Ba(OH)2(aq) + 2 HCl(aq) BaCl2(aq) + 2 H2O()Phenolphthalein can be used as an indicator for this titration.
This method really determines the total ability of the lake water to neutralize acids- not the actual Ba(OH)2 concentration. Other basic substances like carbonate and bicarbonate in the water may not be distinguished from the barium hydroxide:
2 HCl(aq) + CO32-(aq) CO2(g) + H2O()+ 2 Cl-(aq)
HCl(aq) + HCO32-(aq) CO2(g) + H2O() + Cl-(aq)
Ba(OH)2(aq) + H2SO4(aq) BaSO4(s) + 2 H2O()The endpoint of the titration can be detected using any of the methods outlined above. A 20 mL sample of lake water will require about 10 mL of 0.1000 M H2SO4 solution for complete neutralization.
Ba(OH)2(aq) + Na2SO4(aq) BaSO4(s) + 2 NaOH(aq)The BaSO4 can be filtered, dried, and weighed. The mass of BaSO4 obtained can then be related to the molarity of Ba(OH)2 in the lake water by a simple calculation.
The BaSO4 crystals are very fine and difficult to collect by filtration. The crystals can be made larger by heating the solution to a temperature just below boiling for an hour or two. The precipitate is filtered, washed, dried, and then fired to complete dryness in a muffle furnace. It's a very tedious analysis but done with care it is highly accurate. Given the very high concentration of Ba(OH)2 in your sample, gravimetric analysis should work well here.
Carbonates may be a problem again. As the highly alkaline Ba(OH)2 solution is exposed to air, it will absorb carbon dioxide. The carbon dioxide absorbed ultimately precipitates as BaCO3. When you weigh the BaSO4, you may also be weighing some BaCO3.
A better gravimetric method for determining barium in natural waters is given in Official Methods, protocol 33.100. The barium is precipitated as barium dichromate (BaCr2O7) and fired in a muffle furnace to constant weight BaCrO4.
If the lakewater sample is neutralized, adjusted to pH 3.5, and diluted by a known factor prior to this analysis, alizarin red S can be used as an adsorption indicator. Alizarin red S is yellow at this pH, but it forms a pink complex on the surface of BaSO4 crystals. It will only be absorbed when the surface of the particles have a positive charge (when excess barium is present). If a measured amount of Na2SO4 solution is titrated with the Ba(OH)2 unknown, the endpoint is signalled by a sharp color change from yellow to pink. To make the color change more intense, the surface area of the BaSO4 precipitate should be as large as possible. Adding methanol to the sulfate solution befor starting the titration keeps the BaSO4 from forming large crystals.
Official Methods protocol 33.102 recommends precipitating the barium as barium dichromate. The precipitate is filtered off, washed, redissolved in HCl, and treated with 10% KI solution. The dichromate from the precipitate oxidizes the I- to I2 and I3-. Titrating with sodium thiosulfate (Na2S2O3) solution gives the amount of iodine, and so the amount of dichromate and the amount of barium.
Author: Fred Senese email@example.com
Copyright © 1997-2010 by Fred Senese
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Last Revised 08/17/15.URL: http://antoine.frostburg.edu/chem/senese/101/moles/faq/print-baoh2-assay.shtml