PHENOLPHTHALEIN REACTION. 11 



owing to the well-known extreme sensibility of phenolphthalein to 

 this substance. It is also undoubtedly true that most filter papers 

 have a faint acid reaction and with the Pasteur filter air is used 

 containing its full share of carbon dioxid, under pressure. It is a 

 very easy matter to make blanks on the filters with N/100 alkali solu- 

 tions, and though these show a faint result due to the filters it is not 

 sufficient to account for the observed differences. Again, platinum- 

 1 asbestos filters produce precisely the same effect. Finally, to put the 

 filter objection out of court, if orthoclase powder is shaken with water 

 and then allowed to settle quite clear in a stoppered cylinder, the 

 supernatant liquid will not give an alkaline reaction in the cold with 

 phenolphthalein, whereas, after shaking up the sediment, a slight 

 reaction can be obtained. Limestones and marbles behave in exactly 

 the same way as does orthoclase powder, although the alkaline reaction 

 is not entirely inhibited by filtration. Everyone knows that calcic 

 carbonate is distinctly soluble even in pure water free from carbonic 

 acid a and that the solution is alkaline owing to hydrolysis with an 

 accompanying formation of calcic hydroxid. 



Granting that phenolphthalein is sensitive to the small amount of 

 carbonic acid absorbed by the alkaline solutions from the air and 

 also that the slight acidity of some of the filters used affected the 

 results, it is none the less quite certain that there is some other constant 

 cause of this phenomenon at work. The work of other investigators 

 substantiates this conclusion. In commenting on his own qualitative 

 tests with phenolphthalein Clarke b says: 



In nearly every case the reaction was obtained at once, showing a more rapid 

 action of water upon the silicate than had been anticipated. In some instances fad- 

 ing is noticed. This is doubtless due, in general, to the action of light, but in certain 

 cases the colored solution separated in two layers, the color being wholly in the 

 lower. Here the color was really held as a coating on the fine solid particles c and as 

 they subsided the appearance of stratification was produced. 



Following up the qualitative observations of Clarke, Steiger d made 

 an attempt to study quantitatively the extent to which these reactions 

 proceeded. Small weighed samples of finely powdered rocks and 

 minerals were soaked in water for one month, filtered and titrated 

 with dilute acid using methyl orange as indicator. The sort of filter 

 used is not stated. The summing up of the results by the experimenter 

 is very interesting. He points out that there does not seem to beany 

 direct relation between the depth of color originally produced by 

 phenolphthalein solution and the amounts of actual alkali found in the 

 filtrates with methyl orange. Powders that give deep phenolphthalein 



Ellms and Beneker, The estimation 'of carbonic acid in water, J. Am. Chem. 

 Soc., June, 1901, 23 : 422. 

 &Loc. cit. 



c Italics are the writer's. 

 <* U. S. Geological Survey Bui. No. 167, p. 159. 



