Filtering Technique A quantitative evaluation C. E. Meloan Kansas State University, Manhattan. KS 66506
Several years ago when I w a s a s t u d e n t taking quantitative analvsis I was shown how t o filter a ~ r e c i ~ i t aatned told t h a t if it were d o n e in that m a n n e r t h a t consiherahle t i m e would he saved. T h e s e i t e m s were a. Use the proper grade of filter paper. b. Decant. c. Use a long stem funnel. d. Use a narrow diameter stem rather than a wide diameter stem. e. Use a fluted funnel if possible. I. Fold the paper with a 'Is-in. to %-in. offset on the second fold. g. Keep the long stem full of solution. Tear the paper to prevent air intake. h. Touch the end of the funnel to the side uf the beaker. H o w m u c h t i m e would be saved if these things were done was never mentioned nor were any indications given a s to how m u c h time would he saved by each separate step. I have been unable t o find t h e answer t o t h e s e questions published a n y where, s o I decided to see if "good technique" really would m a k e a difference. T h e following s e t of experiments a r e a n a t t e m p t t o p u t a more quantitative aspect t o t h e filtering process.
and poor technique then there is no need tu do any further experiments. The systems used were Good teekniquc Poor t ~ e k n i q u e proper filter paper short stem powder funnel long stem funnel large diameter stem narrow diameter stem no offset when folded I/* in. offset fold paper torn to stop air intake paper not turn soiutims decanted solutions stirred funnel sides smooth I'unnel sides fluted full column of water in the stem no column of water in the stem tip adjacent to the side of beaker tip over the middle of the beaker Part A. Whatman No. 41: a highly porous, fast filtering paper Poor
Good Trial
Time
Trial
Time
1 2 3 ave
4 mi" 35 sec 4 min 35 sec 4 min 14 sec 4 mln 28 sec
1 2 3 aue
7 min 0 sec 9 mi" 10 sec 8 min 47 sec 8 min 19 sec
Part B. Whatman No. 40: a m e d l m porosity paper Good
Poor
Trial
Time
Trial
Time
1 2
5 mi" 42 sec 6 min 20 sec 5 min 59 sec 5 min 22 sec 5 min 51 sec
1 2 3
24 min 10 sec 24 min 24 sec 24 mi" 40 sec
we
24 mi" 25 sec
Experimental . Apparatus: Conventional cummercially available 70 mm, 60°, long stern and ~. both fluted and smooth. The ... . shnrtrtem funnels were used. ~
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3
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funnel; had an msr;q:c I+wg,thul .1w1 ui l i z m w A I mnt I I)., ;and thr .horr ;rent runnrl, h.d i n .nerdre 5rrm lmvlt *..lul~m$durn( n a i 111,111, il % % a r Iwiewd rh.>rrh* s d u n e d i d rid ~ . r nwrr y th8n.I-l mi b tec n , d c l e r m l t m t l m i . i:ompared ic. !In. 2.1, ml #wiylnnls d l r w >w itmc t h ~ i was not considered significant. I
item
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Results and Discussion
The Iron Hydroxide System
Experiment I. T h e Best Technique versus t h e W o r s t Technique with Various Porosity Filter P a p e r s What is the maximum savings in time that can beexpected if proper technique is used? If there is no difference between good technique
4 -
ave
Part C. Whatman No. 42: a low porosity, slow filtering paper GoW
Poor
Trial
Time
Trial
Time
1 2 3 awe
16 min 15 sec 12 min 51 sec 16 min 50 sec 15 min 19 sec
1 2 3 ave
50 min 18 sec 49 min 30 sec 46 mi" 30 sec 48 min 46 sec
p~
The use of goud technique clearly saves considerable time. The proper choice of filter paper is very important. There are two additional factors that occur with good technique that are not apparent from the time measurements alone. The filtered precipitate in the poor case was considerably wetter than in the good case. The comhined weixhts of the precipitate and wet paper for each of the 3 trials (Whatman 40) were 29.78 g fur the poor case and only 22.65 g for the good case. This means that even more time will be saved later on because less water must be driven off the precipitate before the paper can be burned. The second benefit was the nature nfthe placement of the precipitate an the filter paper. In the good technique ease, the precipitate was all % to'% in. below the top of the paper and the paper could be removed, folded over, and immediately placed intoa crucible. In the pour technique case, the precipitate was to 'Inin. from the top of the paper and a further washing would he necessary before the paper could be folded and transferred. The times listed are a minimum time and not a maximum time.
Experiment II. T h e Effect of Decanting 1ih4 ill m e
