Determination of Specific Gravity by Pycnometer and Hydrometer

Determining specific gravity using a pycnometer and hydrometer is a common method for measuring the density of liquids.

Specific Gravity by Pycnometer

Apparatus

( a) Pycnometer fitted with a thermometer of suitable range with 0.1 or 0.2 subdivision or a density bottle

(b) Balance

( c) Water bath maintained at 30 0 - 20°C. 

The thermometer should be checked against a standard thermometer calibrated and certified by the National Physical Laboratory or any other approved institution.

Standardization of Pycnometer

Carefully clean the pycnometer by filling it with Chromic acid cleaning solution and letting stand for several hours. Empty the pycnometer and rinse thoroughly with water, fill with recently boiled water previously cooled to about 20°C, and place in a constant temperature water bath held at 30°C. 

After 30 minutes adjust the water level to the proper point on the pycnometer and stopper, remove from the bath, wipe dry with a clean cloth or towel, and weigh.

Procedure

Fill the dry pycnometer with the prepared sample in such a manner as to prevent entrapment of air bubbles after removing the cap of the sidearm. Insert the stopper, immerse it in a water bath at 300 C 0.20 C, and hold for 30 minutes. 

Carefully wipe off any oil that has come out of the capillary opening. Remove the bottle from the bath, clean it, and dry it thoroughly. Remove the cap of the sidearm and quickly weigh ensuring that the temperature does not fall below 30°C.

Specific gravity calculation formula. 

                                                                A - B
Specific Gravity at 30 °C / 30 °C = ------------

                                                                C - B

Where, 

A = weight in gm of specific gravity bottle with oil at 30°C

B = weight in gm of specific gravity bottle at 30°C

C = weight in gm of specific gravity bottle with water at 30°C

Specific Gravity of Liquids by Hydrometer

 APPARATUS

a). Hydrometers of different specific gravity ranges.

b). 250 ml measuring cylinder.

PROCEDURE

a). Measure the temperature of the liquid sample.

b). Fill in the liquid sample in a 250-ml measuring cylinder up to the 200-ml mark and dip the hydrometer having the desired specific gravity range in the liquid. The hydrometer should float in the cylinder. The mark on the hydrometer coinciding with the top liquid level in the cylinder indicates the specific gravity of the sample at that particular temperature.

The specific gravity of liquid sample = ..................at............ oC