On the Measurement of the pH Value of Snow Water

While staying at the mountain house of our school in Shiga-Plateau (1600 meters high) in winter, for the purpose of studying the relation between mountain vegetation and snowfall, I found that fresh snow has such a very low pH value as 5.8, when the indicator method was used for measurement. Later, I could find that snow and ice have low pH value in general, causes for which are discussed in this paper. First I suspected the CO_2-adsorbing capacity of snowflakes, which is said to be 20 times as much as that of liquid water (Kauko, 1935). But the result of my experiment showed that snow as well as ice did not change their pH value even after heating or boiling, although CO_2 adsorbed by snowflakes ought to have been liberated and lost in these treatments. Next I paid my attention to the fact that, while falling through the air, the snowflakes adsorb nitric acid, nitrous acid, sulfuric acid, sulfurous acid, etc., which are usually counted as trace components of air. But in my experiment the water distilled from snow which had been rendered alkaline by adding NaOH, showed the same pH value as mentioned above, and this result proved that the decrease of pH value of snow is not caused by those trace components of air. Thinking that snow is a purified form of water and must have a similar nature with that of purified water, I made measurements of pH value of the latter and the result showed that its pH value is 5.8. Consequently, my conclusion is that the H-ion of pH-indicator itself must be responsible for the lowering of the pH value of snow and ice, since the acid indicator such as bromcresolpurple dissociates in water as RH→R'+H. In the case of city or well water, this H-ion may be neutralized by its buffer action, the pH value remaining practically unaffected