The use of litmus paper as a quantitative indicator of reaction / by G.S. Walpole.

  • Walpole, George Stanley.
Date:
1913
Catalogue details

Licence: In copyright

Credit: The use of litmus paper as a quantitative indicator of reaction / by G.S. Walpole. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    time taken for a piece of litmus paper to acquire its equilibrium tint was seen to vary very considerably. The same paper changed more rapidly in a solution of greater reaction inertia than in one of less reaction inertia in all cases, and, as one would expect, a paper took longer to reach its equilibrium tint when the colour change from its original to its final tint was great. For instance a red paper (No. 1 red) took several hours to come to its equilibrium tint in the most dilute alkaline solution, while a blue paper did so in a few minutes as it was already almost at that tint. Observation of effect of exposure to the air. It will have been noticed that the two main test mixtures chosen each have a volatile constituent. Pieces of paper that have been immersed for sufficient length of time to assume the equilibrium tint, change that tint rapidly on exposure to the air in such a case. The reddening of blue paper due to the volatilisation of ammonia is naturally more rapid than the blueing of red paper due to acetic acid evaporation. “ Amphoteric solutions.” Dealing with approximately neutral solutions a distinction is frequently made between a solution that is “ merely neutral ” and one that is “amphoteric.” Milk, urine, and solutions of phosphates are regarded as typical examples of the latter class and they are said to “ turn blue litmus paper red, and red litmus paper blue.” What is actually seen may be better described as the “reddening of blue litmus paper and the blueing of red litmus paper.” The “merely neutral ” solution is one of low reaction inertia. A drop placed on paper wets it but only to a very small extent alters its reaction. The colour of the paper therefore remains unchanged. The “amphoteric” solution, on the other hand, has a high reaction inertia. It usually contains a salt of a feebly dissociated acid or base and, being neutral or nearly so, alters the reaction of litmus paper with which it is brought into contact in the direction of neutrality. It would naturally be expected that small pieces of paper, red and blue, would, when immersed in a large volume of an amphoteric solution, ultimately assume the same tint. During the first few hours of observation it is seen that the blue paper is becoming progressively redder, and the red paper bluer, but before equilibrium can be attained certain disturbing influences may be encountered. For instance if protein be present the colour of the paper will be interfered with. And, again, since the range of reaction through which the litmus changes is different in the two cases, the losses of litmus to the solution will not be the same. A blue and a red sample of the quickly reacting paper No. 4 were seen to assume very nearly the same tint in a large volume of milk in 24 hours.