Related Experiment Video
Updated: Jul 13, 2026
08:55
Formulations for Freeze-drying of Bacteria and Their Influence on Cell Survival
Published on: August 3, 2013
Acidity of frozen electrolyte solutions.
Carmen Robinson1, C S Boxe, M I Guzman
1W. M. Keck Laboratories, California Institute of Technology, Pasadena, CA 91125, USA.
The Journal of Physical Chemistry. B
|April 14, 2006
Summary
Freezing electrolyte solutions alters acidity due to ion imbalances. This study uses 3-fluorobenzoic acid with NMR to measure these changes, revealing how different salts impact frozen solution pH.
Area of Science:
- Physical Chemistry
- Materials Science
- Analytical Chemistry
Background:
- Ice formation from electrolyte solutions can lead to significant changes in the unfrozen portion's acidity.
- Electrical imbalances arise from preferential ion incorporation into ice, driving interfacial proton (H+) and hydroxide (HO-) transport.
- These acidity shifts are often overlooked but can critically influence chemical processes in frozen media.
Purpose of the Study:
- To investigate and quantify the changes in acidity of frozen electrolyte solutions.
- To utilize 3-fluorobenzoic acid as an in situ probe for measuring acidity via Nuclear Magnetic Resonance (NMR) spectroscopy.
- To understand how different ionic compositions and freezing conditions affect acidity shifts.
Main Methods:
- Growing ice from various electrolyte solutions (e.g., NaCl, (NH4)2SO4, Na2SO4, zwitterionic buffer).
- Employing solid-state 19F NMR spectroscopy to measure the effective chemical shift of 3-fluorobenzoic acid.
- Correlating NMR chemical shift changes with acidity variations in the unfrozen portion of the frozen solutions.
Main Results:
- Observed significant and permanent changes in acidity upon freezing, dependent on the specific ions present.
- NaCl solutions became more basic, while (NH4)2SO4 and Na2SO4 solutions became more acidic.
- Solutions of 2-(N-morpholino)ethanesulfonic acid showed minimal acidity change upon freezing.
Conclusions:
- The acidity of frozen electrolyte solutions is strongly influenced by ion selectivity during ice growth.
- Solid-state NMR measurements of 3-fluorobenzoic acid provide an effective method for in situ acidity assessment.
- Acidity scales derived from these NMR measurements can potentially determine the ionization state of weak acids/bases in frozen environments.
Related Concept Videos
02:21Electrolyte and Nonelectrolyte Solutions
Substances that undergo either a physical or a chemical change in solution to yield ions that can conduct electricity are called electrolytes. If a substance yields ions in solution, that is, if the compound undergoes 100% dissociation, then the substance is a strong electrolyte. Complete dissociation is indicated by a single forward arrow. For example, water-soluble ionic compounds like sodium chloride dissociate into sodium cations and chloride anions in aqueous solution.
04:02Weak Acid Solutions
Few compounds act as strong acids. A far greater number of compounds behave as weak acids and only partially react with water, leaving a large majority of dissolved molecules in their original form and generating a relatively small amount of hydronium ions. Weak acids are commonly encountered in nature, being the substances partly responsible for the tangy taste of citrus fruits, the stinging sensation of insect bites, and the unpleasant smells associated with body odor. A familiar example of a...
02:54Ions as Acids and Bases
Salts with Acidic Ions
Salts are ionic compounds composed of cations and anions, either of which may be capable of undergoing an acid or base ionization reaction with water. Aqueous salt solutions, therefore, may be acidic, basic, or neutral, depending on the relative acid-base strengths of the salt’s constituent ions. For example, dissolving the ammonium chloride in water results in its dissociation, as described by the equation:
Salts are ionic compounds composed of cations and anions, either of which may be capable of undergoing an acid or base ionization reaction with water. Aqueous salt solutions, therefore, may be acidic, basic, or neutral, depending on the relative acid-base strengths of the salt’s constituent ions. For example, dissolving the ammonium chloride in water results in its dissociation, as described by the equation:
04:08Determining the pH of Salt Solutions
The pH of a salt solution is determined by its component anions and cations. Salts that contain pH-neutral anions and the hydronium ion-producing cations form a solution with a pH less than 7. For example, in ammonium nitrate (NH4NO3) solution, NO3− ions do not react with water whereas NH4+ ions produce the hydronium ions resulting in the acidic solution. In contrast, salts that contain pH-neutral cations and the hydroxide ion-producing anions form a solution with a pH greater than 7. For...
03:38Polyprotic Acids
Acids are classified by the number of protons per molecule that they can give up in a reaction. Acids such as HCl, HNO3, and HCN that contain one ionizable hydrogen atom in each molecule are called monoprotic acids. Their reactions with water are:
01:33Introduction to Electrolytes
In humans, electrolytes play a vital role in various physiological processes. Balancing electrolyte levels is essential for normal body functions; their imbalance can be life-threatening. The major electrolytes include sodium, potassium, chloride, calcium, phosphate, and bicarbonate. They are primarily involved in physiological processes, such as nerve signal transmission, membrane trafficking, muscle contraction, buffering body fluids, and balancing water levels in the body.
Role of Sodium
One...
Role of Sodium
One...