Chemistry of ice: Migration of ions and gases by directional freezing of water

Arabian Journal of Chemistry (2011) xxx, xxx–xxx

King Saud University

Arabian Journal of Chemistry

www.ksu.edu.sawww.sciencedirect.com

ORIGINAL ARTICLE

Chemistry of ice: Migration of ions and gases by

directional freezing of water

Umer Shafique a,*, Jamil Anwar b, Munawar Ali Munawar b, Waheed-uz Zaman b,

Rabia Rehman b, Amara Dar a, Muhammad Salman b, Maria Saleem b,

Naeema Shahid b, Mehwish Akram b, Arooj Naseer a, Nadia Jamil c

a Center for Undergraduate Studies, University of the Punjab, Lahore 54590, Pakistanb Institute of Chemistry, University of the Punjab, Lahore 54590, Pakistanc Earth and Environmental Sciences, University of the Punjab, Lahore 54590, Pakistan

Received 1 June 2010; accepted 14 February 2011

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KEYWORDS

Acidity and basicity;

Directional freezing;

Freezing front;

Ionic redistribution;

Migration of ions on freezing

Corresponding author. Mo

-mail address: umer0101@h

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Abstract Redistribution of anions and cations creates an electrical imbalance in ice grown from

electrolyte solutions. Movement of acidic and basic ions in cooling solutions can permanently

change the pH of frozen and unfrozen parts of the system, largely. The extent of pH change asso-

ciated with freezing is determined by solute concentration and the extent of cooling. In the present

work, redistribution of hydrogen, hydroxyl, carbonate, and bicarbonate ions was studied during

directional freezing in batch aqueous systems. Controlled freezing was employed vertically as well

as radially in acidic and basic solutions. In each case, the ions substantially migrated along with

moving freezing front. Conductometry and pH-metry were employed to monitor the moving ions.

Besides, some other experiments were carried out with molecular gases, such as oxygen, carbon

dioxide, and chlorine and an azeotropic mixture like water–ethanol. Findings can be used to under-

stand possible changes that can occur in preserving materials by freezing.ª 2011 King Saud University. Production and hosting by Elsevier B.V. All rights reserved.

3214990904.

m (U. Shafique).

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Saud University.

lsevier

Shafique, U. et al., Chemistryry (2011), doi:10.1016/j.arabjc.

1. Introduction

In winter, when oceans freeze up (usually at �1.9 �C), the ice

at the top contains much less salt than water underneath thesurface. This is because of the buildup of ice crystals by purewater leaving the dissolved salts in liquid phase. The increaseddensity of seawater beneath the ice causes it to sink towards

the bottom. The process results in ocean currents forming totransport water away from the pole. Freezing of contaminatedwater represents a practicable application of the phenomenon

of solute rejection during the solidification of multi-componentsystems. The phenomenon has been thoroughly studied at

of ice: Migration of ions and gases by directional freezing of2011.02.019

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Figure 1 Gradual cooling in PVC pipe that pushed ions (H+,

OH�, CO2�3 and HCO�3 ) towards bottom.

2 U. Shafique et al.

different freezing conditions i.e., rate of cooling (Kammererand Lee, 1969; Parker and Collins, 1999) and effect of stirring(Gay et al., 2003). Several workers employed this process for

desalination of water (Himes et al., 1959), food processing,such as concentration of milk and fruit juices (Lorain et al.,2001) and elimination of organic contaminants in refinery

effluents (Gao et al., 2009). The effect of rejecting foreignbodies by freezing water is equally valid for soluble salts, or-ganic compounds, and suspended materials.

Various chemical, biological, and environmental processestake place at the interface of ice (Robinson et al., 2006). Re-ports on the unlike evolution of nitrous acid from assortedhigh-latitude snowpacks point to place-reliant acidities (Chen

et al., 2004; Davis et al., 2001). Freeze-induced pH changesare concerned with food, drugs, and tissues during cryogenicstorage (Baicu and Taylor, 2002; Cao et al., 2003; Elford

and Walter, 1972; Eriksson et al., 2003; Yamamoto and Har-ris, 2001). However, pre-freeze treatments (removal of water)can help to reduce the damaging phenomena of loss of shape

and texture decline of fruits during thawing (Huxsoll, 1982).Hanley and Rao (1982) studied the migration of moistureand ions in freezing soil and inferred that at the freezing front,

cations are preferentially rejected from the frozen region intothe unfrozen portion. Li et al. (2007) measured pH and electri-cal conductivity of polar ice cores to rebuild the history of airpollution. Similar studies were carried out on the Tibetan pla-

teau (Sheng and Yao, 1996; Yao and Sheng, 1993), TianshanMountains (Hou et al., 1999; Li et al., 2006) and other regions(Goto-Azuma et al., 2002, 1995, 1993). It was found that pH

and electrical conductivity values were high in spring (Apriland May) and early summer (June) but low in late summer(August and September).

The theory of solute redistribution during freezing of waterhas been the subject of extensive study in recent years. Thepresent work includes study of migrating H+, OH�, CO�23

and HCO�3 ions as well as molecular gases both by verticaland radial freezing. Transfer of ions was noted in graduallyfrozen solutions under controlled conditions by measuringpH and conductance. The acquired information can be helpful

in studying migration of these ions during the preservation offoods and pathological samples at low temperatures.

2. Materials and methods

2.1. Reagents and chemicals

All the chemicals and reagents used during the study were ofhigh purity, availed from Sigma–Aldrich, Inc., were used as

such (by dissolving appropriate quantity in deionized water).Acetic acid, citric acid, formic acid, sulfuric acid, hydrochloricacid, nitric acid, oxalic acid, and tartaric acid were among

acids, while sodium hydroxide, potassium hydroxide, sodiumcarbonate, and bicarbonate were the bases that were selectedfor studying ion migration.

2.2. Unidirectional downward freezing

To carry out unidirectional downward freezing, a 2 mm thickpolyvinyl chloride pipe with an inner diameter of 2 cm and a

height of 30 cm was used. Insulation with polystyrene was

Please cite this article in press as: Shafique, U. et al., Chemistrywater. Arabian Journal of Chemistry (2011), doi:10.1016/j.arabjc.

applied at bottom and outer surfaces of the pipe to ensure

gradual cooling in the downward direction. Ice crystals, prop-agating in the downward direction have pushed the ions to-wards the bottom of the pipe. After continuous freezing for

20 h, the frozen mass was taken out of the pipe and cut intosmall pieces of equal length (3 cm), melted and analyzed.

2.3. Radial freezing

To perform radial freezing, a circular glass tank with an innerdiameter of 30 cm and height of 15 cm was used. Opening (top)and bottom were insulated with thick polystyrene cover (1–

2 in.). Protection was applied to ensure cooling from the sidesthrough the glass. Under such conditions, ice crystals startedto develop from the sides towards the center radially and con-

sequently ions would migrate to the center. After continuousfreezing for 20 h, the frozen mass was taken out of the tank.To record the effect of freezing on the movement of ions, ice

samples were collected by assorted positions, thawed, and ana-lyzed to measure the concentration of ions at that specific po-sition by measuring pH and conductance.

3. Results and discussion

The present work represents results in terms of pH and con-

ductance for solutions carrying H+, OH�, CO2�3 , and HCO�3



Figure 2 Gradual cooling in circular bowl that pushed ions (H+, OH�, CO2�3 and HCO�3 ) towards center.

Chemistry of ice: Migration of ions and gases by directional freezing of water 3

ions, slowly frozen in one direction. Solutions were frozen in acolumnar shape polyvinyl chloride pipe as well as in a roundglass bowl that was insulated from top to bottom with polysty-

rene. Figs. 1 and 2 represents the freezing pattern in the pipeand bowl, respectively. Dotted lines represent the points wherethe ice was cut, thawed, and analyzed. In the case of pipe, side-

walls and bottom were completely insulated while the top wasopen, which allowed the ice to grow from top to the bottom.Freezing front gradually propagating downward has pushedthe ions towards the bottom. During development of crystals

of ice, water molecules come close to each other and alignthemselves in a regular pattern through hydrogen bonding. Ifcooling is rapid, foreign particles will trap in the growing ice

crystals. However, in slow cooling forwarding freezing frontwill push the particles out from the ice texture or crystal lattice.It is for this reason that the bottom of the pipe was rich with

ions while the top was almost vacant. It was the case with a cir-cular bowl as well, where cooling was applied from sides thatpushed the ions to the center.

In Fig. 3, conductance of various segments A–E (labels are

according to Fig. 1), after freezing, cutting and thawing, isshown for 0.1 molar solutions of acetic, citric, formic, sulfuric,hydrochloric, nitric, oxalic and tartaric acid in case of unidirec-


tional downward freezing. Initial conductance (mS/cm) of0.1 mol/L acetic acid, citric acid, formic acid, sulfuric acid,hydrochloric acid, nitric acid, oxalic acid, and tartaric acid

were 0.28, 2.47, 1.33, 40.70, 34.80, 37.60, 16.72, and 19.14,respectively. After continuous freezing for 20 h, conductancefor the topmost portion changed to 0.20, 1.14, 0.78, 30.10,

12.13, 19.26, 11.47, and 4.52 while in the bottom, conductancewas 0.39, 3.23, 1.50, 49.50, 34.96, 41.20, 25.80, and 30.50, inthat order. The values clearly point out the uneven numbersof ions at the top and bottom of pipe. Potentials of hydrogen

ions (pH) were also measured for all the above-mentionedsolutions. After freezing, pHs of the portion A (top) were3.314, 3.502, 3.871, 1.185, 1.595, 1.481, 2.068, and 3.139 while

of portion E (bottom), values were 2.878, 3.016, 3.241, 1.013,1.212, 1.172, 1.622, and 2.351, in that order.

Experiments were also performed with 0.01 and 0.001 mol/L

solutions and change in pH and conductance was noted. For0.001 molar solutions, changes in conductance (mS/cm) beforeand after freezing are shown inFig. 4. It is important to note thatin case of diluted solutions, change in pH or conductance was

more prominent in comparison to that for the concentratedsolution. All the experiments pointed out the same trend, thatis high pH and low conductance (in comparison to original solu-



Figure 3 Conductance (mS/cm) in various segments of 0.1 mol/L solutions of different acids after unidirectional downward freezing in

PVC pipe; Segment labels A to E according to Fig. 1; A.A – acetic acid, C.A – citric acid, F.A – formic acid, S.A – sulfuric acid, H.A –

hydrochloric acid, N.A – nitric acid, O.A – oxalic acid, T.A – tartaric acid.


tions) in segment A because of lesser ions and low pH and high

Figure 4 Conductance (mS/cm) in various segments of 0.001 mol/L so

PVC pipe; Segment labels A to E according to Fig. 1; A.A – acetic ac

hydrochloric acid, N.A – nitric acid, O.A – oxalic acid, T.A – tartaric


conductance in segment E because of more ionic species.

lutions of different acids after unidirectional downward freezing in

id, C.A – citric acid, F.A – formic acid, S.A – sulfuric acid, H.A –

acid.



Figure 5 Conductance (mS/cm) in various segments of 0.1 mol/L solutions of different bases after unidirectional downward freezing in

PVC pipe; Segment labels A to E according to Fig. 1.

Figure 6 Conductance (mS/cm) in various segments of 0.001 mol/L solutions of different bases after unidirectional downward freezing

in PVC pipe; Segment labels A to E according to Fig. 1.


When water begins to freeze, foreign species (dissolved sub-stances) are excluded from the ice crystal in a process called


brine rejection, and the surrounding water becomes more con-centrated and denser. The present experiments point out that



Figure 7 Frozen mass of KMnO4 solution (0.1%) after contin-

uous freezing in circular bowl (insulated from top and bottom) for

24 h.


when the solution is rich with ions, rejection ability of ice de-

creases while in case of diluted solutions, ice front can easilypush out alien species to those portions that are not frozen yet.

Change in pH and conductance of basic solutions (hydrox-

ide, carbonate, and bicarbonate) was also studied for unidirec-tional downward freezing. Changes in conductance (mS/cm)for 0.1 and 0.001 mol/L solutions are shown in Figs. 5 and

6, respectively. It is clear from figures that basic ions also havethe same trend. Like acids, separation efficiency in case of con-centrated solutions was less in comparison to dilute solutions.

Similar experiments were carried out with the acidic and ba-

sic solutions of the same concentration in a circular glass bowl.Bowl was insulated from top and bottom in such a way thatcooling was forced from sidewalls to the center in a radial

manner. Ions noticeably moved towards the center, that is seg-ment D, according to Fig. 2. In the case of a glass bowl, out-ermost segment (A) contains very few ions in comparison to

the topmost segment of columnar pipe (A). In the columnarpipe, rate of cooling was a bit higher as water was directlycooled from the top that trapped few ions in the solidifying

ice but in a bowl, water was cooled slowly through thick glasswalls via conduction. Gradual freezing allowed water mole-cules to push the ions to unfrozen portions expeditiously.

Some other experiments were also carried out with gases

(CO2, O2, and Cl2), 0.1% solution of KMnO4, and brominewater and complex systems like ethanol water solution. Gasessignificantly moved like the previous experiments; to the bot-

tom in unidirectional downward freezing and towards the cen-ter in the case of radial freezing. It was the case with brominewater and KMnO4 solution as well (radially frozen mass is

shown in Fig. 7). However, in case of ethanol, results werenot satisfactory. The sidewalls of the bowl contain slightly lessethanol in comparison to the topmost segment of the columnarpipe but no true separation was achieved in both cases.


Ethanol can form hydrogen bonds with water molecules thatmake its separation difficult since ice formation involveshydrogen bonding between molecules. In spite of the fact that

their freezing points are far from each other, hydrogen bond-ing of ethanol with water causes them to trap liquid ethanol.It was for this reason that frozen water–ethanol solution was

fluffy and soft in comparison to regular ice as there were pack-ets of liquid ethanol trapped between solid ice crystals.

Freezing is a common technique used to preserve food and

biological samples or to extend their storage life. Freezing candowngrade the quality of stored material by mechanicallydamaging the food structure. Cross-linking of proteins andlimited re-absorption of water on thawing can severely affect

the quality and taste (Evans, 2008). Although, Philips et al.(2001) has evaluated that levels of fatty acids were not signifi-cantly lessened by storage at �60 �C for up to 50 months, our

findings are suggesting that slow directional freezing canchange the pH of the frozen materials by pushing ions towardsunfrozen portions. This will lead to various degradation reac-

tions that can occur because of either a change in pH or build-up of ions. For instance, when 0.1% solution of KMnO4 wasfrozen together with NaOH (1%) in a circular bowl, they did

not react (color was violet) but after one day, when thehydroxide ions gathered at the center, rise in pH reduced theKMnO4 to green K2MnO4.

4. Conclusion

Solutions carrying hydrogen, hydroxide, carbonate and bicar-bonate ions, as well as gases like CO2, O2, and Cl2 were grad-

ually frozen in a downward direction as well as in a radialmanner. In each case, ions significantly moved from solidifyingportions to those that were still not solid. Conductance and pH

of portions of frozen mass iced up earlier were clearly suggest-ing that they contain fewer ions in comparison to those por-tions frozen in the end. The resulting information can be

helpful in studying the migration of ions and their possible ef-fect on the quality and life of food and pathological samplesduring their preservation at low temperatures.

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Chemistry of ice: Migration of ions and gases by directional freezing of water

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