observed at the endpoint. Therefore, about 10 drops of diethylamine were added to bring the pH of the diluted sample to approximately 11.5 The color of the Ba2+-methylthymolblue complex in the previously mentioned pH range is deep blue, while the uncomplexed form of the indicator gives the solution a light pink appearance. Thus, when barium is titrated with EDTA, the color change at the endpoint is from blue to pink and is quite sharp. The solution color of the samples containing barium only as solid BaSOI was pink, indicating the absence of free barium ion iu solution. To these samples there was added an excess of standard EDTA solution. The excess added was approximately four times the amount which would have been necessary to complex the barium REILLEY, C.N., SCHMID, R. W., A ~ SADEK, D F. S., J. CHEM. Enuc., 36, 555 (1958). BELCHER, R . , GIBBONS, D., AND WEST, T. S., Chem. Ind., 127 (1'354).
W O R R I A. ~ , G. C., Chemist-Analyst, 48, 76 (1959).
542 / Journol of Chemical Education
present3 as BaSO,. Next about 2 ml of 0.5 M NaOH solution was added to each sample and the samples were heated to boiling. Within 5-15 min all the BaSOndissolved. The solutions were cooled and five more drops of diethylamine and another Metab were added. This solution, which a t this point was a rosy-pink color, was hack-titrated with standard BaC12solution. The color change at the endpoint is from rosy-pink to dark purple. Although this color change is not difficult to detect, i t was found that fluorescent lighting made it even more easily discernible. The samples containing barium as free barium ion and as BaS04were first titrated with EDTA to the blue to pink color change in order to determine the amount of free barium ion. After reading the volume of standard EDTA required to titrate the free barium, the four-fold excess of EDTA was added, and the analysis for barium as BaS04was carried out as described above. In addition to the samples mentioned above, samples containing ignited BaS04were also analyzed for barium. Results and Discussion
The table shows the titrimetrically determined amounts of barium as BaS04to be slightly low in every case, with values ranging from 0.08 to 2.40y0 below the amount weighed out. The error is somewhat greater in some of the larger samples and in the samples which contained ignited BaS04. It is felt that the lowness of the results could perhaps be attributed to incomplete dissolution of the BaS04, since the larger samples and the samples containing ignited BaS04 were more difficult to dissolve than the others. It should be noted that the presence of barium as BaS04 does not interfere with the determination of free barium ion. Thus, this method offers a rapid, accurate means for determining barium as BaSOdand for determining free barium ion and BaSOa in the same system. Data Obtained from Titrations Meq. barmm 8"s
BaS04 added
Meq. barium its ignited Bas04 added
Meq. of free barium ion added
Meq. barium as
Bas04 found
Meq. barium BS
ignited Bas06 found
% error
%
error
Meq. of free ba+m ,on found
%
error
