Hydroxypropyl methylcellulose (HPMC) is a synthetic polymer compound widely used in medicine, food, construction, cosmetics and other fields. It has excellent water solubility, colloidal properties and stability, so it has important applications in various industries. In order to ensure its quality and purity, proper identification is necessary. The following are several simple identification methods for hydroxypropyl methylcellulose (HPMC), covering aspects such as appearance, solubility, infrared spectrum and chemical reaction.

1. Appearance observation
HPMC is usually a white to off-white powder or granular substance, odorless and tasteless. By observing its appearance, you can preliminarily judge whether it is pure HPMC. Any color change or the presence of impurities may indicate that the sample is impure or contaminated.

2. Solubility identification
HPMC has good solubility, especially in water. Put a small amount of the sample into water and stir gently. If it can dissolve quickly and form a uniform colloidal solution, it means that the sample is hydroxypropyl methylcellulose. The speed of dissolution and the viscosity of the solution may be related to the molecular weight of HPMC and the content of hydroxypropyl and methyl chemical groups.

At the same time, the solubility of HPMC in organic solvents can also be used as an identification standard. HPMC is soluble in most organic solvents (such as acetone, ethanol, etc.), but insoluble in fatty solvents. This characteristic can be further confirmed by testing its solubility in appropriate solvents.

3. Infrared spectroscopy (IR) identification
Infrared spectroscopy (IR) is an accurate identification tool that can help confirm the molecular structure of HPMC. The main structural feature of HPMC is the inclusion of groups such as methyl (-CH3) and hydroxypropyl (-CH2CH(OH)CH3). The presence of these groups can be confirmed by the absorption peaks of the IR spectrum.

The characteristic absorption peaks of HPMC’s IR spectrum include:

2920 cm-1 (C-H stretching vibration)

1450 cm-1 (C-H bending vibration)

1100-1200 cm-1 (C-O-C stretching vibration)

3400 cm-1 (O-H stretching vibration, the peak value may change due to the presence of water)

By comparing the IR spectrum of the standard HPMC sample, it can be compared with the spectrum of the unknown sample to confirm the identity of the sample.

4. Chemical reaction identification
HPMC, as an ether compound, has certain chemical reaction characteristics and can be identified by the following simple chemical reactions.

(1) Reaction under acidic conditions:
Dissolve a small amount of HPMC in water, add dilute hydrochloric acid, and heat. If a colloidal substance appears in the solution, it means that it contains HPMC. This reaction can be identified by the structural stability of hydroxypropyl and methyl groups under acidic conditions.

(2) Reaction under alkaline conditions: HPMC dissolves in water to form a colloidal solution. It is not easy to dissolve under alkaline conditions (such as sodium hydroxide solution), which is related to its hydrophilicity and hydrogel properties. If the solution is turbid or precipitated, it means that HPMC is present.

5. Identification by viscosity method HPMC is a substance with viscosity characteristics, so it can be identified by its viscosity in aqueous solution. Generally speaking, HPMC will form a colloidal substance with a certain viscosity after dissolving in water, and the viscosity increases with the increase of its molecular weight.

In order to measure the viscosity, the fluidity of the HPMC solution can be measured by using a rotational viscometer or a glass tube viscometer. According to the molecular weight of HPMC and the concentration of the solution, its viscosity can be estimated. If the viscosity of the sample is significantly lower than that of the standard HPMC solution, it may indicate that its ingredients are impure or the molecular weight is low.

6. Melting point/decomposition point test HPMC, as a high molecular weight compound, usually does not have a clear melting point, but will show softening or decomposition during heating. When HPMC is heated, its changes at different temperatures can be observed. Generally speaking, HPMC will begin to decompose at 180-200℃, releasing some volatile substances (such as water and organic solvents). The change in the decomposition point can further confirm whether the sample is pure HPMC.

7. Solubility and surface tension method
The solution formed after HPMC dissolves generally has a low surface tension. The surface tension of the HPMC solution can be measured using a surface tensiometer or a drip method. If it matches the surface tension of the standard solution, it means that the sample is HPMC.

The above introduces several common and simple methods for identifying hydroxypropyl methylcellulose (HPMC). These methods identify HPMC from multiple angles such as appearance, solubility, infrared spectrum, chemical reaction, viscosity, melting point, etc. Through these means, the authenticity and purity of the sample can be effectively confirmed, providing guarantee for its application in various industries.