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Original article
9 (
1_suppl
); S47-S53
doi:
10.1016/j.arabjc.2011.02.019

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

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

⁎Corresponding author. Mobile: +92 3214990904. umer0101@hotmail.com (Umer Shafique)

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

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 associated 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 understand possible changes that can occur in preserving materials by freezing.

Keywords

Acidity and basicity
Directional freezing
Freezing front
Ionic redistribution
Migration of ions on freezing
1

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 the surface. This is because of the buildup of ice crystals by pure water leaving the dissolved salts in liquid phase. The increased density of seawater beneath the ice causes it to sink towards the bottom. The process results in ocean currents forming to transport water away from the pole. Freezing of contaminated water represents a practicable application of the phenomenon of solute rejection during the solidification of multi-component systems. The phenomenon has been thoroughly studied at different freezing conditions i.e., rate of cooling (Kammerer and 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 foreign bodies by freezing water is equally valid for soluble salts, organic compounds, and suspended materials.

Various chemical, biological, and environmental processes take place at the interface of ice (Robinson et al., 2006). Reports on the unlike evolution of nitrous acid from assorted high-latitude snowpacks point to place-reliant acidities (Chen et al., 2004; Davis et al., 2001). Freeze-induced pH changes are concerned with food, drugs, and tissues during cryogenic storage (Baicu and Taylor, 2002; Cao et al., 2003; Elford and Walter, 1972; Eriksson et al., 2003; Yamamoto and Harris, 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 moisture and ions in freezing soil and inferred that at the freezing front, cations are preferentially rejected from the frozen region into the unfrozen portion. Li et al. (2007) measured pH and electrical conductivity of polar ice cores to rebuild the history of air pollution. Similar studies were carried out on the Tibetan plateau (Sheng and Yao, 1996; Yao and Sheng, 1993), Tianshan Mountains (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 (April and May) and early summer (June) but low in late summer (August and September).

The theory of solute redistribution during freezing of water has been the subject of extensive study in recent years. The present work includes study of migrating H+, OH, CO 3 - 2 and HCO 3 - ions as well as molecular gases both by vertical and radial freezing. Transfer of ions was noted in gradually frozen solutions under controlled conditions by measuring pH and conductance. The acquired information can be helpful in studying migration of these ions during the preservation of foods and pathological samples at low temperatures.

2

2 Materials and methods

2.1

2.1 Reagents and chemicals

All the chemicals and reagents used during the study were of high 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, hydrochloric acid, nitric acid, oxalic acid, and tartaric acid were among acids, while sodium hydroxide, potassium hydroxide, sodium carbonate, and bicarbonate were the bases that were selected for studying ion migration.

2.2

2.2 Unidirectional downward freezing

To carry out unidirectional downward freezing, a 2 mm thick polyvinyl chloride pipe with an inner diameter of 2 cm and a height of 30 cm was used. Insulation with polystyrene was applied at bottom and outer surfaces of the pipe to ensure gradual cooling in the downward direction. Ice crystals, propagating in the downward direction have pushed the ions towards the bottom of the pipe. After continuous freezing for 20 h, the frozen mass was taken out of the pipe and cut into small pieces of equal length (3 cm), melted and analyzed.

2.3

2.3 Radial freezing

To perform radial freezing, a circular glass tank with an inner diameter 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 sides through the glass. Under such conditions, ice crystals started to develop from the sides towards the center radially and consequently ions would migrate to the center. After continuous freezing 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 analyzed to measure the concentration of ions at that specific position by measuring pH and conductance.

3

3 Results and discussion

The present work represents results in terms of pH and conductance for solutions carrying H+, OH, CO 3 2 - , and HCO 3 - ions, slowly frozen in one direction. Solutions were frozen in a columnar shape polyvinyl chloride pipe as well as in a round glass bowl that was insulated from top to bottom with polystyrene. Figs. 1 and 2 represents the freezing pattern in the pipe and bowl, respectively. Dotted lines represent the points where the ice was cut, thawed, and analyzed. In the case of pipe, sidewalls and bottom were completely insulated while the top was open, which allowed the ice to grow from top to the bottom. Freezing front gradually propagating downward has pushed the ions towards the bottom. During development of crystals of ice, water molecules come close to each other and align themselves in a regular pattern through hydrogen bonding. If cooling is rapid, foreign particles will trap in the growing ice crystals. However, in slow cooling forwarding freezing front will 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 circular bowl as well, where cooling was applied from sides that pushed the ions to the center.

Gradual cooling in PVC pipe that pushed ions (H+, OH−, CO 3 2 - and HCO 3 - ) towards bottom.
Figure 1
Gradual cooling in PVC pipe that pushed ions (H+, OH, CO 3 2 - and HCO 3 - ) towards bottom.
Gradual cooling in circular bowl that pushed ions (H+, OH−, CO 3 2 - and HCO 3 - ) towards center.
Figure 2
Gradual cooling in circular bowl that pushed ions (H+, OH, CO 3 2 - and HCO 3 - ) towards center.

In Fig. 3, conductance of various segments A–E (labels are according to Fig. 1), after freezing, cutting and thawing, is shown for 0.1 molar solutions of acetic, citric, formic, sulfuric, hydrochloric, nitric, oxalic and tartaric acid in case of unidirectional downward freezing. Initial conductance (mS/cm) of 0.1 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, conductance for 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, conductance was 0.39, 3.23, 1.50, 49.50, 34.96, 41.20, 25.80, and 30.50, in that order. The values clearly point out the uneven numbers of ions at the top and bottom of pipe. Potentials of hydrogen ions (pH) were also measured for all the above-mentioned solutions. After freezing, pHs of the portion A (top) were 3.314, 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.

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.
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.

Experiments were also performed with 0.01 and 0.001 mol/L solutions and change in pH and conductance was noted. For 0.001 molar solutions, changes in conductance (mS/cm) before and after freezing are shown in Fig. 4. It is important to note that in case of diluted solutions, change in pH or conductance was more prominent in comparison to that for the concentrated solution. All the experiments pointed out the same trend, that is high pH and low conductance (in comparison to original solutions) in segment A because of lesser ions and low pH and high conductance in segment E because of more ionic species.

Conductance (mS/cm) in various segments of 0.001 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.
Figure 4
Conductance (mS/cm) in various segments of 0.001 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.

When water begins to freeze, foreign species (dissolved substances) are excluded from the ice crystal in a process called brine rejection, and the surrounding water becomes more concentrated and denser. The present experiments point out that when the solution is rich with ions, rejection ability of ice decreases while in case of diluted solutions, ice front can easily push out alien species to those portions that are not frozen yet.

Change in pH and conductance of basic solutions (hydroxide, carbonate, and bicarbonate) was also studied for unidirectional 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 have the same trend. Like acids, separation efficiency in case of concentrated solutions was less in comparison to dilute solutions.

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 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.
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.
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.

Similar experiments were carried out with the acidic and basic solutions of the same concentration in a circular glass bowl. Bowl was insulated from top and bottom in such a way that cooling was forced from sidewalls to the center in a radial manner. Ions noticeably moved towards the center, that is segment D, according to Fig. 2. In the case of a glass bowl, outermost segment (A) contains very few ions in comparison to the topmost segment of columnar pipe (A). In the columnar pipe, rate of cooling was a bit higher as water was directly cooled from the top that trapped few ions in the solidifying ice but in a bowl, water was cooled slowly through thick glass walls via conduction. Gradual freezing allowed water molecules 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 bromine water and complex systems like ethanol water solution. Gases significantly moved like the previous experiments; to the bottom in unidirectional downward freezing and towards the center in the case of radial freezing. It was the case with bromine water and KMnO4 solution as well (radially frozen mass is shown in Fig. 7). However, in case of ethanol, results were not satisfactory. The sidewalls of the bowl contain slightly less ethanol in comparison to the topmost segment of the columnar pipe but no true separation was achieved in both cases. Ethanol can form hydrogen bonds with water molecules that make its separation difficult since ice formation involves hydrogen bonding between molecules. In spite of the fact that their freezing points are far from each other, hydrogen bonding 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 packets of liquid ethanol trapped between solid ice crystals.

Frozen mass of KMnO4 solution (0.1%) after continuous freezing in circular bowl (insulated from top and bottom) for 24 h.
Figure 7
Frozen mass of KMnO4 solution (0.1%) after continuous freezing in circular bowl (insulated from top and bottom) for 24 h.

Freezing is a common technique used to preserve food and biological samples or to extend their storage life. Freezing can downgrade the quality of stored material by mechanically damaging the food structure. Cross-linking of proteins and limited 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 significantly lessened by storage at −60 °C for up to 50 months, our findings are suggesting that slow directional freezing can change the pH of the frozen materials by pushing ions towards unfrozen portions. This will lead to various degradation reactions that can occur because of either a change in pH or build-up of ions. For instance, when 0.1% solution of KMnO4 was frozen together with NaOH (1%) in a circular bowl, they did not react (color was violet) but after one day, when the hydroxide ions gathered at the center, rise in pH reduced the KMnO4 to green K2MnO4.

4

4 Conclusion

Solutions carrying hydrogen, hydroxide, carbonate and bicarbonate ions, as well as gases like CO2, O2, and Cl2 were gradually frozen in a downward direction as well as in a radial manner. In each case, ions significantly moved from solidifying portions to those that were still not solid. Conductance and pH of portions of frozen mass iced up earlier were clearly suggesting that they contain fewer ions in comparison to those portions frozen in the end. The resulting information can be helpful in studying the migration of ions and their possible effect on the quality and life of food and pathological samples during their preservation at low temperatures.

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