Gravimetric sulfate: blank correction and a failed duplicate
A 0.5000 g sample of a soluble sulfate salt is dissolved in 150 mL of water and acidified with 2 mL of concentrated HCl. Sulfate is precipitated by adding 10% BaCl2 solution. After digestion on a hot plate, the precipitate is filtered through ashless paper, transferred to a tared porcelain crucible, ignited at 800 °C, and cooled in a desiccator. Crucible tare: 18.4547 g. Crucible plus BaSO4: 18.7421 g. A reagent blank run through the same procedure leaves 0.0008 g of residue. A duplicate sample gives a crucible-plus-precipitate mass of 18.7445 g. The method specification requires duplicate results to agree within 0.15% absolute sulfate. The student added BaCl2 rapidly to the hot solution and filtered after 10 min.
1. Calculate the percent sulfate for both determinations, including blank correction. Do the duplicates meet the 0.15% absolute requirement? Show all steps.
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Precipitate 1: 18.7421 − 18.4547 = 0.2874 g; blank-corrected = 0.2874 − 0.0008 = 0.2866 g. Precipitate 2: 18.7445 − 18.4547 = 0.2898 g; blank-corrected = 0.2890 g. The gravimetric factor is M(SO4)/M(BaSO4) = 96.06/233.39 = 0.4116. %SO4(1) = 0.2866 × 0.4116 / 0.5000 × 100 = 23.59%. %SO4(2) = 0.2890 × 0.4116 / 0.5000 × 100 = 23.79%. The difference is 0.20% absolute, which exceeds the 0.15% requirement; the duplicate fails. The mean is 23.69% SO4.
2. The rapid BaCl2 addition and short digestion are suspect. Explain how each affects particle size, occlusion, and the result, and propose a corrected procedure.
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Rapid addition to a hot, dilute, acidic solution creates high supersaturation, so BaSO4 nucleates as many fine particles. Fine particles have large surface area and are prone to peptization, pass through the filter, and occlude BaCl2 or other ions; occlusion of BaCl2 gives a positive error, while losses give a negative error, so duplicates scatter. Short digestion prevents Ostwald ripening. Correct procedure: add BaCl2 slowly, dropwise, with stirring to a hot but not boiling solution; keep the solution acidic; digest at 80–90 °C for at least 1 h (or overnight) to coarsen the precipitate; test for complete precipitation; filter hot, wash with warm dilute HCl then water until free of chloride, and ignite to constant mass. If the duplicate still fails, reprecipitate the BaSO4.