Filtration filtering pipettes

Filter pipettes and micro filtration devices are available commercially. Thus, the Whatman syringe filters No. 402/0803/06 (with 4 mm effective diameter) and 402/0805/06 (with 13 mm effective diameter) are suitable for micro- or semimicro-work respectively. 

Depending on the amount of solid being filtered and the size of the particles (small particles are more difficult to remove by filtration), it may be necessary to put the filtrate through a second filtering pipette. This should be done with a new filtering pipette rather than with the one already used. 

For the tablets weigh an amount of powdered tablets equivalent to about 0 5 g of cyclobarbitone calcium into a 100-ml glass-stoppered cylinder, add 10 ml of 4 per cent sodium hydroxide solution and 40 ml of water and shake mechanically for thirty minutes. Transfer quantitatively to a 100 ml graduated flask with water, dilute to volume with water, mix and filter. Pipette 25 ml of the filtrate into a glass-stoppered... 

Dissolve 0 5 g of lotion containing 0 2 per cent of chlorocresol in sufficient 72 per cent ethanol (prepared from industrial methylated spirit that has been refluxed for four hours over potassium hydroxide and distilled) to make exactly 50 ml. Shake well, chill in ice-water and filter. Pipette 10 ml of the filtrate into a 20-ml graduated flask and dilute to 12 ml with the 72 per cent ethanol. Add 2 ml of a 0 012 per cent solution of 2,6-dichloro-p-benzoquinone-4-chloroimine in 95 per cent ethanol (refluxed and distilled as above) and 2 ml of a 1 5 per cent aqueous borax solution, mixing after each addition. Allow to stand in the dark for one hour and then dilute to volume with the 72 per cent ethanol. Mix and measure the extinction at 650 m/U in a 2-cm cell subtract the value of a reagent blank and read off the amount of chlorocresol from a calibration curve. 

Dimethoxybenzidine reagent. Mix 0 5 g of 3,3 -dimethoxybenzidine and 0 5 g of activated charcoal (50- to 200-mesh) with 100 ml of wopropyl alcohol, shake well and filter. Pipette 40 ml of the filtrate into a 100-ml graduated flask, dilute to volume with N hydrochloric acid and mix. This reagent should be freshly prepared on the day it is to be used. 

Pipette a volume V, cm of the active solution (with a safety pipette) into a 25 cm beaker. Heat nearly to boiling on a water bath and add V2 cm of 0.4 M iron complex solution dropwise and with stirring. Allow the suspension to settle and filter under suction through GF/F paper in a filter assembly, Fig. 18.2, collecting the filtrate in a clean dry receiver. Pipette 9 cm of the filtrate (safety pipette) into a liquid G-M counter. Transfer it to the lead castle and count for an adequate time (>60 min) setting the H.V.of the counter at 420 V. 

Pipette 25.0 mL of the potassium ion solution (about 10 mg K + ) into a 50 mL graduated flask, add 0.5 mL 1M nitric acid and mix. Introduce 20.0 mL of the sodium tetraphenylborate solution, dilute to the mark, mix, then pour the mixture into a 150mL flask provided with a ground stopper. Shake the stoppered flask for 5 minutes on a mechanical shaker to coagulate the precipitate, then filter most of the solution through a dry Whatman No. 40 filter paper into a dry beaker. Transfer 25.0 mL of the filtrate into a 250 mL conical flask and add 75 mL of water, 1.0 mL of iron(III) nitrate solution, and 1.0 mL of sodium thiocyanate solution. Titrate with the mercury(II) nitrate solution as described above. 

Phase separation of the saturated solution from the excess solid solute is a critical process. If a filter is employed, it must be inert to the solvent, it must not release plasticizers, and its pore size must be small enough to retain the smallest particles of the solid solute. Furthermore, steps must be taken to monitor, minimize, and preferably avoid losses of the dissolved solute by adsorption onto the filter material [27-30] and/or onto the vessels, pipettes, and syringes. Typically, the first small volume of filtrate is discarded until the surfaces of the filter and/or vessels are saturated with the adsorbed solute, to ensure that the filtrate analyzed has not suffered significant adsorption losses. Adsorption can be a serious problem for hydrophobic solutes, for which filtration would not be recommended. 

After shaking the soil suspension in the extraction bottle, a tube of filter-paper folded about the centre to form a V with the open ends uppermost is inserted into the bottle. Clear filtrate collects inside the paper tube and aliquots are removed with a pipette. 

Small volumes of solution (up to 2 ml in one operation) may conveniently be filtered through a dropping (Pasteur) pipette into the constriction of which has been rammed a small piece of paper tissue (about 3 cm square). The pipette is supported vertically and the solution is added from a second Pasteur pipette. Pressure to accelerate the filtration process may then be applied from a rubber bulb attached to the top of the pipette. 

Hunt [340] has described a simple method of filtering soil extracts that eliminates the need for filter funnels and receivers. It therefore reduces the risk of contamination and speeds up the procedure. It also offers a convenient means of obtaining filtrates in the field for subsequent analysis. After shaking the soil suspension in the extraction bottle, a tube of filter paper folded about the centre to form a V with the open ends uppermost is inserted into the bottle. Clear filtrate collects inside the paper tube and aliquots are removed with a pipette. 

Dilute the dark green reaction mixture with 10 mL EtOAc and filter through a Pasteur pipette packed with a cotton plug and a layer of sand (and layer of Celite, optional). Direct the filtrate into a separating funnel charged with 1 M... 

---