INDUSTRIAL A N D ENGINEERING CHEMISTRY
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fourth the molecular weight of the borax, since four molecules of sodium hydroxide are required to react with the boric acid liberated from one molecule of borax. Present Work
The apparatus and the general method of procedure used in determining the p H curves were essentially the same as those described previously.' Titration curves were determined for
Vol. 17, N o . 2
what less highly ionized, could hardly be standardized with satisfactory accuracy. It should be noted that the method of standardizing this acid, with methyl orange as the indicator, made the solution fifth normal with respect to the first hydrogen. The relatively weak, monobasic acetic acid shows a curve still less satisfactory. Borax shows a good curve when titrated with sodium, potassium, or barium hydroxide in the presence of mannitol. A similar curve, shown in an earlier 14
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Figure I-pH Curves Showing Course of Neutralization for 30 MI. of Various 0.2 N Solutions Nos. 1, 2, 3, and 4 are for hydrochloric, sulfuric, phosphoric, and acetic acids, respectively, when titrated with borax. Nos. 6, 6, and 7 are for borax when titrated in the presence of mannitol with barium, potassium, and sodium hydroxides, respectively. Borax solution contained 7 to 8 grams of mannitol.
hydrochloric, sulfuric, phosphoric, and acetic acids when reacting with a solution of borax; for sodium, potassium, and barium hydroxides when reacting with a solution of borax containing mannitol; and for solutions of borax containing different amounts of glycerol or mannitol when reacting with sodium hydroxide. Borax was recrystallized416by allowing a hot, practically saturated solution to drop through a filter paper into a dish placed in cold water, with constant stirring. The crystals were separated on a Buchner funnel and air-dried on filter paper. Constant boiling point hydrochloric acid was weighed directly. The sulfuric and phosphoric acids were standardized against sodium carbonate, with methyl orange as the indicator, and sodium hydroxide free from carbon dioxide was used for the acetic acid, with phenolphthalein as the indicator. The barium and potassium hydroxides were also free from carbon dioxide. Fifth normal solutions were used throughout. The results are shown in the figures. Discussion
An examination of Figure 1will reveal the type of curve obtained for the various substances titrated. It is well known that under ordinary conditions only those substances can be titrated accurately whose p H curves for the titration have a vertical portion of sufficient length. It is evident that hydrochloric and sulfuric acids, representing, respectively, highly ionized mono- and dibasic acids, g!ve the proper type of curve with borax. Tribasic phosphoric acid, some-
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m/ of fitroi'ing solufion Figure 2H Curves Showing Effect of Different Concentrations of Glycerol a n c f w a n n i t o l when Titrating a 0.2 N Solution of Borax with a 0.2 N Solution of Sodium Hydroxide
paper,' is obtained if boric acid is titrated in the presence of mannitol after its liberation from borax by means of hydrochloric acid. Electrometric titration curves enable one not only to ascertain whether a given neutralization reaction will give a sharp end point, but also to select an indicator with the proper range for the titration. I n general, the effective range of a suitable indicator should be opposite the steep portion of the curve. An inspection of the various curves considered as satisfactory will indicate that methyl orange or bromophenol blue, with effective ranges extending from pH 3.0 to 4.6, should be a suitable indicator for either hydrochloric or su1furic acid. Likewise, phenolphthalein, with a pH range of 8.3 to 10.0, is satisfactory for locating the proper end point when using borax as a means of standardizing bases. Assuming that the degree of hydration of borax can be controlled under suitabIe conditions, an objection to its use in certain cases may still be found in the fact that the liberated boric acid cannot be removed by boiling, as in the case of carbon dioxide liberated from sodium carbonate. The advantages of using borax as a standard, as compared with sodium carbonate, are its higher equivalent weight and the possibility of its being used for both acids and bases.
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