APPENDIX. 381 



PIPETTES, ETC. 



Delivery. In using an ordinary single-volume pipette the fluid is drawn 

 by suction just above the mark and closed with the finger. The lower end 

 of the pipette is then allowed to touch the side wall of the bottle 

 or beaker and the fluid run out till the meniscus is exactly at 

 the mark, the eye being level with the meniscus. The fluid 

 is then allowed to run out into the desired vessel and is then 

 drained for 15 seconds with its point touching the wall of the 

 vessel. The majority of pipettes are calibrated for such a 

 delivery, but for certain operations the author prefers to use 

 pipettes which are calibrated in such a way that they have to 

 be blown out after draining as above for 15 sees. The reason 

 why these are sometimes preferable is that the amount left in 

 the nozzle of the pipette after drainage may alter considerably 

 with variations in the surface tension of the fluid measured. 

 It is suggested that such pipettes calibrated for delivery by 

 blowing should be engraved with the letter "B" to distin- 

 guish them from pipettes calibrated for drainage " D." 



Ostwald pipettes (see fig. 48) are always calibrated for 

 delivery by being completely blown out. The orifice must 

 be so narrow that it takes about 30 seconds for the delivery 

 of i cc. They are filled by suction to above the mark and 

 then closed with the finger. The exterior is wiped with a 

 piece of filter paper and the fluid run to the mark by holding 

 the point on the filter paper. The fluid is then allowed to fall 

 out by its own weight, the delivery being completed by blowing 

 Fig. 48. out whilst drawing the point of the pipette up the sides of the 

 Ostwald receiving vessel. 



pipette. Burettes. The chief precautions to be taken are to allow 



time for proper drainage, and to be sure that the meniscus 

 is read in the same way at every operation. The author prefers to use 

 burettes that have the marks engraved as complete circles, and to 

 read the meniscus by means of a piece of black paper held behind 

 the burette. The black paper is pasted on to a piece of white card, 

 which is sharply folded at the black edge (see fig. 49). The black edge 

 is held against the back of the burette a trifle below the meniscus, the 

 slanting white card reflecting the light. The meniscus appears as a very sharp 

 black line. In order to avoid parallax it is important that the eye should be 

 exactly at the level of the meniscus. To ensure this for very accurate work 

 the author has constructed the device shewn in fig. 50. Should the fluid be 

 run rapidly out of a burette ample time must be allowed for proper drainage, 

 the meniscus gradually moving upwards as the fluid runs down the side of the 

 burette. Details of the methods employed for the calibration of pipettes and 

 burettes will be found in most standard works on Quantitative Chemical 

 Analysis.* It is essential that pipettes and burettes should be clean and 

 free from grease. Very considerable errors can be caused by variations in 



* Representative Procedures in Quantitative Chemical Analysis, by F A. 

 Gooch (Chapman & Hall, London, 1916), can be recommended. 



