A simple titration method for determining the specific gravity on one drop of urine.

Samples of urine which have been collected by ureteric catheterization are necessarily small in volume, often only a few drops. In order to determine the efficiency of kidney function it may be necessary to measure the specific gravity of these small samples, but it is impossible to use the hydrometer method, which requires a minimum of about 50 ml. of urine. Methods involving the suspension of drops of urine in solvents, which are immiscible with water, are a possible alternative. Dukes (1939) has described a method based on these principles, but it has some disadvantages. A drop of urine is added to a mixture of carbon tetrachloride and xylol, and the composition of the mixture is adjusted by adding one of the constituents until the drop of urine floats in an equilibrium position. The specific gravity of the mixture is then measured with a urinometer. This requires the use of large volumes of reference liquid, 50 ml., for each test, and errors may be introduced due to the fact that the slight solubility of water in the 50 ml. of liquid may appreciably alter the composition of the drop of urine. The method of Monroe and Hopper (1946) makes use of a series of standard mixtures of xylol and bromobenzene of known specific gravity. Drops of urine are added to these mixtures in turn until the equilibrium mixture is found. This method is open to the same possibility of error, due to the relatively large volume of reference liquid used, as is the first method. Several drops of urine are required for the test and may not always be available. There is also the disadvantage that it is necessary to keep a large number of standard mixtures.