Theory of melting point
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Theory of Melting Point. Indication of purity Physical property used for identification Pure compounds melt within a 1-2 degree range. Theory of Melting Point. Impurities lower melting point: takes less energy to disrupt crystal lattice when impurities are present

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Theory of Melting Point

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Theory of melting point

Theory of Melting Point

  • Indication of purity

  • Physical property used for identification

  • Pure compounds melt within a 1-2 degree range


Theory of melting point1

Theory of Melting Point

  • Impurities lower melting point:

    • takes less energy to disrupt crystal lattice when impurities are present

    • melting point will be lower

    • melting point will be broader


Theory of melting point2

Theory of Melting Point

  • Mixed Melting Point - used to determine identity of compound:

Have unknown compound

that melts at 120-121.

What is the unknown?

How could you tell?

Urea

MP 120-121

Cinnamic acid

MP 120-121


Mixed melting point

Mixed Melting Point

  • Mix unknown compound with a little urea and measure melting point

+ unknown sample

If melting point is still 120-121 degrees, unknown compound was urea:


Mixed melting point1

Mixed Melting Point

  • Mix unknown compound with a little urea and measure melting point

+ unknown sample

If melting point is lower and broader, i.e. 110-116, unknown compound was NOT urea:


Theory of melting point

Eutectic Mixture

It should be noted, however, that there is one unique mixture of two compounds, A and B, that has a lower melting point than any other mixture of the two compounds. This particular mixture is called the eutectic mixture. The melting point of the eutectic mixture is called the eutectic point. A mixture whose composition corresponds exactly to its eutectic mixture will have a relatively sharp melting point..


Melting point of an unknown solid

Melting Point of an Unknown Solid

  • Do one fast determination to get rough approximation of melting point

  • Do next one slower: Go quickly to 10-15 degrees away from melting point, then lower setting.

  • Record temperature at which solid first starts to melt (120)

  • Record temperature at which all solid is melted (121)

  • Report the range: 120-121 oC


Theory of melting point

SAMPLE SIZE

The smaller sample is probably too small to properly observe the mp range.The larger sample may be too large, resulting in a mp range that is too wide and possibly high.The middle sample is about right but it is near the maximum size that should be used.Note that it is firmly packed.


Theory of melting point

WHAT YOU MAY SEE

THIS IS A SAMPLE OF IBUPROFEN ISOLATED FROM COMMERCIAL TABLETS.

TINY DROPLETS APPEAR AT THE START OF THE MELTING POINT RANGE. OFTEN THE SAMPLE APPEARS TO “SHRINK” AWAY FROM THE SIDES OF THE TUBE. HERE, AS SHOWN IN THE INSET (YELLOW BOX), THE SAMPLE HAS STARTED TO MELT JUST ABOVE 72°.

AS THE TEMPERATURE RISES TO 73° THE LIQUID PHASE BECOMES CLEARLY VISIBLE.


Theory of melting point

WHAT YOU MAY SEE

THIS IS A SAMPLE OF IBUPROFEN ISOLATED FROM COMMERCIAL TABLETS.

AT 74° THERE IS A SLUSHY MIXTURE OF CRYSTALS AND LIQUID

FINALLY AT A TEMPERATURE OF 75°, ALL OF THE SOLID HAS MELTED AND A CLEAR LIQUID REMAINS.THE MELTING POINT IS REPORTED AS 72-75°.

THE LITERATURE VALUE (The Merck Index, 12th Ed.) FOR IBUPROFEN IS 75-77°.

THE EXPERIMENTAL RESULT SUPPORTS THE PRESENCE OF IBUPROFEN. BUT THE MP RANGE IS WIDER AND LOWER THAN EXPECTED, PROBABLY DUE TO SMALL AMOUNTS OF IMPURITIES.


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