If a filter paper is folded along its diameter, and again folded along the radius at right angles to the original fold, a cone is formed on opening, the angle of which is 60 deg..  Funnels for analytical use are supposed to have the same angle, but are rarely accurate.  It is possible, however, with care, to fit a filter thus folded into a funnel in such a way as to prevent air from passing down between the paper and the funnel to break the column of liquid in the stem, which aids greatly, by its gentle suction, in promoting the rate of filtration.

Such a filter has, however, the disadvantage that there are three thicknesses of paper back of half of its filtering surface, as a consequence of which one half of a precipitate washes or drains more slowly.  Much time may be saved in the aggregate by learning to fold a filter in such a way as to improve its effective filtering surface.  The directions which follow, though apparently complicated on first reading, are easily applied and easily remembered.  Use a 6-inch filter for practice.  Place four dots on the filter, two each on diameters which are at right angles to each other.  Then proceed as follows:  (1) Fold the filter evenly across one of the diameters, creasing it carefully; (2) open the paper, turn it over, rotate it 90 deg. to the right, bring the edges together and crease along the other diameter; (3) open, and rotate 45 deg. to the right, bring edges together, and crease evenly; (4) open, and rotate 90 deg. to the right, and crease evenly; (5) open, turn the filter over, rotate 22-(1/2) deg. to the right, and crease evenly; (6) open, rotate 45 deg. to the right and crease evenly; (7) open, rotate 45 deg. to the right and crease evenly; (8) open, rotate 45 deg. to the right and crease evenly; (9) open the filter, and, starting with one of the dots between thumb and forefinger of the right hand, fold the second crease to the left over on it, and do the same with each of the other dots.  Place it, thus folded, in the funnel, moisten it, and fit to the side of the funnel.  The filter will then have four short segments where there are three thicknesses and four where there is one thickness, but the latter are evenly distributed around its circumference, thus greatly aiding the passage of liquids through the paper and hastening both filtration and washing of the whole contents of the filter.

!Sample pages for laboratory records!

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Date

Calibration of burette No.

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