edlted by
GEORGE L. GILBERT Den~sanUnwerstty Granv~lie,Ohm 43023
m
Hard Water, Water Softening, Ion Exchange Submitted by:
Checked by:
Ned Egen a n d P e t e r C. Ford University of California, Santa Barbara. California 93106 L. C. Grotz University of Wisconsin-Waukesha Waukesha. Wisconsin
with top w t e r
Preparation
(1) Assemble the apparatus shown. The 40 X 2.5-cm ion exchange columns contain the following resins: I, Dowex 50W-X8 (Na+ form); II, Dowex 50W-X8 (H+ form); III, Bio-Rad Ag3-X4A (OH- form). A combination of glass and Tvgon tubing connects these. The handheld conductivitv ... bridge was ronatrllrt~d hy positioning two semirirr~llar, sintered rarhon disrs (9cm diameter) ~arallelto one s n other -1 mm apart (polyethylene sheetis epoxied in position as a spacer). The carbon electrodes are epoxied to an insulating handle and connected in series with a 6-V lamp and a Variac. Current sufficient to light the lamp was supplied when 20 V was applied across the plates submerged in tap water. (2) Other materials needed are: 1 1 of tap (hard) water (sample A); 1 1 of distilled water ( B ) ;1-1 flask containing 1 I of tan water plus 10 e of sodium carbonate (C): 1-1 flask contaihing 1 I bf tap water plus 0.03 moles EDTA~-(10 g Na7EDTA plus 9 ml 50% NaOH) (D): . .. 100 ml distilled waier containing several drops of universal Indicator; soap solution (add a ouarter of Ivorv .soap. bar to 200 ml distilled water and stir fbr several hours); and two 1-1 Erlenmeyer flasks with stoppers, six 250-ml beakers, eye dropper. Demonstration
Pass 1 1 of tap water through column I and collect in a 1-1 flask (sample E ) (requires -20 min). Simultaneously pass 1 1 of tap water consecutively through the H+ exchange then the OH- exchange columns and collect in a 1-1 flask (sample F ) (requires -30 mini. During this, deliver several ml of water from column I1 via the 3-wav sto~cockto flask containing indicator. Color changes from ;ellow-orange ( p H 6.5) to red ( p H -3) indicating exchange of H+ for other cations. Add 200-ml portions of each of the 6 samples in 1-1 Erlenmeyer flasks to the 250-ml beakers. 1) Test for Ca2+ (hardness): add several ml soap suspension to samples A-F in the flasks; stopper; shake for several seconds; then let settle for -1 min. The presence of Ca2+ is shown hv the ~recioitateof Ca(C,?Hs~01)1 . .. "" -. and no sudsiog. ~ b s e n c oica2; e is shown by sudsing. 2) Test for ions in solution: dip conductivity bridge into 302 / Journal of Chemical Education
sto~coch
Conductivity bridge
beakers containing the six water samples (rinse in distilled water between measurements). Bulb lights when ion concentration is equal to or greater than that in hard water [(Ca2+]-l0W3 Mi. 3) Results of the twelve tests are shown in the table. Summary of Results
Sample
A T a p water (hard)
Re6ultS w i t h Soap
Results w i t h COndUCtiViw Bridge
no rudring. precipitate f o r m a t i o n suds~ng
bulb does n o t
C T a p water softened b y precipitation o f taco,
rudring
bulb lights
Tap water by complexing w i t h EDTA T a p water softened b y exchanging w i t n ~ a +
rudring
bulb lights
sudsing
bulb lights
T ~ P water softened DY deionization
ruaring
bulb d o e l not light
B
Oirtilled water
bulb light5
,:--. ,,Y,,L
F
Remarks This demonstration offers a dramatic display of the operation and effectiveness of ion exchange columns and illustrates precipitation and complexation reactions in relation to hard water. Hard water is softened by five processes: distillation (performed prior to the demonstration), precipitation of Ca2+ (as CaCOa), complexation of Ca2+ (as CaEDTA2-), ion exchange (exchanging 2Na+ for Ca2+), and deionization. The Dresence of Ca2+ in hard tap water and its objectionable-property of. forming preipitates (mainlv calcium stearate) with soaD is shown. The presence bf ion; species in certain water samples is indicated by a conductivity device.
Raoult's Law and Vapor Pressure Measurement Submitted by:
Checked by:
Ned Egan a n d P e t e r C. Ford University of California Santa Barbara, 93106 A. R. Burkett Dillard University New Orleans, Louisiana 70122
Preparation (1) The mercury barometers (illustrated) are constructed from 110 mm long, 8 mm i.d. Pyrex tubes, flared at the bottom and with 3-mm vacuum stopcocks sealed to the top. These are wired onto a rigid hoard in parallel to a meter stick. The flared ends are submerged vertically into a pool of mercury in a beaker. Use a vacuum pump to evacuate the tubes uia the stopcocks, then close the stopcocks and adjust the height so the bottom of the meter stick is a t the Hg pool surface. The height of the Hg column should "
..
(2) Prepare stoppered flasks
df pure
ethyl ether and of 50 mole % etberltoluene. Also needed are two hook-shaped eyedroppers (bent Pasteur pipets can be used). Demonstration (1) Read the height of the Hg column then inject (using a pipet) -0.5 ml ether into the lower end of one barometer so that it can rise to the top of the column. Read the new height. The difference of the initial and final heights is the vapor pressure of ether in equilihrium with its pure liquid (-400 mm (Hg) at 2OoC). (2) Use Raoult's Law to calculate the vapor pressure of ether in equilihrium with a 50% mole solution of ether and the relatively non-volatile solute toluene. Introduce -0.5
ml of this solution into the second barometer. Allow about 30 sec for the system to come to equilibrium then measure the new height of the barometer. Compare the measured vapor pressuie to the calculated value. Remarks Upon introducine a volatile liauid (ether) into a barorneter sipporting a H; column equal to the atmospheric pressure, the height of that column quickly drops an amount approximately equal to the vapor pressure of the liquid. Introducing an ether solution of a non-volatile solute into an identicibarometer causes a smaller drop in the Hg column which can be compared to the vapor pressure calculated ac= P,~,.,t(l - x.,I,~)~. cording to Raoult's Law, [Pa,~.tian Agreement of measured and calculated values is within 10%. This demonstration offers a simple and dynamic method to reinforce the student's understanding of the term "vapor pressure in mm Hg" by direct measurement of the height of a mercury column to show how vapor pressure can be measured, and to show how Raoult's Law can apply to a real system.
Volume 53, Number 5, May 1976 / 303
