2 4 MOISTURE DETERMINATION OF WATER BY VACUUM

METHOD 2.4 Printed with effect from 1st January 2003 MOISTURE - DETERMINATION OF WATER BY VACUUM SAND DRYING 1: 1:1 1:2 1:3 APPARATUS Vacuum oven drying (1) Balance Aluminium dishes and lids 1:4 1:5 1:6 Stirring rods for beakers Mixing rods for drying dishes Beakers 2: 2:1 REAGENTS Sand - purified by acid, 40-100 mesh 3: 3:1 TEST PROCEDURE Place 30 to 35g of treated sand in the aluminium dish together with a small mixing rod Rest the dish in the lid and dry for hours in an oven at 110oC or in the vacuum oven at 65oC under the vacuum conditions After drying place the dish and contents in a desiccator to cool to ambient temperature Weigh the beaker and glass-stirring rod, add about 20g of molasses and re-weigh Pipette 10cm3 of water into the molasses and thoroughly stir to ensure a uniform mixture It may be desirable to warm the molasses and water up to 50oC to improve the degree of mixing Weigh the beaker containing the molasses solution and glass-stirring rod; if the solution has been warmed then cooling is necessary before weighing is attempted Weigh the dish, sand, lid and mixing rod together Stir the molasses solution in order to suspend insoluble material and then transfer approximately 2g of the solution on to the sand contained in the dish Re-weigh the dish, sand, lid, rod and molasses solution (3) Thoroughly mix the molasses solution and sand with the mixing rod ensuring that no loss of material occurs Rest the dish in its lid and place in the vacuum oven for final drying The oven is to be operated at 65oC at a pressure not exceeding 50mm of mercury After 18 hours drying the lid is replaced on the top of the dish which is transferred to a desiccator for cooling before being weighed 3:2 3:3 3:4 3:5 3:6 3:7 3:8 3:9 4: Thermostatically controlled Accuracy: 0.1mg 60mm diameter 25mm deep (approximately) To have flattened ends Capacity: 60cm3 EXAMPLE OF CALCULATION The solids content of the molasses is tabulated on the following basis: (a) Weight of beaker and rod = (b) Weight of beaker, rod and molasses = (c) Weight of beaker, rod, molasses and water = (d) (b - a) Weight of molasses = (e) (c - b) Weight of water = (f) (c - a) Weight of molasses solution = (g) Weight of dish, lid, sand and rod = 2:4/1 36.2485g 56.3346g 66.2530g 20.0861g 9.9184g 30.0045g 47.8365g (h) (i) Weight of dish, lid, sand, rod and molasses solution Weight of dish, lid, sand, rod and molasses solution after drying (j) (h - g) Weight of molasses solution for drying (k) (h - i) Loss of weight on drying (m) (i - g) Weight of residual solids of molasses = = 49.8852g 48.9692g = = = 2.0496g 0.9160g 1.1336g % Solids in original molasses = m x f x 100 j x d % Moisture in original molasses = 100 (1 - m x f ) (j x d ) In the above example: Moisture = 100 (1 - 1.1336 x 30.0045 ) (2.0496 x 20.08681) = 17.38% NOTES Vacuum ovens tend to vary a little in design The Townson and Mercer oven (40A - 600) 1: has proved to be very satisfactory for this type of determination Dried sand absorbs moisture from the atmosphere very rapidly and the dried 2: sand/molasses mixture even more so It is essential therefore to expose the dishes, containing these materials, to the atmosphere for as little as possible and carry out the weighing quickly In order to minimise effects created by evaporation it is advisable to avoid delay between 3: weighing diluted molasses and the transfer of the required amount to the sand If delay is unavoidable then the beaker containing the molasses solution should be protected from evaporation effects with a cover glass The bleed air should be dried by passage through a drying tower containing a suitable 4: desiccant, e.g phosphorous pentoxide or silica gel In order to gauge the volume of air being bled into the oven it is an advantage to precede the drying tower with a Drechsel bottle (gas washing bottle) This vessel should contain concentrated sulphuric acid and serves the double purpose of partially drying the air and indicating airflow For dilute materials i.e 78% solids or less 5: a) Weigh 2g sample into the dish containing the sand and stirring rods b) Place in the 65oC oven, without vacuum, for minutes, then mix in the molasses as in 3.7 This procedure is extremely accurate, cuts out unnecessary weighings and creates a much simpler calculation i.e a) Wt of dish + sand + molasses = 59.625 Wt of dish + sand = 57.208 Wt of molasses = 2.417 (a) Wt of dish + sand + molasses after drying = 59.072 Wt of dish + sand = 57.208 Wt of dried molasses = 1.864 (b) % Solids = b x 100 = 1.864 x 100 = 77.1 a 2.417 2:4/2 ... molasses = 59. 625 Wt of dish + sand = 57 .20 8 Wt of molasses = 2. 41 7 (a) Wt of dish + sand + molasses after drying = 59.0 72 Wt of dish + sand = 57 .20 8 Wt of dried molasses = 1.8 64 (b) % Solids... on drying (m) (i - g) Weight of residual solids of molasses = = 49 .8852g 48 .9692g = = = 2. 049 6g 0.9160g 1.1336g % Solids in original molasses = m x f x 100 j x d % Moisture in original molasses... In the above example: Moisture = 100 (1 - 1.1336 x 30.0 045 ) (2. 049 6 x 20 .08681) = 17.38% NOTES Vacuum ovens tend to vary a little in design The Townson and Mercer oven (40 A - 600) 1: has proved