HOW MOLECULES MAKE MASSES 43 



Crystals and Colloids 



Water is commonly referred to as H 2 0, and many know that 

 there are also present a small number of molecules of "heavy 

 water" (deuterium oxide, D 2 0), the average ratio being one mole- 

 cule of deuterium to 6,500 of hydrogen. The physical chemist, 

 however, is aware that under ordinary conditions water molecules 

 undergo dissociation into H + and OH", with recombination at such 

 a rate that with very pure water at 22° C we have at any instant a 

 concentration of these oppositely charged ions of 1/10,000,000 



mole per liter, which equals — - or 10 7 mole per liter. This is 



commonly expressed by saying that pure water has a pH of 7, 

 which means that its concentration of hydrogen ions is expressed 

 by the mathematical exponent 7, deprived of its minus sign. 

 The French expression first used by S0rensen was "pouvior hydro- 

 gene," or hydrogen power (in the mathematical sense of the word 

 "power"). As a consequence, the intervals on the pH scale, being 

 exponential, are vastly wider than the figures themselves might 

 indicate. This may be seen from the following table: 



pH Number of times H + or OH - concentration 



exceeds that of pure water at 22° C 



1 1,000,000 



2 100,000 



Acid side 3 10,000 



(excess of H + ions) 4 1 ,000 



5 100 



6 10 



Neutrality > 7 



8 10 



9 100 



Alkaline side 10 1,000 



(excess of OH - ions) 11 10,000 



12 100,000 



13 1,000,000 



In most living units the pH hovers near neutrality, the pH of the 

 blood in man ranging from 7.30 to 7.45, the former approaching 

 "acidosis" which is really a dimished alkalinity. The cytoplasm of 

 immature starfish eggs shows a pH of about 6.6 to 6.8, while that of the 

 nucleus is 7.6 to 7.8. However, the skin is usually quite acid (about pH 

 4.2 to 5.6) and Professor Frank C. Combes states 2 that this acid mantle 

 is an important factor in the protection of the organism against 

 invasion by bacteria and fungi. 



In certain protected spots, (e.g., the axilla and between the smaller 



