Comprehensive Guide to Calibration of Melting Point Apparatus

Introduction:

A melting point apparatus is used to measure the melting point of a solid (powder) substance. The melting point is the temperature at which a powder or solid substance gets converted into a liquid state. This is a very important characteristic property of a compound. Melting point is used to identify a substance that is mentioned in different pharmacopeias.

Melting points apparatus usually have two types of specifications: Manual and Automatic. In manual type, an operator observes the sample as it is heated and melts at a specific temperature, and then the operator notes down the melting point. Whereas, Automatic apparatus uses a variety of sensors to detect the melting point of a substance.

SOP of operation and calibration of Melting Point Apparatus

1. Objective: To lay down a procedure for the Operation and calibration of the Melting point Apparatus. [Make: ANALAB, Model: µThermolab]

2. Scope: The procedure of this SOP shall be applicable to the Q.C. Department of XYZ Pharma company.

3. Responsibility: Quality Control Chemist

4. Accountability: Quality Control Manager

5. Abbreviations :

S.O.P.: Standard Operating Procedure

QC: Quality Control

QA: Quality Assurance

6. Procedure :

6.1 Ensure that the instrument is clean and free from dust.

6.2 Connect the instrument with the main cord & switch on the instrument.

6.3 The instrument lamp will glow up after it is switched on.

6.4 Ensure that all knobs are in normal position

6.5 Fill the capillary with material.

6.6 Put the capillary in the instrument.

6.7 Allow to rise the temperature.

6.8 Watch the material through a magnifying glass in the instrument & note down the stage temp. at which the material changes into a liquid.

6.9 Repeat the procedure a second time for correct and confirmed results.

7.0 Calibration Method :

7.1 The melting point apparatus should be calibrated using a certified thermometer & AR Grade/certified substances, mentioned below:

S.NO. Melting Point Standards Melting Range

Vanilline: 81°C– 83°C
Caffeine (Anhydrous): 234°C – 237°C
Acetanilide: 114.0°C – 116.0°C
Urea (finely crushed): 132.0°C – 135.0°C
Sulphanilamide: 164.5°C – 166.5°C
Diclofenac Sodium: 277.0°C – 283.0°C

7.2 Follow the operating procedure to calibrate the instrument and note down the data (For Manual Apparatus), if the readings are identical to the limits given above then the instrument is working correctly.

7.3 If the thermometer varies by more than 2°C from any one of the melting ranges, the thermometer should be replaced.

7.4 The error should not exceed 1°C to record the data.

Time Interval for Calibration: Once a month

Troubleshooting and Adjustments:

Common problems in Melting point Apparatus:

Inaccurate melting point readings: Such issues are most common in melting point, it is due to various reasons like; Improper calibration of the melting point, Impurities in the sample, Incorrect sample filling in the capillary, and Excessive heating rate of samples.
Decomposition of samples: When samples are heated too quickly in an open capillary tube then such problems exist.
Sample Cracking: If the sample is not filled till the ground of the capillary then the sample cracking issue persists.

Troubleshooting:

If you are facing above said problems with your melting point apparatus, follow the following troubleshooting tips:

Recalibrate the apparatus: Make sure the apparatus is calibrated correctly as per SOP. Check the standards are not moisturized.
Check the sample purity first: Impurities can vary the melting point, so ensure the sample does not have impurities more than its prescribed limits.
Check the sample preparation: Ensure that the sample is ground into a fine powder then fill in the capillary tube. It will prevent sample cracking.
Adjust the heating rate: To avoid sample decomposition reduce the heating rate.
Use one end closed capillary tube: Closed capillary tube helps to prevent sample decomposition.

References:

Melting-Point Apparatus-[Wikipedia(.org)]
SOP of SOPs
Melting point chemistry-[Britannica(.com)]