Removal of Electrolytes from Solutions by Ion ~ k c h a n ~ e ' A Lecture Demonstration Experiment FREDERICK C. NACHOD and SIDNEY SUSSMAN The Permutit Company, New York City
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NTIL recently, the only method by which water could be almost completely freed from electrolytes was the familiar and ancient process of distillation. Within the past eight years there has been developed and brought into widespread industrial use an ion exchange method for producing water of distilled water quality. This method, commonly called demineralizing, does not require the use of heat. In addition to its ability to produce water substantially free from electrolytes, the demineralizing process can remove electrolytes from aqueous solutions of nonvolatile nonelectrolytes, (such as sugar or gelatin); a separation which could not be carried out by distillation and which heretofore could be carried out only by the relatively expensive process of dialysis. The objects of this paper are to review briefly the chemistry of the demineralizing process and to present details of a simple lecture demonstration experiment illustrating various applications of the process. Most elementary chemistry texts describe the zeolite process for softening water by base exchange. This is an ion exchange process which depends upon the replacement of the calcium and magnesium ions in hard water by sodium ions obtained from the zeolite by cation exchange. When the zeolite is no longer able to furnish sodium ions for exchange, the reaction may be reversed and the zeolite regenerated by treatment with a relatively strong solution of common salt. NaZ 2Na+ (1) . C . Ca++ G= CaZ ..
all metallic cations in solution by an equivalent amount of hydrogen ion. HsZ
+ 2NaC = Na.Z + 2H+
(2)
Many water supplies contain bicarbonate as virtually the only anion. Treatment of these waters with acidregenerated cation exchangers, followed by aeration to remove the carbon dioxide formed, resulted in the formation of a water almost equivalent in quality to distilled water.8 The chlorides and sulfates present in most other water supplies were converted into hydrochloric and sulfuric acids by treatment of these waters with acid-regenerated cation exchangers. The discovery of insoluble amino compounds4capable of removing these acids from solution and of being regenerated by alkaline solutions when exhausted (so-called anion exchangers) marked the last step necessary for the establishment of the demineralizing process. It is now possible to remove any salts from solution by contacting the solution successively with an acid-regenerated cation exchanger and with an anion exchanger.
+
= CaZ + 2HC1
CaCL HIZ H U f R9N
* RrN.HCl
(3) (4)
Carbonates, if present, are converted into carbon dioxide which is readily removed by aeration. Some conception of the quality of water obtained by the demineralizinz orocess mav be obtained from the table on page 57 %ch shows tke performance of indusThis process has been used commercially for well over trial demineralizing installations operating on a wide 30 years, during which time the zeolites (or, more corvariety of raw waters. These results compare very rectly, the cation exchangers) have undergone a number of improvements resulting in materials of higher ca- favorably with those obtained in commercial distillation installations. pacity and greater ruggedness. However,the first fundaThe demineralizing process has economic limitations mental change in the cation exchange process was made in cases where the electrolyte concentration is high.' possible in the early years of the last decade by the development of organic cation exchangers, such as the These limitations have been discussed elsewhere. It is sufficient to note here that demineralizing by means of sulfonated coalsP$8and synthetic resin^.^,^ Because bf their acid resistance, this class of cation exchangers can cation and anion exchangers is not applicable to the be operated with acid regeneration as well as with salt production of potable water from sea water.8 Introduction of the organic cation and anion exregeneration. This made possible the replacement of changers intensified interest in the application of ion exPresented before the Division of Chemical Education of the change reactions to fields other than water treatment.*
+
American Chemical Society, 106th meeting, Pittsburgh, Pennsylvania. Seotember 6. 1943. APPLEBA~, "~pplicationsof carbonaceous zeolites to water softening." I.Am. Water Works Assoc., 30, 947-73 (1938). "TIGER,, ','Carbonaceous zeolites, an advance in boiler feedwater cond~troning,"Trans. Am. Soc. Mech. Engrs., 60, 31525
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I,A"w->. tO?PI
' ADAMSAND
Ho~aws,"Absorptive properties of synthetic
resins," 5.Soc. Chem. Ind., 54, 1+T (1935). 6 M~sns, EASTES, a m MYERS, "Synthetic resins as exchange adsorbents." I d . Eng. Chem., 33, 697-706 (1941).
* APPI.EBA~
AND I~ILBY "Zeo-Karb . H, new methd of conditioning water to remove sodium bicarbonate chemically instead of by distillation." Ind. Enn. Chm.. 30. 80-82 (1938). 'TIGERAND SUSSMAN, "~emiu&&zing soiutio& by a trostep ion exchange process." I d . Eng. Chem.. 35, 186-92 (1943). TIGER AND SU~~MAN. loc. cit., pp. 189-90. Gnr~sss~cn."Ueber die Herstellung und Anwendung neuerer Austauschadsorbentien, insbesondere auf Harzbasis." Beihcfte zu Zeit. Verein Dcut. Chm., pp. 1 3 4 , Verlag Chemie. Berlin (1939).
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TABLE
I
COXFDI~ION OI RAWA N D DBUINB-IZBD W ~ T B B AT
SOY*INDDB-IL
PL*NT.
(All valves expressed in p. p. rn.
PIo.1 A
Plonl B
Plnnl C
PP
CPCOd
Plan1 D
PIonl E
Row Dcmin. Row Demin. Row Demin. R e v Dcmix. Row Darnin.
tube. Each bed should be backwashed in order to eliminate air bubbles and should be rinsed briefly. B a b a s h i n g may be carried out conveniently by use of an overflow tube, such as that shown in Figure lB, which may be made of glass or rubber tubing.
Total 3
Cations 64
68
2
149
3
4
197
277
2
Many processes involving the recovery or removal of ions present in low concentrations may now be carried out by ion exchange. Among these applications may be mentioned the recovery of various metals, removal of ash and impurities from beet sugar and other carbohydrate solutions, removal of ash from gelatin solutions, and others. Demonstration Apfiaratus. The demineralizing process applied to water and to solutions of nouelectrolytes, as well as other ion exchange applications, may be readily demonstrated using- the simple apparatus illustrated -. in Figure 1A. The anoaratus consists of two vertical class tubes connetted by glass or rubber tubing and equipped with a feed funnel, outlets for each tube, a dropper in the top of the second tube, and screw clamps for controlling the flow from either tube or between the tubes. The entire apparatus may be mounted on a single ring stand using condenser or buret clamps to support the tubes and a ring to support the inverted flask used as a feed reservoir. With two exceptions, the dimensions of the apparatus are not critical. The ion exchanger beds should be a t least nine to ten inches deep and if backwashing and regeneration of the ion exchangers are to be carried out in the tubes, these should preferably be a t least one inch in diameter and tall enough to permit a 50 to 100 per cent expansion of the ion exchanger bed during backwashing. In filling the tubes, the bottom stopper is put in place and a layer of fine gravel or very coarse sand about one inch deep is added as a support for the ion exchanger bed. A plug of glass wool instead of the gravel bed may be used if desired. After partly filling the tube with water, the ion exchanger is added until a bed depth of a t least nine inches is obtained. A cation exchanger, such as Zeo-Karb,IQis used in the first tube and an anion exchanger, such as De-Acidite,lQis used in the second
. A
Trademark Reg. U.S. Pat Off. lo
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Small quantities of ion exchangers are often furnished in the regenerated condition, in which event the tubes are now ready for operation. However, if the materials are not in the regenerated condition or if their previous history is unknown they may be readily regenerated in the tubes. Regeneration of the cation exchanger may be effected by feeding three volumes of 0.4 N sulfuric or hydrochloric acid for each volume of cation exchanger bed. The flow rate should be adjusted to give an average time of contact of 15 to 20 minutes, and the acid treatment should be followed by one or two liters of rinse. I n
practice, when water is being demineralized the raw feed is used for rinsing, but when solutions of nonelectrolytes are being demineralized the ion exchanger beds are rinsed with the available water supply before introducing the solution to be treated. Regeneration of the anion exchanger may be effected by feeding two volumes of 0.75 N sodium carbonate or sodium hydroxide for each volume of anion exchanger bed. The flow rate should be adjusted to give an average time of contact of 30 minutes and the alkali treatment should be followed by one or two liters of rinse, using distilled or demineralized water. In practice, large anion exchange units are rinsed by the effluent of the cation exchange unit when demineralizingwater and by softened or demineralized water when demineralizing solutions. The above directions for regeneration are simplified for the purposes of this experiment and do not include the more rigid control of regeneration and rinse used in large demineralizing installations, or in tests involving determinations of capacity and regeneration efficiency. Certain ion exchangers require larger regenerant dosages or longer times of contact for optimum performance. After completion of the regeneration and rinse, the apparatus may he assembled as shown in Figure 1. Operation. The tubes should be operated a t a flow rate of about 50 ml. per minute. A variety of feed solutions may be used to demonstrate various aspects of the demineralizing process. Tap water may be used. If a higher electrolyte content is desired in order to make the demonstration clearer, a solution of sodium chloride or other salt may be used. A 0.01 N solution is adequate. If a conductivity instrument is available, the conductivity of the feed solution and of the treated water may be shown. Commercial demineralizing installations make use of a conductivity instrument called the Solu-Bridge1' which is calibrated directly in concentration of dissolved salts. Conductivity instruments of very simple construction have been des~ribed.'~Ja
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" Manufactured by Industrial Instruments, Inc., Jersey City, New Jersey. LAPP, "A conductivity kit for fifty cents," Scimce Counselor.
5, 79, 91 (1939).
A conductivity instrument is useful for demonstrating the fact that the efficiency of demineralizing is dependent upon the efficiency of the cation exchanger in removing metallic cations, because the anion exchanger functions solely as an acid remover and does not remove cations. Thus, after operating the demineralizing apparatus until an effluent of constant low conductivity is obtained, a small amount of the feed solution may be introduced directly into the anion exchange tube by means of the dropper. This action will be reflected in an increase in the conductivity of the effluent. The action of the exchangers may be followed by the pH change, using a pH meter or indicators. At the start of the run the dropper a t the top of the second tube may be charged with methyl orange or other indicator solution and during the run an occasional drop of indicator solution may be added to the liquid to show the acidity of the cation exchanger effluent. If desired, a sample may be withdrawn from the base of the first tube and its acidity measured by titration. The final effluent may be treated with the same indicator to demonstrate its neutrality. The final pH will generally be somewhat lower than 7 because of the presence of dissolved carbon dioxide. The application of demineralizingto solutions of nonvolatile nonelectrolytes may be demonstrated by adding several per cent of sugar to the feed. The tests mentioned in the preceding paragraph may be applied in this case and, in addition, the final solution may be tasted in order to demonstrate the presence of the sugar in the treated solution. Other special applications, such as removal of heavy metal ions and of organic acids, may be demonstrated by using a cupric acetate feed. A 0.01 N feed is sufficiently concentrated so that the audience may clearly see the blue color in the influent and its absence in the cation exchanger effluent. This effluent may be tasted in order to establish the presence of acetic acid. An equivalent amount of citric acid may be added to the feed, in order to enhance the taste of the effluent from the cation exchanger tube. The final anion exchanger effluent may also be tasted to demonstrate that the citric acid has been removed. -
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LUDER AND VERNON, "A flexible student conductivity bridge assembly," J. CHEM.EDUC.,17,229-31 (1940).
