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`A BUFFER SOLUTION FOR COLORIMETRIC
`COMPARISON.
`
`Br T. C. MGILVAINE.
`
`(From the Department of Soils, West Virginia University, Mcrgcmtotim.)
`
`(Received for publication, September 23, 1921.)
`
`In the use of standard buffer solutions for colorimetric compari-
`son where more than a restricted range of reaction is required,
`it has been necessary in the past to make use of several solutions.
`The author has developed a system requiring but two stock solu-
`tions and covering a range of from pH 2.2 to pH 8.0 which ap-
`proximately includes the limits" of reaction for arable soils and
`physiological media. The materials used are as follows: 0.2 M
`disodium phosphate‘ and 0.1 M citric acid, combined in such
`volumes as to make 20 cc. of the mixture.
`
`The disodium phosphate employed was recrystallized three
`times. A 0.2 M solution was prepared by titration against I'I:CI,
`using methyl orange as indicator. Although the titration end-
`point was not distinct, the error in the resulting mixtures was
`usually not appreciable, seldom exceeding 0.01 pH. On account
`of the variable water of crystallization content of the phosphate
`salt” the stock solution was standardized by titration in order to
`have a reproducible method.
`‘Upon exposure to the air for a
`period of 2 weeks the water of crystallization of disodium phos-
`phate is reduced to 2 molecules.
`If, after exposure, a quantity
`of the salt is kept on hand in a closed container and the correct
`weight of the salt required to produce the proper concentration
`of stock solution determined by titration, it is possible at any time
`to make up the stock solution by simply weighing out the salt.
`The citric acid was recrystallized at least twice before using.
`
`1 Molds may develop in phosphate solutions under suitable conditions,
`but this source of trouble has been obviated by Martin’ by shaking the
`solutions with a little calomel for a few minutes and then filtering.
`3 Martin, C. J., Bicchem. J., 1920, xiv, 98.
`183
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`184
`
`A Buffer Solution
`
`The 0.1 M stock solution was standardized by titration against
`NaOH solution which had been prepared with boiled water and
`protected from 00;. Barium hydroxide was also used as recom-
`
`511 required.
`
`TABLE 1.
`
`0.2 M NMHPO4.
`6G.
`
`0.1 mcitric acid.
`00.
`
`2.2
`2.4
`2.6
`2.8
`3.0
`3.2
`3.4
`3.6
`3.8
`4.0
`4.2
`4.4
`4.6
`4.8
`5.0
`5.2
`5.4
`5.6-
`5.8
`6.0
`6.2
`6.4
`6.6
`6.8
`7.0
`7.2
`7.4
`7.6
`7.8
`8.0
`
`0.40
`1.24
`2.18
`3.17
`4.11
`4.94
`5.70
`6.44
`7.10
`7.71
`3.28
`8.82
`9.35
`9.86
`10.30
`10.72
`11.15
`11.60
`12.09
`12.63
`13.22
`13.85
`14.55
`15.45
`16.47
`17.39
`18.17
`18.73
`_ 19.15
`19.45
`
`19.60
`18.76
`17.82
`16.83
`15.89
`15.06
`14.30
`13.56
`12.90
`12.29
`11.72
`11.18
`10.65
`10.14
`9.70
`9.28
`3.85
`8.40
`7.91
`7.37
`6.78
`6.15
`5.45
`4.55
`3.53
`2.61
`1.83
`1.27
`0.85
`0.55
`
`mended by Sorensen and given by Clark,“ but sodium hydroxide
`was found to be as accurate and more convenient. The correct
`
`weight of citric acid required to make the stock solution can also
`be determined by titration.
`
`3 Clark, W. M., The determination of hydrogen ions, Baltimore, 1920.
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`186
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`'
`
`A Bufier Solution
`
`The pH values of the mixtures were determined electron1etri-
`cally by use of -the chain; Hg l HgCl | NKCL | saturated solution
`of KCl I H2 1 Pt. No allowance was made for liquid potential. No
`attempt was made to maintain a constant temperature. How-
`ever, the temperature of both calomel electrode and buffer solu-
`tion were taken into account in computing the pH values. Clark’s’
`extension of S01-ensen’s values for the normal calomel electrode
`
`was used with the necessary interpolations. Three extra calomel
`electrodes were employed for checking the accuracy of the one in
`general use. A Leeds and Northrup type ‘K potentiometer and
`type R sensitive galvanometer were used for making the elec-
`trometric measurements. The electrode was of the platinum
`wire variety. The hydrogen was generated electrolytically and
`passed first through an acid permanganate solution, next through
`a hot tube, and finally through a wash bottle containing distilled
`water.
`.
`
`A graph was constructed in which the pH values (determined
`electrometrically) of various mixtures of phosphate and citric
`acid solution, so arranged that the total volume was in all cases
`20 cc., were plotted against the volumes of the two solutions.
`By interpolation, using the curve so obtained, it was possible to
`arrive at the proper volumes of the two solutions which when
`mixed would give 20 cc. of a solution having any desired reaction.
`The values given in Table I were obtained in this way and checked
`by actually preparing the solutions and measuring the pH values
`by the electrometric method.
`In-all eases the variation of the
`observed from the calculated pH was 0.01 or less.
`Fig. 1 gives the titration curve for the foregoing system of
`bufiers. "For comparison, there is also given the titration curves
`of Well known standard buffer solutions.“ _
`' The shape of the phosphate-citric acid curve of Fig. 1 indi-
`cates that the mixtures are well suited for colorimetric determina-
`
`tions of pH.
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