CH. I.] DETERMINATION OF P H . 29 



8. The determination of the P H of urine. 



Apparatus and reagents required. 



(1) A number of clean, dry test-tubes of thin clear glass and of 

 uniform bore, f inch is a suitable external diameter. 



(2) A comparator for holding the tubes. This is shown in 

 figure 4. 



(3) A series of buffer solutions of known P H prepared according 

 to directions given above. It is convenient to have a series of these 

 prepared and contained in bottles fitted with a rubber stopper, 

 through which passes the stem of a 5 cc. pipette. 



(4) Solutions of appropriate indicators (see page 23). Methyl 

 red, brom-cresol-purple, brom-thymol blue cover the range of the 

 majority of specimens of urine. 



(5) A screen to cut out blue rays when using brom-cresol-purple 

 (see page 23). 



Method. To 5 cc. of the filtered urine add 5 drops of methyl red. 

 If the mixture is red, the P H is in the neighbourhood of 5. If it is 

 yellow, the P H is nearer 6. In the latter case treat another 5 cc. with 

 5 drops of brom-cresol-purple. A deep purple tint suggests that 

 the P H is higher than 6, in which case it may be necessary to use 

 brom-thymol-blue. Having . roughly obtained the range and the 

 necessary indicator, place about 5 cc. of the specimen into two of the 

 special tubes and place them in the holes marked 2 and 6. Into a 

 tube in 4 place some distilled water. 



Measure exactly 5 cc. of the urine into another tube, and to it add 

 5 drops of the indicator (measured with a Dreyer's dropping pipette, 

 fig. 5). Mix by rotating the tube between the palms of the hands, 

 and place the tube in the hole marked 3. Measure 5 cc. of one of the 

 buffer solutions into a tube, add 5 drops of the indicator mix, and 

 place the tube in the hole marked i. Hold the comparator to the 

 source of light with the ground glass screen towards the observer 

 and note the appearance opposite the slots x and y. It will then 

 be ascertained whether the buffer chosen is acid or alkaline to the 

 urine. In either case another buffer solution must be taken, the 

 indicator added, and the tube placed in the hole 5, and the tubes 



