OR/14/067 Filtering and drying the silver phosphate precipitate

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Lapworth, D J, Surridge, B, Williams, P J, Heaton, T H E, and Gooddy, D C. 2014. Method for analysing phosphate 18O/16O ratios for waters with high C:P ratios. British Geological Survey Internal Report, OR/14/067.

To recover the silver phosphate crystals from the beaker it is necessary to filter them through a polycarbonate filter. Polycarbonate filters are used as they are strong enough to allow the crystals to be carefully scraped from their surface without disintegrating. A 0.2 mm polycarbonate filter was used, but 0.4 mm is adequate.

The contents of each sample beaker were filtered through the polycarbonate filter using a 25 mm narrow necked Buchner filtration funnel attached to a vacuum pump. The precipitate floating on the surface of the meniscus was easily recovered, but some crystals needed to be scraped from the sides and bottom of the beaker with a narrow spatula. These were then flushed from the beaker into the filtration funnel with Ultrapure water.

Once all the precipitate had been rinsed onto the polycarbonate filter, the precipitate caked polycarbonate filter was removed (with the utmost care) from the filtration assembly and placed on a watch glass before being put in the oven for drying. The oven was set at 50°C.

When dry, the precipitate was scraped from the filter paper onto a folded glacine weighing paper. This is important as the weighing paper has anti-static properties and prevents the precious crystals flying away. The silver phosphate crystals settle safely into the crease of the fold in the paper and can be transferred easily to a small glass vial. The vials were weighed before and after the addition of the silver phosphate crystals, thus allowing the weight of the crystals to be measured.

Plate 6 Final silver phosphate crystals visible in the bottom of sample vials.