lyze in a spectrophotometer, scanning from 380 to 500 nm, vieldine the characteristic visible snectrum for B-carotene. +he ch~orophyllfraction can be chased down the column by changing the eluting solvent from petroleum ether to acetone. Continue adding acetone to the column until the chlorophyll fraction is collected in another test tube. This fraction is also of sufficient concentration to analyze in a spectrophotometer, scannine from 380 to 700 nm.. vieldine . - the characteristic visible spectrum for chlorophylls. In conclusion, this procedure is a very handy way of illustrating the concept of extraction, separation, and analysis of a biochemical mixture, in less than an hour, without cumbersome and expensive glassware and materials. I t can be adapted to suit the needs of a student laboratory exercise or a lecture demonstration, and we encourage experimentatioq with different plant extracts and solvent mixtures.
"Cathodic Protection by Impressed Current SUBMITTED BY
R. Celdran P. Gonzalo Departamento de Eledroqvirnlca Unlversidad Autbnoma de Madrld Canto Blanco Madrid 34, Spain CHECKED BY
Wayne L. FeHy Penn State Unire,slly Wilker-Barre Carnous
Several articles concerning cathodic protection have been published in THIS JOURNAL.^.^ We describe here an illustrative demonstration by impressed current, which shows the protective effect depending on the current i n t e n ~ i t y . ~ Materials Three beakers, three sheets of iron (carbon steel) of different sizea": (1)0.08X3.9X1.3nn,(2)0.08X7.8X3.7cm,(3)0.08X9.5X6.9cm.
Four graphite bars and a calomel (sat.) referenceelectrode. Solution 1M in acetic acid with NaCl(3%)and Hz02 (0.05%).A de source,an ammeter, and an electrometer. The metals should first be cleaned and weighed. The graphite bar has a copper sleeve to which is soldered the conductive wire. The steel sheet is perforated on the center of the upper end where the conductive wire is attached. A wooden or olastic holder in which holes have been made to suoport rhr rlertrodes is usrd. In [his way, dl the elcrrmdes nrc submerged sirnultanrously in the solurions, whilr rrsring rhr holder ton the hrim of the veasels. Demonstration Three beakers are filled with the solution. Sheet (3) is immersed in one beaker and, in the remaining two iron sheets (1) and (2) are immersed with graphite bars on either side (see figure). The two beakers are connected in series to the dc source and the ammeter is placed between them. The iron
sheets act as the cathode and the graphite as the anode. Once : i d r s e d in the solution, the electrode of least surface area will be immediatelv orotected bv sheet (1). Potential measurement betw;en sheet (1) and the calomel was used as a criterion of protection, the sheet having to reach a potential value of -0.776 VISCE. In practice, the value imposed was -0.800 V and the current applied to achieve protection was approximately 6 mA. The potential of sheets (1) and (2) was read from time to time by means of the reference electrode. Sheet (1)maintains a constant potential while sheet (2) acauires a value of E = -0.552 V. which correswnds to the corros'ion potential of iron in this medium and is due to its larger surface area (small polarization). With time, the solutions containing sheets (2) and (9) acquirrd a reddish-hn~wn tinge, the latter with greater intensity. The reactions that take place (pH = 2.4) on the different sheers are the following:
+
2H+ 2eFe -Fez+ 0 2 4Hf
+
-+
+
Hz (gas) 2e4e2H20
-
E = -0.14 V E = +0.44 V E = +1.09 V
(1) (2) (3)
Sheet (I),full protection: reactions (1)and (2). Sheet (Z), partial protection: reactions (1).(Z),and (3). Sheet (3),no protection: reactions (11, (21, and (3). After 90 min the three sheets are removed, washed, and weighed. I t is found that sheet (1) has lost no weight, while sheets (2) and (3) have undereone a weieht loss. Sheet (3) shows fascer rate of corn&. With t c s experiment the orotection of an iron sheet t.1 .1 that is within the area of im&unity is observed. Likewise. the ~ a r t i a~l m t e c t i o nof sheet (2) was verified bv measurement oiits potkntial and its weight loss with regard to another sheet with no protection. Visual observation of this fact is also possible by the color acquired by the solution. This reddish-brown color is due to the oxidation of Fez+ to Fe3+, which, in an aerated solution, is produced to some extent and is favored by the presence of HzOz.
a
'
Skinner,J. F., J. CHEM. EDUC., 58, 802 (1981). 'Lauren. P. M.. J. CHEM. EDUC.. 55,319 (1978). Basic Corrosion Course. Ed. NACE. Houston (19751. Sheets of intermediate, or larger, sizes than those described here, can be used to extend the experiment.
Volume 62 Number 6 June 1985
531
