Transition+Temperature+of+Sodium+Thiosulphate+Pentahydrate

toc

** __Transition Temperatures of Hydrates__ ** A research project by: Lowell Kwan, Kristie Lau, Kam Ganesan, Sandy Hu

When substances change from one solid phase into another the change is accompanied by absorption or release of heat. The temperature at which such a change takes place is called the transition temperature. This change can be thought of as a change from one compound to another. In this experiment, the compound used is Sodium Thiosulphate Penta Hydrate (Na2S2O3∙5H20). Hydrates are formed when ionic compounds are formed in water and then isolated as solids, therefore the water remains trapped in the compound. Hydrates are written with a dot between the ionic compound and the amount of water molecules involved with the compound. This indicates the number of water molecules per one molecule of the ionic compound. Though water is involved in the chemical formula, this does not indicate that the substance is wet. In fact, many hydrates have dry appearance and touch. When water is removed from the hydrates, one is left with the anhydrous compound (dehydrate). When these hydrates are heated (change in temperature), the water will evaporate and thus the compounds will no longer be hydrates. When water vapour is added to the dehydrates, the dehydrates absorb the water vapour to form a hydrate.

In this experiment, an attempt will be made to find the transition temperature of a specific compound: Sodium Thiosulphate Penta Hydrate (Na2S2O3∙5H20). Sodium Thiosulphate Pentahydrate (Na2S2O3•5H2O) is a colourless crystalline compound with a variety of uses. It is commonly used in photographic processing, gold extraction, the manufacturing of bleach, the manufacturing of hand warmers and heating pads, as well as an antidote to cyanide poisoning.

A variety of methods can be used to determine the transition temperatures of salt hydrates. The basis of the method used in this experiment is that the temperature will rise constantly until the hydrate evaporates, thus changing state. Once the compound appears to be completely liquid, it is removed from heat and cooled, such that the temperature falls at a definite rate. When cooling the hot solution in an undisturbed manner, super cooling usually occurs below the transition point. After falling 4 or 5 degrees C, the temperature of the compound suddenly rises as a result of adding a single crystal of the compound, and crystallizes. At this time, the point where the temperature remains constant thereby provides an accurate transition temperature.

Thus, on heating crystals of Na2S2O3. 5H2O, the temperature should rise normally until the hydrate begins to change into an anhydrous salt (i.e. water evaporates). Then the temperature remains constant until the transformation is complete. If the reverse change is allowed to take place, there is an evolution of heat. The temperature will fall at a definite rate until the transition point is reached. Then the temperature stays constant until all of the substance has been transformed. Therefore, by plotting a time vs. temperature curve, the transition point can be determined. For this experiment, the compound will be placed in a test tube and heated in a water bath. An air jacket, a container placed surrounding the test tube will prevent outside air flow while cooling the compound. As the compound cools to 40 degrees C after the compound is heated, a single crystal of Sodium Thiosulphate Penta Hydrate (Na2S2O3∙5H20) will be added, acting as a seed crystal. Seed crystals are used to expedite the crystallization of the compound as they eliminate the need for random molecular collision/interaction. By introducing an already pre-formed basis of the target crystal to act upon, the intermolecular interactions are formed much more easily than relying on random flow. Thus, the temperature at which it stabilizes at after the see crystal is added is the transition temperature.

__Materials__

 * Retort stand
 * Test tube clamp
 * Ri ng clamp
 * Wire gauze
 * Bunsen burner
 * Flint lighter
 * Beaker tongs
 * Thermometer
 * Boiling tube
 * 15 g of Sodium Thiosulphate Pentahydrate
 * Electronic scale
 * 150 mL of water
 * Temperature Probe
 * Computer
 * Scupula
 * 1 L beaker
 * Safety goggles

__Procedure   __
1) Wear safety goggles 2) Set up a temperature probe attached to a computer via the CBL 3) Set up computer and open program LoggerPro. Ensure that LoggerPro will record temperature in relation to time.      4) Set up retort stand and clamp a ring clamp 25 cm above the base 5) Place a piece of wire gauze over the ring clamp     6) Place a 1L beaker by the retort stand opposite the ring clamp 7) Measure 150 mL of water into a 250 mL beaker 8) Place the beaker on top of the wire gauze 9) Measure 15 g of Sodium Thiosulphate Pentahydrate with use of an electronic scale. 10) Place the sodium thiosulphate pentahydrate in the boiling tube as well as the stirring rod and the thermometer probe. Ensure that the thermometer probe and stirring rod are submerged in the compound. 11) Place the boiling tube into the beaker and ensure that the so dium thiosulphate pentahydrate is completely surrounded by water      12) Securely fasten the boiling tube in place with a test tube clamp 13) Record the initial temperature

14) Place a Bunsen burner under the ring clamp 15) Ignite the Bunsen burner with a flint lighter and adjust to a moderate flame 16) Start data collection on LoggerPro with the use of the temperature probe  17) As the substance is heating, keep the temperature rising stedily by stirring well with the stirring rod. 18) When the compound changes into a liquid state, allow the temperature to rise another 5˚C 19) Turn off the flame. Record the time at which the flame was turned off. 20) Quickly adjust the test tube clamp in order that the boiling tube is placed in the air jacket, the 1L beaker. 21) Allow the compound to cool in the air jacket 22) When the temperature decreases to 40˚C, add one crystal of sodium thiosulphate pentahydrate to the boiling tube and stir. 23) Once the temperature remains constant, record the temperature and stop data collection. This is the transition temperature.

Safety Precautions

 * Sodium Thiosulphate Pentahydrate is slightly toxic and harmful to skin. It is an eye irritant and can cause health complications if ingested. It is incompatible with acids and oxidizing agents. Handle with care.
 * When using a Bunsen burner make sure flammable materials do not surround the burner's area.
 * Make sure you wear safety goggles.
 * Be cautious when handing hot objects to avoid burning- if skin is burnt, rinse under cold water.
 * When finished with the burner, turn gas off completely.
 * Safety goggles must be worn at all times, contact lenses should not be worn.
 * Long hair must be tied back and loose clothing must be secured.
 * Wash your hands after handling chemicals.
 * Dispose of the compound carefully, following teacher’s directions
 * Make sure you know where the fire extinguisher, chemical shower and eye wash station are, in case of an emergency.

__Observations__
The following table displays the relationship between time and temperature of sodium triosulphate pentaphydrate over the period of 30 minutes – 0 to 1800 seconds. The time is shown at 30 second intervals. The table also displays the occurring procedural steps. Data in blue is mathematically approximated.

Relationship Between Time and Temperature (0s to 1800s)
For a greater understanding, below is a table displaying the temperature, recorded at 30 second intervals, of sodium thiosulphate pentahydrate after the heat was turned off. At time 331, the heat was turned off and the boiling tube was placed in an air jacket. Once the temperature reached 40˚C, one crystal of sodium thiosulpahte pentahydrate was added.
 * = Time (s)  ||=  Temperature (˚C)  ||=  Procedural Step  ||
 * = 0  ||=  21.1  ||=  Initial Temperature  ||
 * = 30  ||=  20.9  ||=   ||
 * = 60  ||=  21.9  ||=   ||
 * = 90  ||=  24.5  ||=   ||
 * = 120  ||=  29.9  ||=   ||
 * = 150  ||=  44.7  ||=   ||
 * = 180  ||=  50  ||=   ||
 * = 210  ||=  55.9  ||=   ||
 * = 240  ||=  68.6  ||=   ||
 * = 270  ||=  67  ||=   ||
 * = 300  ||=  73  ||=  At 294s, all of the substance has become liquid. Substance is let to heat for to an additional 5˚C  ||
 * = 330  ||=  65.6  ||=  Temperature peaks at 73.8˚C having risen another 5˚C. Heat is turned off. Boiling tube is placed in an air jacket. Substance begins to cool.  ||
 * = 360  ||=  64.3  ||=   ||
 * = 390  ||=  64.8  ||=   ||
 * = 420  ||=  63.3  ||=   ||
 * = 450  ||=  62.4  ||=   ||
 * = 480  ||=  61  ||=   ||
 * = 510  ||=  60.1  ||=   ||
 * = 540  ||=  58.4  ||=   ||
 * = 570  ||=  56.8  ||=   ||
 * = 600  ||=  56.2  ||=   ||
 * = 630  ||=  55.3  ||=   ||
 * = 660  ||=  54.7  ||=   ||
 * = 690  ||=  53.8  ||=   ||
 * = 720  ||=  53.1  ||=   ||
 * = 750  ||=  52.2  ||=   ||
 * = 780  ||=  51.4  ||=   ||
 * = 810  ||=  50.8  ||=   ||
 * = 840  ||=  50  ||=   ||
 * = 870  ||=  49  ||=   ||
 * = 900  ||=  48.2  ||=   ||
 * = 930  ||=  47.5  ||=   ||
 * = 960  ||=  46.8  ||=   ||
 * = 990  ||=  46  ||=   ||
 * = 1020  ||=  45.5  ||=   ||
 * = 1050  ||=  45  ||=   ||
 * = 1080  ||=  44.2  ||=   ||
 * = 1110  ||=  43.5  ||=   ||
 * = 1140  ||=  43.1  ||=   ||
 * = 1170  ||=  42.4  ||=   ||
 * = 1200  ||=  41.8  ||=   ||
 * = 1230  ||=  41.1  ||=   ||
 * = 1260  ||=  40.8  ||=   ||
 * = 1290  ||=  40.3  ||=  The substance reaches 40˚C at 1294s. One crystal of sodium thiosulpahte pentahydrate is added to the boiling tube  ||
 * = 1320  ||=  42.7  ||=   ||
 * = 1350  ||=  47.2  ||=  As the temperature begins to stabilize, 47.6˚C is the transition temperature.  ||
 * = 1380  ||=  47.9  ||=   ||
 * = 1410  ||=  48.2  ||=   ||
 * = 1440  ||=  47.9  ||=   ||
 * = 1470  ||=  47.9  ||=   ||
 * = 1500  ||=  47.8  ||=   ||
 * = 1530  ||=  47.7  ||=   ||
 * = 1560  ||=  47.7  ||=   ||
 * = 1590  ||=  47.6  ||=   ||
 * = 1620  ||=  47.8  ||=   ||
 * = 1650  ||=  47.6  ||=   ||
 * = 1680  ||=  47.8  ||=   ||
 * = 1710  ||=  47.7  ||=   ||
 * = 1740  ||=  47.6  ||=   ||
 * = 1770  ||=  47.6  ||=   ||
 * = 1800  ||=  47.6  ||=   ||

Relationship Between Time and Temperature After Heating (311s to 1781s)

 * = Time (s)  ||=  Temperature (˚C)  ||
 * = 311  ||=  73.8  ||
 * = 341  ||=  65.1  ||
 * = 371  ||=  63.7  ||
 * = 401  ||=  64.2  ||
 * = 431  ||=  63.1  ||
 * = 461  ||=  61.9  ||
 * = 491  ||=  60.6  ||
 * = 521  ||=  59.5  ||
 * = 551  ||=  57.7  ||
 * = 581  ||=  56.5  ||
 * = 611  ||=  55.7  ||
 * = 641  ||=  55.1  ||
 * = 671  ||=  54.4  ||
 * = 701  ||=  53.5  ||
 * = 731  ||=  52.7  ||
 * = 761  ||=  51.8  ||
 * = 791  ||=  51.1  ||
 * = 821  ||=  50.2  ||
 * = 851  ||=  49.5  ||
 * = 881  ||=  48.7  ||
 * = 911  ||=  47.9  ||
 * = 941  ||=  47  ||
 * = 971  ||=  46.6  ||
 * = 1001  ||=  46  ||
 * = 1031  ||=  45.1  ||
 * = 1061  ||=  44.7  ||
 * = 1091  ||=  43.9  ||
 * = 1121  ||=  43.6  ||
 * = 1151  ||=  42.8  ||
 * = 1181  ||=  42.1  ||
 * = 1211  ||=  41.6  ||
 * = 1241  ||=  41.1  ||
 * = 1271  ||=  40.7  ||
 * = 1301  ||=  40  ||
 * = 1331  ||=  44.3  ||
 * = 1361  ||=  47.7  ||
 * = 1391  ||=  48.1  ||
 * = 1421  ||=  47.9  ||
 * = 1451  ||=  48.1  ||
 * = 1481  ||=  47.9  ||
 * = 1511  ||=  47.9  ||
 * = 1541  ||=  47.9  ||
 * = 1571  ||=  47.8  ||
 * = 1601  ||=  47.8  ||
 * = 1631  ||=  47.8  ||
 * = 1661  ||=  47.8  ||
 * = 1691  ||=  47.6  ||
 * = 1721  ||=  47.6  ||
 * = 1751  ||=  47.6  ||
 * = 1781  ||=  47.7  ||

Graph: Temperature of Sodium Thiosulphate Pentahydrate
Below is the plotted graph displaying the visual relationship between time (s) and temperature (˚C). The full duration of the experiment is shown.

Graph: Temperature of Sodium Thiosulphate Pentahydrate After Heating (311s to 1781s, 30 Second Intervals)
The following graph visually represents the relationship between time and temperature during the time period of 311s-1781s at intervals of 30 s. During this time period, the heat is turned off at 311s and a crystal is added at 1294 s.



__Conclusion__
For this experiment, the transition temperature was to be found on the basis that adding a small crystal of the compound after it was cooled (i.e supercooled state) would cause evolution of heat and a rapid increase in temperature, and also cause the liquid compound to rapidly crystallize. Since it was known that Sodium Thiosulphate Pentahydrate is especially prone to the effects of using a seed crystal to speed up crystallization, this method was decided upon as the best way to determine an accurate transition temperature. Based on our experiment and analysis of our results, we found the transition temperature of Sodium Thiosulphate Pentahydrate to be approximately 47.6 ˚C. Since this result is very close to the theoretical transitional temperature of Sodium Thiosulphate Pentahydrate, which is approx. 48˚C, it was concluded that our experiment showed fairly accurate results and any discrepancies could be attributed errors during the experiment.

Endothermic vs. Exothermic
The single crystal of sodium thiosulphate acts as a seed crystal and speeds up the crystallization process, as the compound releases heat when the crystal is added, and the temperature of the compound rapidly rises, showing that the reaction is an exothermic one. The seed crystal allows the intermolecular forces to react and collide instead of colliding slowly in a random flow. In this manner, seed crystals increase the speed of recrystallization.

The temperature changes occurring show a steady fall as the liquid cools, and once a crystal is added to the supercooled liquid, the temperature rapidly rises as crystallization takes place. Exothermic reactions release heat, while endothermic reactions absorb heat. In this experiment the compound is heated until it changes states, then cooled. A crystal is added to the supercooled liquid, which in turn releases heat. As crystallization takes place, the temperature rises, confirming the process is exothermic.

Transition Temperatures
The transition temperature is the temperature at which a compound changes from one solid to another. This was determined to be the point where temperature stayed constant for a short period after rising rapidly when the crystal was added. Essentially, the transition temperature of a compound is the temperature at which two states of the compound can exist in equilibrium.

Practical Uses
The transition temperature of a compound can be useful when synthesizing various other products with the compound. For this compound specifically, it is useful to know the transition temperature when making things with sodium thiosulphate so as to know the optimum temperature at which to combine other things. For example, sodium thiosulphate is used in a variety of hand warmers and electric heating pads that utilize exothermic crystallization to produce heat. When making such products, it would be useful to know the temperature at which they crystallize, produce heat, etc.

In order to successfully carry out this experiment, it was done three or four times with a few modifications made each time in order to better the result and make the experiment simpler and more efficient. Firstly, the use of a normal thermometer was replaced with the use of a temperature probe so that measured temperatures would be much more accurate and continuous. Secondly, instead of manually writing down data, temperatures were collected every second using Logger Pro software. Thirdly, an air jacket was added to the experiment in order to minimize outside air flow.

In terms of suggestions to improve this experiment further, Logger Pro can be replaced with better software that allows for continuous data collection, unlike Logger Pro, where a new file had to be opened every 5 minutes. Also, a better method may be utilized to judge whether the compound had liquefied, as the method used for this experiment was mere visual judgment which is not very accurate.

Sources of Experimental Error
In determining the transition temperature of hydrates such as sodium thiosulphate pentahydrate, knowledge of experimental errors are important as they may have lead to inaccurate results. Some sources of experimental error can be attributed to the equipment used. For instance, the results from the experiment were staggered due to the capabilities of the program "Logger PRO." The program was only capable of collecting data for only five minutes. As a result, approximately 29 seconds between each new graph was not recorded as a new graph needed to be prepared for data collection. The missing data for the 29 seconds between each new graph were obtain through mathematical calculations by averaging the trends. As well, LoggerPro only recorded temperatures to one decimal place which is not accurate. Inaccurate information was recorded during the transportation time of transferring the boiling tube into the air jacket. Since the test tube was heated, it was slowly handled with caution. Due to human error, there was no way to tell accurately when the sodium thiosulphate pentahydrate has transformed fully into a liquefied state. It was judged by vision and it could have been incorrect. As well, this was inhibited by the condensation on the sides of the beaker, obstructing the view of the boiling tube. Lastly, contamination could have occurred in any of the materials used in the experiment such as the beakers, the scoopula, or the sodium thiosulphate pentahydrate.