A student was given an unknown pure solid and was asked to determine the identit
ID: 712969 • Letter: A
Question
A student was given an unknown pure solid and was asked to determine the identity of the unknown by doing a mixed melting point analysis. The student was also told that the solid was one of the six solids in the table below. The student took the melting point of his unknown and found it to be 112-113 °C. (6 points) a. Suggest which mixed melting points should the student perform? Explain b. Is there a mixed melting point that is not needed to be performed given the data? c. How would the student determine the identity of the unknown solid? Explain 4.Explanation / Answer
Determining the MP is a simple and fast method used in many diverse areas of chemistry to obtain a
first impression of the purity of a substance. This is because even small quantities of impurities change
the melting point, or at least clearly enlarge its melting range.
the capillary method as the standard technique for melting point determination. In this methodology, a
thin glass capillary tube containing a compact column of the substance to be determined is introduced
into a heated stand (liquid bath or metal block) in close proximity to a high accuracy thermometer. The
temperature in the heating stand is ramped at a user-programmable fixed rate until the sample in the
tube transitions into the liquid state. While determining a melting point, several observations and the
temperatures are recorded.
The accuracy of a melting point record is assured by: (a) careful sample preparation, (b) proper
instrument setup, and (c) routine calibration of the instrument’s temperature scale against certified
melting point standards.
Careless preparation of a sample is the leading cause of inaccurate and irreproducible results in melting
point determinations. Any substance being loaded into a melting point capillary must be:
1. Fully dry
2. Homogeneous
3. In powdered form
A sample height between 2.0 mm and 3.0 mm is recommended for optimum results and reproducibility.
Visual Observations Several noticeable changes take place in the capillaries during a melting point
determination.
Clear (or Liquefaction) Point The clear point corresponds to the stage along the melt at which the
substance becomes completely liquid no more solid is left (i.e. the last crystals are melted).
Definition of MIXED MELTING POINT
The fusion temperature of a mixture of two components that in the case of
two different substances is usually lower than that of either component or
that in the case of a mixture of two samples of the same substance prepared
by different methods as a check on their identity is not lower than that of
either sample.
If two substances melt at the same temperature, a Mixed Melting Point determination can reveal if
they are one and the same substance.
The phenomenon of melting point depression can be applied to the identification of unknown pure
substances. For example, if you measure the melting point of a sample at 160 °C, you will find from the
MP tables that this is the same melting point for several different reference compounds. The substance
can be identified by determining its Mixed Melting Point the sample is mixed one-by-one with small
amounts of the references and the mixed melting point is determined in each case. Whenever the
melting point of the sample is depressed by mixing a small amount of a reference with it, the two
substances cannot be identical. If, however, the melting point of the mixture does not drop, the
reference substance that was added was identical to the sample (i.e. the sample has been identified).
The mixed melting point technique is an important reason why all high-quality melting point
instruments accommodate at least three capillaries in their heating blocks.
In its most common implementation, three melting points are determined: (1) sample, (2) reference and
(3) reference and sample in a 1:1 mixing ratio. If the melting point of the mixture remains the same, the
two substances are identical. If the melting point is depressed, they are two different substances.
Tips
• The requirements for precision and reproducibility are not as high here as when doing a high-
precision, single MP determination. Heating rates as large as 10 °C/min are acceptable for mixed MP
determinations.
• A few pairs of substances show no melting point depression when mixed, but more frequently the
failure to depress may be observed only at certain compositions. It requires little additional effort to
measure the melting point of several compositions. Typically a 20/80, 50/50 and 80/20 % mixture of
sample and reference is prepared, and the three tubes are run in the MP apparatus. If the three melt at
the same temperature, it is very likely the two compounds are one and the same!
Answer (a) The student should perform mixed M.P. analysis with first four compounds given in table because mixed M.P. is always less than equal to the M.P. of pure substance to be identified.
Answer (b) The mixed M.P. analysis with last two compounds given in table should not be performed.
Answer (c) The mixed melting point record by the student should match with the reference data given in
the table. By comparing the student will conclude that the mixed mixture is either acetanilide or
benzoquinone.
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