DETERMINATION OF H-ION CONCENTRATION 595 



of any double color indioator, which would be useful and work- 

 able are: 



1. The alkaline and acid solution shall be such as will bring out 

 the alkaline and acid colors of the indicator in a well defined way. 



2. The solutions used shall be of a strength that will not fade 

 quicklj\ 



3. They shall be of a strength that will not bring out secondary 

 colors. 



4. The indicator solutions shall be of a strength such that 

 when made use of to prepare the color standards in the manner 

 described thej^ shall produce definite, strong and unmistakable 

 color differences between the adjacent pairs at an interval of 

 pH 0.2. 



5. The amounts of indicator used in preparing the color stand- 

 ards shall be measured in a way that can be correctly duplicated 

 at any time. This can best be done by using fractions of a 

 cubic centimeter from a graduated pipette. 



6. The difTerent colors, as represented by each pair of tubes 

 of the color standards, shall be calibrated with buffer mixtures, 

 of known H-ion concentration containing the same amount of the 

 buffer mixture as is present in each tube of the pair and exactly 

 the same amount of indicator solution present in both tubes of 

 the pair. 



7. The work of calibrating the "standard colors" shall be done 

 with a comparator block having 9 holes, permitting of having on 

 each side of the "unknown" a tube with 10 cc. of buffer mixture 

 of known H-ion concentration containing 0.8 cc. of indicator 

 watery solution. 



When measuring an unknown solution that is turbid or colored, 

 an extra tube of this unknown should be placed in front of each 

 standard color in the comparator block. 



8. The test tubes used for the color standards must be of 

 clear glass and of a caliber (14 to 15 mm. inside dimensions) 

 such as to give sufficient density of color and to be as nearly 

 alike in diameter as possible. 



All these requirements the author has tried to meet and has 

 met in the "color standards"^ — as previously described. 



