Chapter 4, Note 10. Calcium In Urine

1 cc. of the urine was diluted in a test tube with 8 cc. of distilled water and the optical density of the mixture was determined. To the mixture, 1 cc. of a 1% solution of potassium oxalate and 3% of oxalic acid was added. After standing for 5 minutes, the tube was shaken and the optical density again was read. The difference, multiplied by 10, was divided by the two figures of the specific gravity of the sample. The value obtained was called the calcium index.

Chapter 4, Note 11. Urinary Surface Tension (ST)

The role of changes in the surface tension of various body fluids in normal and abnormal physiology has become of increasing interest. Some authors have gone so far as to consider the surface tension forces present at the interfaces separating entities, to be the most important factors in the boundary formations which serve to individualize these entities.

Considering multiple aspects of the problem, it appeared interesting to attempt, as a first step, to obtain information about the surface tension of different body fluids. It was as part of this program that urinary surface tension was investigated with the intention of utilizing the data to gain insight into changes related to the dualistic offbalances. Before we could proceed, it was necessary to resolve several problems, including the technical difficulties in measuring surface tension that result from the special constitution of the urine.

Technical Problems In Measuring Surface Tension

Successive measurements of surface tension, when made on fluids formed by a single substance, consistently furnish the same value. But for fluids composed of two or more constituents, values vary from one moment to the next. This is explained by the fact that molecules of constituent substances have a tendency to migrate in the fluid, some accumulating at the surface, others concentrating in the bulk. (Gibbs dictum) The surface tension of different complex fluids has been found to vary according to the nature and amount of tensio active substances present. And, study of the variations has furnished information about the nature of these substances.

In a fluid such as urine, containing many different substances, the problem of variations in surface tension is a major one. ST measurements, made without considering these variations, would be subject to serious errors. Examination of different samples of urine has shown great differences between values obtained at different times. Using Lecomte du Noiiy's tensiometer (215) it could be seen that, for the same urine sample, values vary according to the length of time the sample is left to stand. Values progressively decrease as standing time increases. Similar changes are seen when the pendant drop method is used. (216)

Because of the fact that a certain time is needed for changes to take place, the relationship between change and time was investigated. The study of various urine samples has emphasized the inequality which exists between them not only in the intensity of changes but also in the time necessary for the changes to take place. This fact has rendered useless the measurement of the surface tension of different samples if all are made at some given moment. Except for measurements made at frequent intervals, use of du Noiiy's tensiometer has appeared to be inadequate for urine. Traube's stalagmometer also is unable to furnish values that take these changes into account.

Theoretically, it would appear possible to obtain measurements that would correspond to the surface tension for each drop at a desired moment by changing the rate of flow of the urine through the apparatus. But the differences between urines, related to changes in distribution of components, have made this inadequate.

With the pendant drop method, progressive changes which occur in the shape of the drop would appear to indicate the changes in surface tension. (216) Technically, it would appear necessary to obtain data as frequently as possible in order to follow changes which occur at various times. By using serial pictures, the changes, the moment of their occurrence, and their intensity can be studied accurately. Unfortunately, the complexity of the method, with the need for frequent pictures and involved calculations, prohibits its use for routine measurements and, consequently, for any broad clinical and experimental research.

It was under these circumstances that we returned to the capillary method which we considered capable of furnishing the desired data. Classically, the height of the ascending column in a calibrated capillary is used to calculate the surface tension. Height alone, however, is unsatisfactory, since it does not reveal the changes that take place. It was by studying the descent of the column in a capillary that we were able to obtain the data which we were seeking. We could show that the column does not descend with uniform velocity. It stops or slows down perceptively several times before it comes to rest at a fixed value. We could recognize that, for most urine samples, there is a first stop usually of several seconds duration. In some urines, this first stop is replaced by a marked slowdown in velocity of descent. The stop or slowdown is followed by renewed but slower descent and a second stop somewhat longer than the first. After another descent, often lasting more than 20 minutes, a new stop occurs.

The time of descent, the duration of the stops, and especially the heights of the column at which the stops occur, while reproducible for the same urine, vary widely with different samples. They would thus indicate different repartitions and the times when they occur. This technique of using the capillary consequently appears to be adequate for the study of the surface tension of complex solutions and particularly for the study of urine.

Each of the heights at which the descending column stops would indicate the surface tension for a particular stage in the repartition of the constituents. In studying this problem further, it appeared advisable to try to have the capillary so calibrated as to permit a direct reading of surface tension values at these stops. The study of the relationship between the surface tension of a fluid and the height of the column has indicated the nature of intervening factors, their values, and under what conditions a direct reading is possible.

The fluid column remains stationary in a capillary tube when the surface forces which bind the column of fluid to the wall of the capillary are equal to the weight of the fluid column.

With σ representing the surface tension; r, the radius of the capillary tube; h, the height of the column; A, the specific gravity of the fluid; and g, the acceleration of gravity, we have 2 π r σ = π r^2 hg Δ. It can be seen that the specific gravity is the only factor related to the sample, other than the surface tension, which intervenes in determining the height of the fluid column.

According to this formula, the relationship between the surface tension and the specific gravity of the specimen is: σ = Δrgh/2 The same height of the column is obtained if the relationship between surface tension σ and σ' of two different liquids with the specific gravities Δ and Δ' fulfills the condition.

Technical Problems In Measuring Surface Tension 221

If measurements with a capillary tube having a bore radius of 0.5 mm. are made in New York City, where the acceleration of gravity is 981 upon water which at 18°C has a surface tension 73 dynes/cm., the height of the fluid column is found to be 6.0 cm. and the relationship between σ and Δ, expressed in the cgs. system is σ = 73 A.

A capillary tube thus can be calibrated to permit the direct reading of the surface tension in dynes/cm. for any liquid having the same specific gravity. For fluids of different specific gravity, the same capillary tube can be used if a correction of 0.073 dynes/cm., is made for each 0.001 increment of the specific gravity.

Urinary specific gravity values encountered clinically range between 1.001 and 1.035, with an average value around 1.015. Tubes calibrated to measure urine specimens with specific gravity values at either extreme can yield errors in the surface tension of as much as 2 dynes/cm. In order to minimize the degree of error for routine laboratory use, the capillary tube has been calibrated to correspond to a fluid with a specific gravity of 1.015. The maximum error of the surface tension values for the extremes of specific gravity clinically observed will be reduced to approximately ± 1 dyne/cm. in this way. Furthermore, the fact that sodium chloride concentration is one of the important factors inducing different values for urinary specific gravity reduces the influence exercised by specific gravity upon the height of the column. Sodium chloride represents a negative surface active substance. It will raise the surface tension values as its concentration increases because of its tendency to migrate from the surface toward the bulk of the fluid. This will partially decrease the influence exerted by the specific gravity of the urine. Since the surface tension values of human urine specimens measured by this method have been found to vary between 73 and 50 dynes/cm., the error for extreme values of specific gravity is less than 5% and is not clinically significant. If more precise values are desired, the necessary correction can be made by adding or subtracting 0.073 dynes/cm. for every .001 difference in the sample above or below the specific gravity for which the tube is calibrated (i.e. 1.015).

The temperature of the urine to be tested is another factor which intervenes. Although the fluid rapidly attains the same temperature as the capillary walls, it is advisable to perform measurements when the temperature of the fluid is around 18°C, since the surface tension of a liquid decreases as its temperature increases. For clinical use, corrections for differences in temperature are not considered necessary.