Relative density of urine/determination

Relative density of urine can be determined using a urinometer, refractometer or diagnostic strips.

Urinometer

A urinometer is a simple device for determining the density of urine. It is a suitably calibrated hydrometer, i.e. actually a special float made so that the depth of immersion corresponds to the density of the liquid. Commonly used urinometers are calibrated to a temperature of 15 °C, as this temperature approximately corresponds to the temperature of urine standing for 1 hour in the cold. If we measure the density at a different temperature, the values ​​must be corrected. For every 3°C above the calibrated temperature, 0.001 is added and vice versa. A larger volume of urine (10-15 ml) is required for the examination.

Refractometer

The refractometer determines the density optically based on the refractive index measurement. It has some advantages over the urinometer. 1-2 drops of urine are sufficient for examination. No correction for temperature is required.

Diagnostic strips

__By the relative density of urine we mean the ratio of the density of urine and the density of water. The density of water is practically equal to 1 kg/l, so the difference between the density of water (in kg/l) and the relative density of urine is negligible. In the SI system, density has the dimension kg·m -3 . The density of the sample in relation to the density of water is a relative quantity and is therefore given by a dimensionless number.

Determination of urine density

The density of urine is estimated indirectly by the concentration of cations using diagnostic strips. The indicator zone of the strip contains a suitable polyelectrolyte as an ion exchanger and the acid-base indicator bromothymol blue. The principle of diagnostic strips is based on the exchange of cations from urine, especially Na + , K + , NH 4 + , for H + ions of the polyelectrolyte in the indicator zone. The released H + acidifies the weakly buffered acid-base indicator, which is in alkaline form. Acidification is accompanied by a change in color to bromothymol blue. The disadvantage is that examination with diagnostic strips does not take into account substances of a non-electrolyte nature such as glucose , proteins, urea ,creatinine and some others.