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crucible. Insert a 360° thermometer in the bicarbonate and heat the sand bath so that the thermometer registers a temperature between 270° and 300° for an hour. At this temperature the bicarbonate is converted to the carbonate according to the reaction

NaHCO3 + A→ Na2CO3 + H2O + CO2

The heating must be watched carefully, as frequently the temperature begins to mount very fast and will go beyond 300° unless checked, and at temperatures much above 300° we have enough decomposition of the carbonate into oxide according to the reac

tion

Na2CO3+A→ Na2O + CO2

8

to vitiate its use as a standard for purposes of titration. The carbonate is cooled in a desiccator for thirty minutes and weighed. The heating is repeated for half-hour periods until constant weight is obtained, when the carbonate is ready for use. It is kept in its containing crucible in the desiccator and, as needed, small portions are withdrawn by means of a spatula.

If a supply of pure sodium bicarbonate is not at hand, a good grade of "Analyzed" sodium carbonate may be used in its place.

130. Preparation and Standardization of Approximately 0.1 Molar Hydrochloric Acid by Means of Sodium Carbonate.

Preparation of the Acid. Pour into a clean liter flask about 600-700 c.c. of distilled water, then add from a burette 8.5 c.c. of concentrated (12 M) hydrochloric acid. Make up to the mark with water and thoroughly mix the solution by shaking. This will give a solution which is approximately 0.1 molar. Transfer the solution to a clean two-liter, glass-stoppered bottle.10

Standardization of the Acid. Three beakers of about 300 c.c. capacity each are selected as nearly alike in form and color as

beaker for the purpose of supporting the crucible to be heated is placed a small triangle with its legs bent at right angles and cut to such length that the plane of the triangle shall be about 2 cm. above the floor of the silica beaker.

8 W. C. Ferguson, J. S. C. I. 24, 781 (1905).

'Concentrated hydrochloric acid, sp. gr. 1.19, 36-37%, as generally supplied is usually about 12 molar. The constant boiling point hydrochloric acid, i.e., that which boils at 110° under 760 mm. pressure, is about 6.1 molar.

10 The two-quart bottles in which acids are usually supplied for the laboratory serve excellently for this purpose.

practicable. The sodium carbonate is weighed out upon a tared watch-glass in successive portions of about 0.160 g., 0.175 g., and 0.190 g., to the nearest tenth of a milligram, and transferred to the several beakers by means of a camel's-hair brush. In handling the watch-glass upon which the sodium carbonate is placed, the watch-glass should not be touched by the fingers but by means of tongs, because the fingers even though freshly cleaned will leave enough perspiration or grease upon the watchglass to cause an error of several tenths of a milligram in the weighing.

After all the portions of sodium carbonate have been weighed out, add to each beaker 125 c.c. of water and 2-4 drops of methyl orange solution. Fill a 50 c.c. burette to the zero mark with the acid to be standardized; place the first beaker upon an unglazed piece of white paper12 under the burette and while stirring the solution in the beaker run in the acid slowly from the burette, until the color of the methyl orange becomes a very faint yellow, wash the walls of the beaker down with distilled water and resume the titration very carefully by adding the acid a drop at a time until the color of the methyl orange becomes a faint pink. This marks the end point of the titration. The burette reading is noticed after waiting the customary thirty seconds for drainage. There is often difficulty in judging the color change of methyl orange because of the fact that too much of this indicator has been employed. Methyl orange is peculiar in that it gives the sharpest color change when the amount of it that is used is the least amount that is plainly noticeable; if too much of it is used the color change becomes indefinite owing to the development of an intermediate claret-brown tint during the color transition.

The contents of the second and third beakers are similarly titrated. From the weights of the sodium carbonate and the volumes of hydrochloric acid employed calculate to four significant figures for each titration the number of grams of sodium carbonate equivalent to 1 c.c. of the acid.13 The average deviation of these

11 Pyrex glass, owing to its greenish yellow tint, is not desirable for use in the present titration.

12 A 121⁄2 cm. filter paper answers admirably for this purpose.

13 In making this calculation it is not customary to correct for the amount of solution necessary to affect the indicator; failure to make such correction introduces a slight error in the

three results should not be greater than one part per 1000. If the average deviation is greater than this, further titrations must be run until the average deviation, excluding those results which are doubtful in accordance with our criterion for the rejection of doubtful observations, is within one part per 1000. The arithmetical mean is then taken as the value of the solution.

131. Labeling Standard Solutions. — Immediately upon obtaining the value of any solution, the analyst should paste upon the stock bottle containing the solution, a label bearing the name of the reagent, the value of the solution, the date of standardization and the initials of the person making the standardization, thus:

Hydrochloric Acid

0.1098 M

8.12.25 H. A. F.

Cautions about the Use of Standard Solutions. Before withdrawing any standard solution from its stock bottle, always do two things: first, always shake the stock bottle vigorously because very often some water distils from the solution and collects upon the inner upper walls of the bottle, thereby changing the value of the solution unless remixed with it; secondly, always wipe the mouth of the bottle carefully with a damp cloth to remove any dirt or foreign matter that might have collected.

Having once withdrawn some standard solution from its stock bottle, never return any unused portion of it to the bottle, as such practice is extremely threatening to the reliability of the solution. 132. Standard Alkalies. It is to be observed that all alkaline solutions are subject to deterioration upon standing because of the absorption of carbon dioxide from the air and the action of alkaline solutions on the glass of the containing vessels. Therefore it becomes necessary to redetermine or check the value of all alkaline

value of the solution but such error cancels out as explained in § 57, if the same volume of standard solution is used in the subsequent titration of an unknown as was used in the standardization.

solutions practically every new day that they are used. From what has been said under the theory of acidimetry-alkalimetry, § 113, it follows that a strong base is the only one that is suitable for the general purposes of titration. There are three strong bases available, namely, potassium hydroxide, sodium hydroxide, and barium hydroxide; of these sodium hydroxide is more generally used because it can be more easily secured on the market. Even the best grades of this chemical, however, contain small amounts of chlorides, sulphates, silica and alumina, besides more or less sodium carbonate and water, so that it is advisable to weigh out about 10% more than the calculated amount when using it for the preparation of a standard solution.

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133. If a solution of sodium hydroxide, quantitatively free from carbonate, is desired, the sodium hydroxide solution is first made up in a bottle of suitable size, and enough barium hydroxide (or barium chloride) solution added to precipitate the carbonate. The precipitate is allowed to settle overnight or so, and the clear solution then siphoned off into another bottle which has a two-hole rubber stopper, one hole to accommodate a guard tube filled with soda lime, the other hole to accommodate the long arm of the siphon used for conveying the solution from the stock bottle to a burette after the manner represented in Fig. 21.

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FIG. 21

STOCK BOTTLE

The end of the siphon should be about one inch away from the bottom of the stock bottle so that the solution may be withdrawn without disturbing any slight precipitate of barium carbonate that may have formed. The burette

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is filled by applying suction at N. It is advisable to have the guard tube bent at right angles, as enough moisture is sometimes absorbed to cause the soda lime to dissolve and if the tube were not at right angles the concentrated alkaline solution would drip into the stock bottle.

In many determinations the presence of a small amount of carbonate in the sodium hydroxide still allows a precision sufficient for the work in hand. When such is the case it is not necessary to go to the precautions described above, but only necessary to dissolve a good grade of sodium hydroxide and make up to volume forthwith. To assure himself of the amount of carbonate which may be present in the solution and whether such amount is allowable or not, the analyst should not merely accept the published figures giving the manufacturers' analysis of the sodium hydroxide but should run a differential titration using phenolphthalein and methyl orange (or methyl red) as indicators.

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134. The primary standard for the standardization of either sodium hydroxide or potassium hydroxide solutions is benzoic acid in conjunction with phenolphthalein as indicator. Standard hydrochloric or sulphuric acids are also used for the purpose of standardizing the alkaline solutions, but they can only be regarded as secondary standards, since by their use we are basing a standardization upon a solution which itself has been standardized.

Directions for Use of Benzoic Acid as Acidimetric Standard.14 The benzoic acid should not contain more than 0.1 percent of chlorine as chlor-benzoic acid.15 This material may be safely used as an acidimetric sample in work requiring an accuracy of not more than one part in 2000, provided the following precautions are carefully observed:

1. The acid should be carefully fused before use in a glass or platinum vessel placed in an air bath. The temperature during fusion must not rise above 140° C; it is best to keep it below 130° and to cease heating as soon as fusion is complete.

2. The acid should be dissolved in 95% alcohol (about 20 c.c. for a 1-gram sample) and a blank test made upon the same volume of alcohol so that the necessary correction may be applied in the subsequent titration.

14 As outlined by the U. S. Bureau of Standards.

15 Benzoic acid of the requisite purity can be obtained from the Bureau; their Standard Sample No. 48, Benzoic Acid, C6H5COOH 99.9%, Acidimetric Standard.

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