Melting point determination:

Purpose:

• To find out the purity of a material using melting denote as physical property 
• To recognize an unknown compound using its melting point 
• To identify an unknown compound using mixture melting point 
• To study how to attain an accurate melting point using a Mel-Temp apparatus 

Discussion: 

Identifying an unknown compound can be a tiresome and exacting task. In identifying a compound, chemists often determine numerous physical properties (melting point, boiling point, density, and so on.) and study a few chemical properties ('reactivity, acidity, basicity, and so on'.) of the compound.

The cause for determining numerous chemical and physical properties of the compound is that it is fairly feasible for 2 different compounds to contain a few physical and or chemical properties in general; but it is extremely unlikely for the 2 compounds to have extremely many identical physical and chemical properties. Helpful physical properties that are frequently utilised via chemists in identifying an organic compound contain colour, odour, physical state, melting point (M.P.), boiling point (B.P.), infrared (IR) spectrum, density (d), nuclear magnetic resonance (NMR) spectrum and ultraviolet (UV) spectrum.

Physical constants are mathematical values computed at the time certain physical properties are examined. As long as the physical constants are measured under typical conditions (temperature, pressure, etc.), they are invariant and, hence, useful in helping to find out the identity of unknown materials.

Chemists reflect on a table of physical properties and physical steady to be extremely helpful in identifying unknown compounds. There are a number of mention books that have tables of physical properties and physical constants of compounds. If the physical properties of an unknown compound are identical to the physical properties of a compound recorded in the tables, the 2 compounds are almost certainly similar. Therefore, a colourless, liquid compound through a melting point of 5.5^oC, a boiling point (at 760 mm) of 80.1^oC, is probable to be benzene, even though we might desire to build a few more examinations to be sure. It should be spotted out, though, that it isn't possible to precisely expect the physical properties of recently synthesized or isolated compounds. Hence, tables of physical properties are only helpful in identifying earlier known compounds. Though, useful information as to the compound's character and its purity can frequently be attained from its melting point.

The melting point of a solid is described as the temperature at which the liquid and solid phases are in equilibrium.  The freezing point of a liquid is similar temperature as the melting point of its solid. Though, freezing points are hardly ever computed in practice since they are harder to find out. One cause for this is that solidification might not happen at the accurate temperature due to the phenomenon of supercooling. Supercooling happens whenever a liquid is cooled below its freezing point and doesn't solidify.

Determination of the temperature at which the solid and liquid phases of a material are in symmetry is tiresome and time consuming; it is as well quite hard through a small amount of example. Therefore, in practice, most melting points are concluded as capillary melting points that can be complete rapidly through a small amount of sample in a capillary tube. A capillary melting point is described as the temperature range over that a small amount of solid in a thin walled capillary tube primary visibly softens (first drop of liquid) and then entirely liquefies.

A solid is said to melt harshly if the melting point range is 0.5 - 1.0^oC (narrow melting point range). A pure solid will usually melt sharply since the forces of attraction between its elements are similar. Though, the presence of a foreign particle in a crystal lattice interrupts its regular structure and the forces of attraction are weakened.

A contaminated solid melts at a inferior temperature and over a wider variety. Therefore, a solid's melting point is helpful not only as an assist in identification but as well as an indication of purity. Assume 2 compounds X and Y have identical melting points of 131- 132^oC and show to be identical. We can simply find out whether or not X and Y is actually similar compound via mixing a small amount of Y by X (or vice versa) and taking the melting point of the mixture. (The melting point of a mixture is termed the mixture melting point). If X and Y is similar compound, the mixture melting point will be equal as the melting point of pure X or pure Y. If  X and Y aren't similar compound, one will act as an impurity in the other and the mixture melting point will be lower and more extend out (wide range 120-125^oC in this case) than the individual melting points of pure X or pure Y.

It should be noted, though, that there is one exclusive mixture of 2 compounds, X and Y, which has a lower melting point than any other mixture of the 2 compounds. This particular mixture is termed the eutectic mixture. The melting point of the eutectic mixture is termed the eutectic point. A mixture whose composition communicates exactly to its eutectic mixture will have a moderately pointed melting point. Therefore, there is a risk that a eutectic mixture could be incorrect for a pure compound. Though, if a little amount of either X or Y (assuming they are both known) is adjoined to the mixture, the melting point of the consequential mixture will be superior or more extend out than the melting point of the eutectic mixture.

Several solids pass openly from the solid state to the gaseous state with no 1^st liquefying; this phenomenon is termed sublimation.  The temperature at which sublimation happens is termed the sublimation point. Additional solids decompose rather than melt. The temperature at which a solid decomposes is the decomposition point. Whilst together sublimation points and decomposition points are helpful as they help in identifying compounds, neither is extremely useful in beginning the purity of a compound.

Several solids start to "sweat" a few degrees beneath their true melting points. Additional solids abruptly contract just previous to melting. These shrinkage of a solid being heated is termed sintering. Soluble impurities (that is, impurities that are included inside the crystal matrix) tend to lower the examined melting point and broaden the melting point range. Unsolvable impurities have no consequence on the melting point.

Mel-temp operating instructions caution: 

a. Don't suppose the chapter is cold Wait for the heating block to cool if the temperature isn't less than 20^oC beneath the melting point (m.p.) of our compound. Permit the block to cool to room temperature if we have no idea what the approximate melting point of our compound is.

b. Set voltage to attain the wanted heating rate at the predicted melting point range .The voltage control controls the rate of heating, not the temperature! The superior the setting, the faster the temperature increase. Utilize the heating rate charts under to select the accurate voltage and approximation the amount of time to attain a temperature inside 20^oC of the melting point.

NOTE: It is constantly a good idea to find out the estimated value for the melting point in a literature source prior to laboratory work. If the approximate value is not possible to attain, save time via 1^st gaining a quick melting range, and then do again gradually through another example. 

c. Obtain a sample or prepare a sample by packing capillary tube 3-4 mm high with thoroughly dried, finely powdered, densely packed sample. Larger, loose samples will heat unevenly! Insert the loaded m.p. tube in one of the (three) channels in the opening at the top of the unit. 

d. Insert thermometer into thermometer fine of instrument (bulb first). 

e. Turn on power switch, making sure that apparatus is plugged in. Set the power according to the heating rate chart (beneath). 

f. Examine examples through the eye about 6" from lens. Turns down the voltages manage to obtain a 2^oC per minute increase whenever we are within 20^oC of the melting range (see instance below right). Be patient! 

g. Verification the temperatures of the melting range. 

T1: Temperature at which 1^st drop of liquid appears.

T2: Temperature at which the last crystal just disappears.

Fig: Mel-Temp Apparatus

Set the voltage control to zero and turn off the power switch when terminated. Leave apparatus on lab bench until cool. Reject the example in the glass disposal box.

Table: Chart for Setting the Mel-Temp Melting Point Apparatus

Experimental procedures:

A. Preparing the example situate a pea-size mound of one of the listed compounds on a piece of paper and grind it to a well powder using a spatula. Utilize the spatula to push a little amount of the solid into the release end of a capillary tube. After that drop the capillary down numerous times to fill and pack the example well to reason the solid to fall to the bottom. Do again these steps until we contain accumulated an example 2-4 mm (≈ 0.5 cm) high in the bottom of the tube (Figure).

B. Taking a Melting Point:

To verification the melting point range of a compound fill up 2 capillary tubes each to a intensity of 2 -4 millimeters through the compound of attention. First, obtain a rough idea of the estimated melting point range (Figure). Place one of the tubes in the Mel-Tem apparatus and increase the temperature a comparatively fast rate, about 10°C/minute. Record the range of temperature from the 1^st observable evidence of liquid (the example shows moist, or a tiny drop of liquid is examined) to the complete liquefaction of the example. Note the temperature at which the compound first starts to melt. Permit the melting point apparatus to cool to about 20°C below that temperature, and then place in the 2^nd tube. Increase the temperature more gradually this time, at the rate of about 2°C/min.

Note the temperatures at which: I. the first crystals melt, and; II. At which the sample has completely melted. This is the melting point range. For example, amp range of 164-168°C average of 166°C indicates the sample softened or began to melt at 164°C and that transition to a liquid was complete at 168°C.

C. Mixture Melting Point

Take roughly equivalent amounts of the 2 compounds for which we have already measured melting points and grind them jointly extremely methodically. Get ready a capillary enclosing a example of this mixture as explained in Part B and find out its melting point.

Table: List of Melting Points for Standard Compounds

D. Determination of the identity of an unknown:

Attain an unknown compound from our instructor. Get ready 2 capillaries enclosing the unknown. Find out an estimated melting point for it using the 1^st tube and a heating rate of 15-20^o/minute. Then let the thermometer and Mel-Temp apparatus cool to at least 20 degrees below this estimated melting point and utilize the 2^nd tube to attain an accurate melting point through a heating rate of no more than 3 degrees per minute. From the found melting point of our unknown, choose which of the listed compounds it might probably be. There might well be more than one reasonable possibility. Prepare a mixture of our compound through the compound that is the most likely of the possible choices and get a melting point of this mixture. If its melting point is similar as that of the unknown, it is probable that our unknown and the compound with which we blended it are similar. If the melting point of the mixture is lower than that of the unknown, we should get ready a mixture of our unknown through the next most likely choice. We must carry on experimenting until we have found a material that doesn't lower the melting point of our unknown. Whenever we have verified the identity of our unknown record, our discovering in the report sheet.

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