Examining the Deliquescence of Lithium Salts and Those of Other Alkali Metals Through a Range of Humidity

Introduction

The use of lithium chloride as a means to control the humidity of air in some commercial buildings is a well-established practice;( Fumo and Goswami, 2002) and numerous lithium-chloride-based dessicant packages are commercially available. As well, in some semi-tropical and tropical climates, a few grains of rice are often added to salt shakers to keep the salt dry and easy to pour, as a common practice, because even sodium chloride can extract moisture from the ambient air in humid environments. And yet the examination of deliquescence, while it has a relatively long history, (Steinbach, 1943) does not appear to have been brought into the classroom or the teaching lab in any meaningful way. Most studies of the deliquescent nature of a material appear to be focused on a specific material, or on some application of the property.( Tereschenko, 2015; Salameh and Taylor, 2006; Hiatt et al., 2011; Kallenberger and Froeba, 2018)

Our own recent work has been centered on a comparison between the deliquescent properties of several lithium salts, and their sodium counterparts, as well as their potassium counterparts, to determine which takes up more water from the surrounding air under the same conditions.( Mather et al., 2020) Table 1 shows a summary of the salts used.

Table 1. Salts used to extract moisture from air

As can be seen from Table 1, several of these salts have some use that impacts the general public, although such uses may not be widely known. The use of lithium salts to treat bi-polar disorder, for example, has been studied for decades, at this point.( O’Connell et al., 1991) Similarly, common table salt as well as sodium sulfate, potassium sulfate, and potassium nitrate, are produced on massive scales each year.

Discussion

An extremely simple experiment, one suitable for early college lab classes, as well as those in secondary schools, is simply to measure amounts of lithium chloride and other salts, place them in pre-weighed containers, and allow them to sit, open to the surrounding atmosphere for some amount of time. In many environments, within one hour some change in the mass of the salts occurs, routinely an increase in the mass. But the salts can also be allowed to sit for longer periods. Students can take measurements of changes in mass over the course of a day, week, or even a semester, if the instructor chooses to extend the observations for this long a period of time. Graphs can be made of the changes in mass of each material over the course of time, allowing students practice in both data gathering and analysis.

Concentrations, Molarity and Molality

An interesting outgrowth of this straightforward set of comparisons is that both molarity and molality can be determined for each of the salts. Molarity is certainly one of the most common measures of concentration taught in general chemistry classes. Although molality is less commonly taught, the mass of the starting salt, and the mass of the material after some time has elapsed provides the mass of the water that was extracted from the air, and thus what is required to determine molality as well.

Weight Percentage

Gathering the changes of mass for each salt over some period of time also allows an instructor to discuss, and to have students compute, weight percentage of water and salt in each of the samples. After looking through a number of general chemistry textbooks, it is fair to say that weight percentage is taught in less detail in general chemistry classes than molarity, although it has not vanished from the curriculum.

Dry Environments

While comparing the mass of water taken up from the air by various lithium, sodium, and potassium salts over time is a useful one, and works in many environments, how can this experiment be performed in extremely dry locales? There are numerous schools, colleges, and universities that exist in or near the deserts of all six of the inhabited continents. It seems logical that the uptake of water by salts in such dry environments will be tiny, and may not be noticeable at all, simply because there is very little water in the air in such places.

To illustrate this phenomenon in a dry environment, we have found that a simple, inexpensive environmental chamber can be made using a large beaker, two small containers, and some improvised lid. The experimental steps can be listed broadly as follows, below.

Experimental

All salts were purchased from scientific supply houses such as Aldrich or Flinn Scientific, and used as purchased.

Experimental steps are:

1. Weigh some amount of a salt (lithium chloride, or one of the others listed in Table 1) and place it in a small beaker or Erlenmeyer flask.

2. Weigh some amount of water, and place it in a second small beaker or Erlenmeyer flask.

3. Place both small containers in a much larger beaker.

4. Cover the beaker with aluminum foil, plastic wrap, or other material. Tape this “lid” onto the large beaker. An example is shown in Figure 1.

5. After an hour – although the experiment can run for more or less time – remove the two small containers, and again weigh the salt in the first, and the water in the second.

Figure 1: Simple environmental chamber for deliquescence experiments

Elevating the Temperature of the Chamber

The use of this simple set up can be further adjusted to optimize the uptake of water by any salts in the chamber. The entire apparatus can be immersed in warm to hot water bath, often simply a large crystallizing dish or a beaker larger than the others. This can be held in place with a variety of weights, and the amount of time for which this elevated temperature experiment can be varied, as has been mentioned above, for other iterations of the experiment and comparisons.

Once again, at the conclusion of some pre-set amount of time, the inner container of salt, and the inner container of water can be weighed, providing the needed data for the just-mentioned discussion and computing of concentrations. If the instructor does not have a single container large enough to run this experiment with the nine salts listed in Table 1, perhaps obviously, several environmental chambers and warm or hot water baths can be constructed.

Conclusions, Educational Applications, Recommendations

Simplicity

This remains a very simple experiment, but one with numerous connections to concepts taught in general chemistry classes,(NGSS; Next Generation Science Standards; Science Education for Responsible Citizenship) such as: gases, and the behavior of matter as it changes from liquid to gas, then back to liquid; concentrations of salts, as molarity, molality, or weight percent; and the phenomenon of deliquescence by a variety of ionic salts.

Importantly, the experiments, and especially the construction of isolated environmental chambers allows for the adjustment of the environment each of the salts is in. This permits the student to maximize the uptake of water, and to make the deliquescence of salts easy and straightforward to observe.

Connections to Gas Phase Chemistry

This set of experiments allows discussion of how water moves as a vapor. This lab therefore allows students to see, understand, and discuss the movement of some material as a gas. Since gases that are visible all tend to be toxic (such as bromine or iodine vapor), it is difficult to present experiments to students in which they can see direct or indirect evidence of a gas moving. By weighing an amount of water and an amount of salt prior to this experiment, then weighing the container of water and of salt after it has been in a controlled humidity chamber, students have direct evidence that a material – water – has moved by being transformed into a gas, then back to a liquid. It can be recommended that any discussion of gas behavior include this phenomenon.

Adaptability

We opted for the use of beakers with nothing more than aluminum foil or plastic coverings as environmentally-controlled chambers because they are simple, very inexpensive, and the easiest to adapt to any teaching lab or classroom. There are certainly other sealed containers that can be used to create a small environment in which the humidity can be adjusted and increased. But many appear to be more expensive than a beaker and plastic cover, and thus might be too expensive for some schools and school districts. One of our aims was to develop a set of experiments that is easy to adapt to virtually any classroom; and we believe this has been accomplished. It can be recommended that for teaching faculty or instructors, those introducing this to their classes for the first time, that some comparison between different means of making such chambers be presented and become part of the learning experience.

References