Nicholson's Hydrometer

This instrument is used (1) to determine the specific gravity of small solids which can be immersed in water; (2) to determine the specific gravity of a liquid.

(1) To find the Specific Gravity of a Solid.

Taking care that no air-bubbles adhere to it, place the hydrometer in a tall vessel of distilled water recently boiled, and put weights on the upper cup until it just sinks to the mark on the stem.

To avoid the inconvenience caused by the weights falling into the water, a circular plate of glass is provided as a cover for the vessel in which the hydrometer floats. This has been cut into two across a diameter, and a hole drilled through the centre, through which the stem of the instrument rises.

It will generally be found that with given weights on the cup the hydrometer will rest in any position between certain limits; that there is no one definite position of flotation, but many. The limits will be closer together and the experiment more accurate if the surface of the instrument, especially that of the stem, be thoroughly clean and free from grease. It is well therefore carefully to rub the stem and upper part of the bulb with some cotton-wool soaked in methylated spirit.

Suppose now it is floating with the mark on the stem just below the surface. Take off some weights until the mark just rises past the surface; let the weights then on be 8.34 grammes. Put on weights until the mark just sinks below the surface, and then let the weight be 8.35 grammes. Do this several times, and take the mean as the weight required to sink the mark to the surface.

Let the mean be 8.345 grammes.

Remove the weights and put the solid in the upper cup. Then add weights until the mark again just comes to the surface, estimating the weight required as before. Let this be 2.539 grammes. The weight of the solid in air is the difference between these, or 5.806 grammes.

Now put the solid in the lower cup¹ and weights in the upper one until the mark sinks to the surface. Estimate this as before. Let the mean of the weights be 5.462 grammes. The difference between this and the weight 8.345, put on originally to sink the hydrometer, gives the weight in water.

Thus, the weight in water = 2.883 grammes.

And the weight of water displaced = weight in air - weight in water = 2.923 grammes.

The specific gravity, therefore, referred to water at the temperature of experiment

To determine the true specific gravity - water at 4° C. being taken as the standard - we must multiply this number by the specific gravity of the water at the time of the experiment.

This may be taken from the table (32), if we know the temperature. Thus, we must observe the temperature of the water at the time of the experiment. Let it be 15°. Then the specific gravity required

(2) To determine the Specific Gravity of a Liquid.

Let the weight of the instrument itself be 11.265 grammes. This must be determined by weighing it in a balance.

Place it in the water, and put weights on the upper pan until it just floats up to the mark on the stem. Let the weight be 8.345 grammes. This of course must be estimated as in experiment (1).

The sum of these two weights is the weight of a volume of water equal to that of the instrument up to the mark on the stem. Thus, the weight of this volume of water is 19.610 grammes.

Now place the instrument in the liquid and add weights till the mark is just in the surface. Let the weight be 9.875 grammes.

Then the weight of the volume of liquid displaced is

The specific gravity of the liquid referred to water at the temperature of the experiment is therefore

Let the temperature of the water be 15°C.; that of the liquid 11.5°C. Then the specific gravity of liquid at 11.5°C. is

1.078 x 0.99917 = 1.077.

Experiments

(1) Determine the specific gravity of sulphur by Nicholson's Hydrometer.
(2) Make a 20 percent solution² of common salt in water, and determine its specific gravity by Nicholson's Hydrometer.

Enter results thus:

(a) Specific gravity of sulphur.

Mean weight required to sink the hydrometer to the mark	8.345 gms.
Mean weight required to sink the hydrometer with sulphur on upper pan	2.539
Mean weight required to sink the hydrometer with sulphur on lower pan	5.462
Temperature of the water	15° C.
Sp. gr. of sulphur	1.985

(b) Specific gravity of salt solution.

Weight of salt used	539.0 gms.
Weight of water used	2156.0 gms.
Weight of hydrometer	11.265 gms.
Weight required to sink the instrument to the mark in water at 15°	8.345 gms.
Weight required to sink instrument in solution at 11.5°C	9.875 gms.
Specific gravity of solution	1.077

1	If the solid be lighter than water it must be fastened down to the cup either by a wire or by being enclosed in a cage fixed to the instrument.
2	A 20 percent solution is one which contains 20 parts by weight of salt in 100 parts of the solution. It may therefore be made by adding the salt to water in the proportion of 20 grammes of salt to 80 grammes of water.