Investigating Dissolution and Precipitation Phenomena with a

Laboratory Experiment pubs.acs.org/jchemeduc

Investigating Dissolution and Precipitation Phenomena with a Smartphone Microscope Gregg J. Lumetta*,† and Edgar Arcia‡ †

Pacific Northwest National Laboratory, PO Box 999, Richland, Washington 99352, United States Tri-Cities Prep, 9612 St. Thomas Drive, Pasco, Washington 99301, United States

‡

S Supporting Information *

ABSTRACT: A novel smartphone microscope can be used to observe the dissolution and crystallization of sodium chloride at a microscopic level. Observation of these seemingly simple phenomena through the microscope at 100× magnification can actually reveal some surprising behavior. These experiments offer the opportunity to discuss some basic concepts such as how the morphological features of the crystals dictate how the dissolution process proceeds, and how materials can be purified by recrystallization techniques.

KEYWORDS: Elementary/Middle School Science, High School/Introductory Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Inquiry-Based/Discovery Learning, Crystals/Crystallography, Laboratory Equipment/Apparatus, Phases/Phase Transitions/Diagrams, Physical Properties, Precipitation/Solubility

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example, in situ optical microscopy3 and transmission electron microscopy (TEM)4 have been used to investigate nucleation of calcite, and in situ and cryo-TEM techniques have been applied to the study of the crystal growth of various iron oxide minerals.5 The experiments described in this paper could be done on a conventional optical microscope. However, we chose to use a smartphone microscope that was recently developed at Pacific Northwest National Laboratory (PNNL).6 Three primary reasons led to this choice. First, the microscope is inexpensive (on the order of a few U.S. dollars, provided one has access to a 3D printer and a smartphone). Second, middle and high school students, the target demographic for these experiments, often have great affinity for their smartphones. Indeed, the insertion of cell phones and other mobile computing platforms into the classroom is the subject of recent educational research.7

issolution and precipitation of solid phases are familiar to the practitioners of chemistry at all levels. We regularly dissolve solid compounds to make up reagent solutions and isolate desired products by precipitation methods. Students typically encounter these phenomena early in the study of chemistry. Indeed many time-tested qualitative analysis schemes are based on a series of dissolution and precipitation steps. This methodology is encountered as early as the middle school and high school levels as evidenced by the qualitative analysis aspects of the Science Olympiad Crime Busters and Forensics events.1 Exposing students to such schemes also supports numerous Science and Engineering Practices, Crosscutting Concepts, and grade-level specific Disciplinary Core Ideas across multiple grade levels, as described in the Next Generation Science Standards (NGSS).2 Table 1 describes the applicability of the experiments described in this paper to selected NGSS. However, these phenomena are usually observed at the macro scale; that is, we visually watch things dissolve or precipitate with the naked eye. Recently, advanced microscopy tools have been employed to look more closely at these phenomena, especially for precipitation processes. For © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: April 4, 2016 Revised: June 17, 2016

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DOI: 10.1021/acs.jchemed.6b00248 J. Chem. Educ. XXXX, XXX, XXX−XXX

Offering them the ability to perform scientific experiments directly on their phones is appealing. Indeed, several reports have appeared in this journal describing novel colorimetric or spectrophotometric experiments utilizing smartphone technology.8 Third, fortuitously, the focal length for the lens used here (the 100× version of the PNNL Smartphone Microscope) corresponds to the thickness of a typical glass microscope slide (∼1 mm). This allows for a very convenient method of focusing on the specimens, which will be described below. For the experiments described here, we believe this built-in ability to focus at the interface between the microscope slide surface and the salt particles is an advantage over traditional optical microscopes or USB digital microscopes9 because students do not need to spend time focusing the microscope. Described here is the application of the smartphone microscope to observe both the dissolution and crystallization of sodium chloride (NaCl). All the experiments use readily available, nonhazardous chemicals, so they are appropriate for the middle and high school levels. These experiments can be used to demonstrate the principles of how surface topographical features affect the progression of dissolution of solid particles, and purification by recrystallization.10 They also provide direct connections to the Next Generation Science Standard performance expectations at the middle and high school grade levels (e.g., DCI PS1.A, Structure and Properties of Matter, DSI PS1.B, Chemical Reactions, and the crosscutting concept, Scale, Proportion, and Quantity). We also describe an unexpected phenomenon that we observed involving air bubbles released during the dissolution of NaCl crystals. This phenomenon is not always observed, but when it is, it provides an excellent illustration to students that, if they are looking closely enough, interesting things can be observed even in seemingly straightforward experiments.

The experiments directly probe the effects of the interaction of water and NaCl.

Again, the notion of the cubic crystal structure of NaCl leads to the visual observation of cubic-shaped crystals.

The crystallization of NaCl from supersaturated solution as the solution cools provides opportunity for students to formulate models as to the nature of crystallization processes.

Laboratory Experiment

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EXPERIMENTAL SECTION The smartphone microscope used in this work was designed by scientists and engineers at PNNL, but can be replicated anywhere with a 3-D printer and access to high-quality glass beads.6 The microscope consists of a glass bead that serves as the lens, which is held in a plastic clip. The clip is designed such that the bead can be oriented over the lens to the smartphone camera. The Web site provided in ref 6 provides suggestions on where to obtain the requisite glass beads, and 3-D printer files for producing the clips needed to hold the beads. The smartphone microscope is based on the same optical principle used by Antonie van Leeuwenhoek in his pioneering studies of microbiology.11 Other low-cost microscopes have been reported that are based on this principle, such as the Foldscope.12 The bead used in this work produces a 100× magnification. The field of view is approximately 1250 μm (across the short side of the rectangular phone screen). This value was determined by measuring a 1 mm increment on a transparent plastic ruler. There are number of options for exactly how the microscope is configured, but we found that the most convenient and reproducible option is to place the lens over the camera on the front side of the smartphone and operate the camera in the “selfie” mode. A glass microscope slide can be placed on top of the clip containing the glass bead, and ambient light is sufficient for illumination (Figure 1), although the lens position should be adjusted to obtain maximum illumination on the screen. When configured in this way, the specimen to be measured will be in focus when placed directly on a 1 mm thick

Performance Expectation MS-PS1-2: Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred. Performance Expectation MS-PS1-4: Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed. Performance Expectation HS-PS1: Matter and Interactions. HS-PS1-3: Plan and conduct an investigation to gather evidence to compare the structure of substances at the bulk scale to infer the strength of electrical forces between particles. Performance Expectation HS-ESS2: Earth’s Systems. HS-ESS2-5: Plan and conduct an investigation of the properties of water and its effects on Earth materials and surface processes.

Disciplinary Core Idea PS1.B: Chemical Reactions (applicable at middle school and high school levels)

Applicability Next Generation Science Standard

Disciplinary Core Idea PS1.A: Structure and Properties of Matter (applicable at middle school and high school levels)

Table 1. Applicability of Selected Next Generation Science Standards2

The manner in which NaCl crystals dissolve (kinks and steps dissolving more rapidly than terrace sites) can be related to the crystal structure. The bulk properties of NaCl (such as its cubic shape) are determined by the electrical forces between the sodium and chloride ions. The NaCl solid is formed from repeating subunits consisting of sodium and chloride ions. Dissolution of a solid salt can be driven by either (or both) (a) enthalpy, in which the energy gained by solvation of the dissolved ions exceeds the crystal lattice energy; or (b) entropy, in which the entropy gained by destruction of the ordered crystal lattice exceeds any unfavorable enthalpic term. The latter is the case for dissolution of NaCl in water. Students analyze and interpret observations related to the solubility properties of NaCl.

Journal of Chemical Education

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°C. The formation of NaCl crystals were recorded over a 30 min period. Redissolution of Sodium Chloride

After the crystallization experiment described above, dissolution of the resulting crystals was observed in the microscope by first starting the video camera and then adding two drops of distilled water (at the ambient temperature of 14 °C) to the slurry of crystals that had formed.

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HAZARDS There are no significant chemical hazards associated with these experiments. If saturated salt solutions are prepared in boiling water, care should be taken to avoid burns, e.g., handling hot containers with tongs or with appropriately insulated gloves. There is a danger of embarrassment if the microphone on the smartphone is not disabled during recording of video images.

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Figure 1. Configuration of smartphone microscope apparatus; the CD is used as a shim so that the glass slide is level.

RESULTS AND DISCUSSION Most of the results described here are those obtained when first developing the ideas for simple experiments utilizing the smartphone microscope. The experiments were later performed by high school chemistry students, and the lessons learned from their performance in the high school setting are discussed.

glass slide. In this work, the measurements were made under ambient fluorescent lighting because this is likely to be the case in most laboratory settings. However, lighting is an obvious parameter that can easily be manipulated to optimize the performance of the smartphone microscope. For example, LED “book” lights could be adapted to enhance the illumination of the specimens. The clip-on microscope bead will work with most smartphones or tablet devices as long as the thickness of the device does not exceed the opening of the clip. In this work, a Samsung Galaxy S4 mini was used. To ease viewing of the image, it is recommended that the screen on the device be set to maximum illumination. Videos of the dissolution processes were recorded with the camera application that came with the phone. For slower processes, such as precipitation phenomena, it is useful to employ time-lapse photography. There are a number of smartphone applications available to perform timelapse photography. In this work, LapseIt Pro13 was used to take images every 15 s at a resolution of 720 p. In the classroom setting, students were simply instructed to take a still image every 15 s.

Static Images

To get a feel for using the smartphone microscope, it is useful to have students capture static micrographs of solid materials. Figure S1 displays micrographs obtained with the smartphone microscope for a number of household chemical items including table salt, sea salt, sugar, vitamin C crystals, baking soda, and Epsom salts. A particularly interesting case is that comparing table salt to sea salt (Figure 2). The classic cubic shape of NaCl was clearly evident in the table salt, but this was not the case for the sea salt. In the latter case, irregularly shaped

Experiment 1: Dissolution of Sodium Chloride

A few grains of table salt (Morton) were placed on the microscope slide, and the slide was adjusted until one or more crystals were in view. After the video recorder was started, a drop of distilled water (store bought) was placed on the table salt crystals. This action typically resulted in the crystals being moved out of the view of the microscope, so the slide was immediately readjusted to bring the table salt crystals into view. Recording was stopped once the primary crystal had dissolved. This process was also performed using laboratory grade NaCl from Fisher Scientific, with similar results. The ambient temperature during these experiments was 11 °C. Experiment 2: Crystallization of Sodium Chloride

A saturated solution of NaCl (Fisher laboratory grade) was heated to boiling in a microwave oven, with excess solid NaCl present to ensure supersaturation upon cooling. After the undissolved NaCl had been allowed to settle, one drop of the hot NaCl solution was transferred to the microscope slide and the time-lapse application was started. For best results, the microscope slide was adjusted so that the image was focused close to the edge of the drop. The ambient temperature was 14

Figure 2. Static micrographs of (a) Morton table salt, (b) Alessi Mediterranean Sea salt, and (c) recrystallized sea salt. C




Figure 3. Still images take from Movie S1 showing the dissolution of a table salt crystal in distilled water; ambient temperature was 11 °C.

so it is not entirely surprising that insoluble material was observed. However, since the rhombohedral particle appeared to have been imbedded in the NaCl crystal, it seemed likely that it was not attributed to an additive introduced after the NaCl had formed. Of course, this second observation is specific to the particular table salt crystal dissolved in this experiment, but any nonspherical material remaining after dissolution of the primary NaCl crystal can be attributed to insoluble impurities (which we found to be fairly common for commercial table salt). A third observation can be made regarding the manner in which the dissolution proceeds. Dissolution appears to be faster along the corners and edges of the cube, compared to at the face of the cube. This leads to a gradual rounding of the NaCl particle. This observation is consistent with the terrace−step− kink (TSK) model of mineral dissolution.14 In this case, a terrace site would correspond to the flat face of the cube, a step would be at the intersection of two faces, and a kink would be a corner at which three faces intersect. The TSK model can intuitively be understood by envisioning the ease with which a solvent molecule can approach a potential solute ion in the crystal. At a terrace site, the solvent can only approach from one direction, whereas at a step the solvent can approach from two directions, and the solvent can approach from three directions at a kink site. Therefore, the expected relative dissolution rates would be kink > step > terrace, which is qualitatively what was observed in this work. Finally, at ∼47 s into Movie S1, an air bubble near the upper left of the primary NaCl crystal begins to pulsate. This pulsation accelerates the dissolution of the crystal surface adjacent to that air bubble. The air bubble also expands during

crystals were observed, suggesting significant differences in the manner in which these crystals formed, or perhaps the sea salt particles observed were broken off larger regularly shaped crystals during transportation and handling. Regardless, the sea salt can be converted to the classic NaCl cubic shape (Figure 2c) by preparing a supersaturated solution in boiling water, transferring a drop of the hot supersaturated solution to a microscope slide, and allowing the salt to recrystallize upon cooling. This crystallization process can be followed in situ in a manner similar to the experiment described below. Experiment 1: Dissolution of Sodium Chloride

The dissolution of a crystal of table salt in distilled water was followed in situ, with the results shown in Figure 3 and Movie S1. This very simple experiment resulted in a number of interesting observations. First, at the initiation of the dissolution process a significant expulsion of gas occurred; much of this gas remained as spherical bubbles. This was attributed to air trapped near the surface of the NaCl crystal; the amount of gas evolved decreased as the outer part of the crystal was dissolved away. This phenomenon was also observed when laboratory grade NaCl (Fisher Scientific) was dissolved in a similar manner (Movie S2). Second, other material emerged that appeared to be insoluble impurities in the salt. For example, in Figure 3, a transparent rhombohedrally shaped particle clearly emerged from the upper right corner of the NaCl crystal at approximately 35 s into the dissolution process. The chemical identity of this particle was not determined. The package from which the table salt was taken indicates that calcium silicate was added as an anticaking agent, D




Figure 4. Still images take from Movie S3 showing the crystallization of sodium chloride in distilled water; ambient temperature was 14 °C.

Student Performance of the Experiments

this pulsation. The phenomenon appears general in that it was also observed in Movie S2 for an air bubble near the top of the screen in that video. This observation was quite surprising and unexpected. As such, it provides potential opportunity to instruct students that, even for what might at first glance appear to be a relatively straightforward experiment, interesting observations can be made if they look carefully enough. Although we do not have a definitive explanation for the phenomenon, the observed pulses are most likely acoustic waves emitted from a pulsating air bubble. We have not been able to find reference to a situation specific to the location of an air bubble adjacent to a dissolving solid, however phenomena associated with acoustic waves emitted from pulsating air bubbles have been documented since the 1950s.15

Experiments 1 and 2 were performed in two separate sections of a high school chemistry class. Approximately 40 students participated, and they were divided into groups of two. The experimental procedures provided to the students are given in the Supporting Information. For both class periods, the students were able to complete experiments 1 and 2 within 30 min. However, when the students performed the NaCl crystallization step, it proceeded much more rapidly than that shown in Movie S3 and resulted in NaCl crystals that were much smaller. We hypothesize that the rapid crystallization was due to carryover of some undissolved NaCl that acted as seed crystals accelerating the precipitation. To avoid this, filtration of the saturated NaCl solution before providing it to the students is recommended. Slowing of the NaCl crystallization process could add another 10 min or so to the time required to complete the experiments. The students were surveyed in order to gain insight into their impressions of these experiments. A total of 36 students responded to the survey. Of these, 25 students indicated a positive impression and appeared to enjoy performing the experiments, 4 students had overall negative comments, and 7 students were ambivalent. Twenty-seven of the students indicated that they enjoyed using the smartphone microscope. Further evaluation of the students’ survey responses suggested that most of the negative impressions of the experiments stemmed from frustrations in getting good quality images from the smartphone microscope. These difficulties were also observed during their actual performance of the experiments. This was a very important lesson in that it revealed two issues. First, the successful application of the smartphone microscope used in this work is somewhat dependent on the actual smartphone used. If the students are struggling in this regard, a potential solution is to provide them with an alternative smartphone or tablet (or if they are working in pairs, they can perhaps switch to the other partner’s smartphone). Related to this was the observation that if a student has a protective case on their smartphone, it might be necessary to remove this so that the lens seats correctly over the smartphone camera.

Experiment 2: Crystallization of Sodium Chloride

The crystallization of NaCl was observed by time-lapse photography as a supersaturated solution cooled directly on the microscope slide (Figure 4 and Movie S3). Well formed, transparent crystals were formed during this process. Evidence for crystallization was seen within the first 15 s, and crystallization was mostly complete within 15 min, although some subtle changes to the system continued past that point in time. Redissolution of Sodium Chloride

The NaCl crystals formed from the saturated solution described in the preceding paragraph were redissolved by adding two drops of distilled water. Again, the progress of the dissolution was monitored by video recording (Movie S4). In this case the NaCl crystals dissolved cleanly with very little undissolved residue remaining (Figure S2). It is also interesting that no gas evolution was observed during dissolution of the freshly recrystallized NaCl indicating that no air was trapped within the crystals during the crystallization process. Comparison of the results such as those seen in Movies S2 and S4 provides students with a visual indication of the effectiveness of recrystallization as a means to purify materials. E




2008, 3, http://eric.ed.gov/?id=EJ1067124 (accessed June 2016). (b) Graham, E. Using Smartphones in the Classroom; http://www.nea. org/tools/56274.htm (accessed June 2016). (c) Abend, L. L. Commonalities in Pedagogy Situating Cell Phone Use in the Classroom. PhD Dissertation; Northcentral University: 2013. (8) (a) Montangero, M. Determining the Amount of Copper(II) Ions in a Solution Using a Smartphone. J. Chem. Educ. 2015, 92, 1759−1762. (b) Campos, A. R.; Knutson, C. M.; Knutson, T. R.; Mozzetti, A. R.; Haynes, C. L.; Penn, R. L. Quantifying Gold Nanoparticle Concentration in a Dietary Supplement Using Smartphone Colorimetry and Google Applications. J. Chem. Educ. 2016, 93, 318−321. (c) Knutson, T. R.; Knutson, C. M.; Mozzetti, A. R.; Campos, A. R.; Haynes, C. L.; Penn, R. L. A Fresh Look at the Crystal Violet Lab with Handheld Camera Colorimetry. J. Chem. Educ. 2015, 92, 1692−1695. (d) Koesdjojo, M. T.; Pengpumkiat, S.; Wu, Y.; Boonloed, A.; Huynh, D.; Remcho, T. P.; Remcho, V. T. Cost Effective Paper-Based Colorimetric Microfluidic Devices and Mobile Phone Camera Readers for the Classroom. J. Chem. Educ. 2015, 92, 737−741. (e) Grasse, E. K.; Torcasio, M. H.; Smith, A. W. Teaching UV-Vis Spectroscopy with a 3D-Printable Smartphone Spectrophotometer. J. Chem. Educ. 2016, 93, 146−151. (9) Lopes, F. S.; Baccaro, A. L. B.; Santos, M. S. F.; Gutz, I. G. R. Oxygen Bleach under the Microscope: Microchemical Investigation and Gas-Volumetric Analysis of a Powdered Household Product. J. Chem. Educ. 2016, 93, 158−161. (10) (a) Sharpe, A. G. Solubility Explained; http://www.rsc.org/eic/ sites/default/files/Solubility-explained.pdf (accessed June 2016). (b) Recrystallization; http://www.wiredchemist.com/chemistry/ instructional/laboratory-tutorials/recrystallization (accessed June 2016). (11) Antonie van Leeuwenhoek; https://en.wikipedia.org/wiki/ Antonie_van_Leeuwenhoek (accessed June 2016). (12) Cybulski, J. S.; Clements, J.; Prakash, M. Foldscope: OrigamiBased Paper Microscope. PLoS One 2014, 9 (6), e98781. (13) Lapse It home page; http://lapseit.com/ (accessed June 2016). (14) Tromans, D.; Meech, J. A. Enhanced Dissolution of Minerals: Microtopography and Mechanical Activation. Miner. Eng. 1999, 12, 609−625. (15) (a) Strasberg, M. Gas Bubbles as Sources of Sound in Liquids. J. Acoust. Soc. Am. 1956, 28, 20−26. (b) Devin, C. Survey of Thermal, Radiation, and Viscous Damping of Pulsating Air Bubbles in Water. J. Acoust. Soc. Am. 1959, 31, 1654−1667.

Second, the ambient lighting in the laboratory can have an effect on the quality of the image. In our laboratory, we found that the lighting was not evenly distributed. In some cases, the problem of image quality was resolved simply by having the students relocate to a more well-lit location in the laboratory.

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CONCLUSION The smartphone microscope offers a convenient way for students to explore, in microscopic detail, dissolution and precipitation phenomena that they are normally familiar with on a macroscopic scale. Examining these seemingly simple systems at a microscopic scale can lead to some surprising observations. The smartphone microscope has further appeal in that students can perform actual scientific experiments using their cell phones. The experiments described can easily be performed within a 50 min period. The smartphone microscope should also be viewed as a useful tool for inquiry-based learning. Because of its ease of use, students should be encouraged (and given the freedom) to explore other chemical systems with the smartphone microscope.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.6b00248. Images of crystals and experimental procedures (PDF, DOCX) Video files of dissolution and crystallization (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Frances Smith of Pacific Northwest National Laboratory for reviewing this manuscript and providing many helpful suggestions.

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REFERENCES

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Investigating Dissolution and Precipitation Phenomena with a

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