platinum dish and dried to constant weight at 130°, after which it should be put in a glass-stoppered bottle, and the bottle kept in a desiccator. Sodium chloride is not very hygroscopic, so that (unless the atmosphere is unusually damp) it can be weighed out directly.

Procedure. Weigh out to the nearest tenth of a milligram about 0.250 g. sodium chloride, dissolve in 150 c.c. water, acidulate the solution with 5 c.c. 0.1 M HNO3, and proceed thenceforth as directed either in Exercise No. 7, or (less preferably) in Exercise No. 8. Calculate the percentage of chlorine actually found.

175. Volumetric Determination of Chlorine in Soluble Chlorides. Mohr's Method.5 General Remarks. In this method the solution of the soluble chloride is treated with a few c.c. of a dilute solution of potassium chromate to serve as indicator, and then titrated with standard silver nitrate solution. The end point is evidenced by the change in the color of the solution from a lemon yellow to a faint orange tint. Hydrogen ion concentration plays a very important rôle in this titration and must be between the limits of P 4.0 and P 7.0. For this reason, if the method is used to determine chloride in a sample of hydrochloric acid, the p of the solution must be brought within the range mentioned. Either ammonium hydroxide solution or powdered calcite (natural calcium carbonate) can be used for this purpose.

=

=

To show the application of theory to this method, let us consider the actual standardization of an approx. 0.1 molar silver nitrate solution by means of sodium chloride. We weigh out a suitable amount of sodium chloride, say 0.1850 g., and then dissolve it in about 125 c.c. water; to this solution we add 3 x.c. of 0.1 molar potassium chromate solution, after which we are ready to begin the titration. The silver nitrate solution is added dropwise from a burette, and the scdium chloride solution stirred

4 If it is not convenient to prepare a purified sodium chloride as described and a good grade of "analyzed" sodium chloride is at hand the purification may be dispensed with, but the heating to constant weight at 130° cannot be omitted, because the sample is very likely to contain some hydrochloric acid and some water, particularly the latter if the sample has been opened for any length of time.

5 A. Classen, Frederick Mohr's Lehrbuch der Chemisch-Analytischen Titrirmethode., 1896, 7th ed., p. 425.

vigorously at the same time. When an equivalent amount of silver nitrate (0.5377 g.) corresponding to the equation

NaCl AgNO3 :: 0.1850 g. : x
58.46 169.89

has been added, the concentration of silver ion in the solution will be equal to the concentration of chloride ion because of the fact that the total number of moles of silver in both phases will be equal to the total number of moles of chlorine, and an equal number of each has precipitated out as the solid phase. By virtue of this equality of concentration of the silver ion and the chloride ion at the stoichiometrical point, the actual value of the two concentrations is easily derivable from the solubility product relationship for silver chloride. We have for this relationship

CAg+ X CCI- = 10-10

and since this is true when CAg+Cc- it follows at once that when the two concentrations are equal their respective values must be cag+= CCI = 10-5.

Now the concentration of potassium chromate which we have employed is not sufficient to cause a precipitation of silver chromate at the above point, because to cause such a precipitation the concentration of chromate ion must exceed 2 X 10-2 in accordance with the solubility product relationship for silver chromate and the existent concentration of silver ion, viz.,

(CAg+)2 X Ccro > 2 X 10-12

or putting in actual values

Whence

(10-5)2 X Ccro > 2 X 10-12

Ccro 2 X 10-2

The actual concentration which we have employed is 2.5 X 10-8 or one-eighth that demanded by theory. Two pertinent questions might now well be asked: why was not the theoretical amount of chromate used to give a precipitation of silver chromate when the concentration of silver ion became 10-5, and how much more silver nitrate solution will have to be added beyond that point to give such a precipitation? The answer to the first question is a practical one: if the theoretical amount of indicator

were used the color of the solution would be such a deep orange that the appearance of the silver chromate would be almost undiscernible. The answer to the second question is that the addition of 0.0009 g. AgNO3 (= 0.05 c.c. 0.1 M AgNO3) is sufficient for the satisfactory establishment of the end point; this excess amount of precipitant increases the concentration of silver ion to 4.1 X 10-5, whereas the necessary concentration for the precipitation of the silver chromate is 2.8 × 10-5 as can be seen by solving the relationship which must be satisfied that

(CAg+)2 X (2.5 X 10-3)= 2 × 10-12
X

So far we have said nothing about the hydrogen ion concentration of the solution which is being titrated because with sodium chloride, which is a salt that shows no hydrolysis, the p of the solution would be around p = 5.86 and this is all right. If, however, the solution were more acid than p1 = 4.0, the concentration of chromate ion would be so repressed by the disturbance of the following equilibrium to the left

that as a result the solubility product of the silver chromate would come no ways near being exceeded and we would fail to get an end point. If, on the other hand, the solution were much more alkaline than p = 7.3, the concentration of hydroxyl ion would be such that silver oxide might be precipitated, owing to the solubility product of silver hydroxide (S.P.AgOH 10-8) being locally exceeded

CAg+X COH-> S.P. ⚫AgOH

=

2.3 X

These considerations always necessitate careful attention to the establishment of the proper concentration of hydrogen ion in Mohr's method, namely, a concentration lying between p = 4.0 and p = 7.0.

176. Exercise No. 10. Volumetric Standardization of Approx. 0.1 M AgNO3 Solution by Means of Sodium Chloride (Mohr's Method). Prepare about 500 c.c. of approx. 0.1 molar silver nitrate solution and about 50 c.c. 0.1 molar potassium chromate

6 See § 115.

solution. Standardize the silver nitrate solution as follows: pulverize about 1 g. of pure fused sodium chloride, spread it in a thin layer on a watch-glass and dry at 110°-120° for an hour. Select three 250 c.c. casseroles' and weigh out accurately to the nearest tenth of a milligram varying amounts of the sodium chloride, say, about 0.160 g., 0.170 g., and 0.180 g., and transfer to each casserole, respectively, the casseroles being marked I, II, III for identification. Add to each casserole 125 c.c. of water and 3 c.c. 0.1 M K2CrO4 solution and then titrate with the silver nitrate solution which should be added slowly with attendant stirring of the solution in the casserole. When the silver nitrate forms a precipitate of AgCrO4 which changes slowly into AgCl as the mixture is being stirred, the end point is not far away, and the further addition of the AgNO3 solution must be made with caution. The end point of the titration is marked by the transition in color from a lemon yellow to a faint orange tint which does not disappear on stirring. This change of color can only be judged satisfactorily by having at hand for comparison another casserole containing a similar volume of water, the same amount of indicator, and about 0.100 to 0.200 g. NaCl, to all of which has been added 10-15 c.c. of the silver nitrate solution; these quantities and conditions serving to establish the lemon yellow color in regard to which the end point must appear to be a faint orange tint. As the end point is difficult to recognize, it is a good plan at first for students to get the assistance of the instructor. The duplicate and triplicate titrations are run in the same manner. The individual results of each titration are then calculated, and if the average deviation of the series is not more than 2 parts per 1000, the average of the determinations is taken as the value of the silver nitrate solution. If the average deviation is greater than that mentioned, further titrations must be made until, excluding doubtful determinations (§ 57), the average deviation is brought down to the desired figure. If after repeated trial a student finds it impossible to obtain an average deviation better than 7-10 parts per 1000, it is very likely that he is troubled with color blindness.

7 It will be found preferable to employ casseroles rather than beakers as it is easier to judge the end point when casseroles are used.

8 Very often at this junction, but not always, the precipitated AgCl will coagulate; it is not advisable, however, to count on this coagulation as an index of the approaching end point.

4

177. Exercise No. 11. Volumetric Standardization of Approx. 0.1 M HCl Solution by Means of Standard Silver Nitrate Solution (Mohr's Method). Use the standard silver nitrate solution. prepared above and run the determinations in triplicate. Measure accurately from a burette into a casserole about 30 c.c. of the HCl solution and dilute with 100 c.c. distilled water; add 1-2 drops methyl orange solution and neutralize the acid with dilute NH4OH until the end point of methyl orange is just reached.10 To this neutralized solution add 3 c.c. 0.1 molar potassium chromate solution and titrate with the standard silver nitrate solution as described in the preceding exercise. Calculate the value of the acid and compare the value with those obtained by the procedures of §§ 172, 173. The average deviation for this determination should not be more than 2 parts per 1000.

178. Exercise No. 12. Determination of Potassium Chloride in a Mixture of Sodium Carbonate and Potassium Chloride (Mohr's Method). — Weigh out into a casserole such a quantity of sample as will require about 30 c.c. of 0.1 molar silver nitrate solution this presupposes an approximate knowledge of the composition of the sample. If the analyst does not possess such knowledge, he should first run a rough preliminary determination in order to obtain it. Having weighed out the appropriate amount of sample," dissolve it in 100 c.c. of distilled water, add 1-2 drops of methyl orange solution, and neutralize the alkaline solution with 0.1 molar nitric acid solution until the methyl orange end point is reached. To the neutralized solution add 3 c.c. 0.1 molar potassium chromate solution and titrate with standard silver nitrate solution as described in § 175. The percentage of potassium chloride in the sample should be reported on the dry basis (§ 7); this will necessitate a separate determination of the moisture. To make this determination, clean a porcelain crucible and dry it in the oven at 110°-115° for an hour. After allowing it to

If a standard HCl solution has already been prepared for Exercise No. 3, § 136, or Exercise No. 7, § 172, it can be used here.

10 The presence of the methyl orange does not seem to interfere in any way with the precision of this determination.

11 Whenever a sample of powdered material is being weighed out it should be critically examined as to its homogeneity. If there is any evidence of non-homogeneity, such as large or strange particles or unevenness of color, etc., the acceptance of the sample for analysis is open to severe criticism.