Part 10

Reagents.—1. Standard calcium chloride solution. Dissolve 0.2 gram of pure calcite (calcium carbonate) in a little dilute hydrochloric acid, being careful to avoid loss of solution by spattering. Evaporate the solution to dryness several times with distilled water to expel excess of acid. Dissolve the residue in distilled water and dilute the solution to 1 liter. One cc. of this dilution is equivalent to 0.2 mg. of calcium carbonate.

2. Standard soap solution. Dissolve 100 grams of dry white Castile soap in 1 liter of 80 per cent alcohol, and allow this solution to stand several days before standardizing. Pure potassium oleate made from lead plaster and potassium carbonate may be used in place of Castile soap.

First method of standardization.—Dilute 20 cc. of the calcium chloride solution in a 250 cc. glass-stoppered bottle to 50 cc. with distilled water which has been recently boiled and cooled. Add soap solution from a burette, 0.2 or 0.3 cc. at a time, shaking the bottle vigorously after each addition until a lather remains unbroken for five minutes over the entire surface of the water while the bottle lies on its side. Then adjust the strength of the stock solution with 70 per cent alcohol so that the resulting diluted soap solution will give a permanent lather when 6.40 cc. of it is properly added to 20 cc. of standard calcium chloride solution diluted to 50 cc. Usually 75 to 100 cc. of the stock soap solution is required to make 1 liter of the standard soap solution. The quantity of calcium carbonate equivalent to each cubic centimeter of the standard soap solution consumed in the titration is indicated in Table 6.

Table 6.—TOTAL HARDNESS IN PARTS PER MILLION OF CaCO_{3} FOR EACH TENTH OF A CUBIC CENTIMETER OF SOAP SOLUTION WHEN 50 CC. OF THE SAMPLE IS TITRATED.

───────────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬─────┬───── Cubic │ │ │ │ │ │ │ │ │ │ centimeters│ 0.0.│ 0.1.│ 0.2.│ 0.3.│ 0.4.│ 0.5.│ 0.6.│ 0.7.│ 0.8.│ 0.9. of soap │ │ │ │ │ │ │ │ │ │ solution. │ │ │ │ │ │ │ │ │ │ ───────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼───── 0.0│ │ │ │ │ │ │ │ 0.0│ 1.6│ 3.2 1.0│ 4.8│ 6.3│ 7.9│ 9.5│ 11.1│ 12.7│ 14.3│ 15.6│ 16.9│ 18.2 2.0│ 19.5│ 20.8│ 22.1│ 23.4│ 24.7│ 26.0│ 27.3│ 28.6│ 29.9│ 31.2 │ │ │ │ │ │ │ │ │ │ 3.0│ 32.5│ 33.8│ 35.1│ 36.4│ 37.7│ 38.0│ 40.3│ 41.6│ 42.9│ 44.3 4.0│ 45.7│ 47.1│ 48.6│ 50.0│ 51.4│ 52.9│ 54.3│ 55.7│ 57.1│ 58.6 5.0│ 60.0│ 61.4│ 62.9│ 64.3│ 65.7│ 67.1│ 68.6│ 70.0│ 71.4│ 72.9 │ │ │ │ │ │ │ │ │ │ 6.0│ 74.3│ 75.7│ 77.1│ 78.6│ 80.0│ 81.4│ 82.9│ 84.3│ 85.7│ 87.1 7.0│ 88.6│ 90.0│ 91.4│ 92.9│ 94.3│ 95.7│ 97.1│ 98.6│100.0│101.5 ───────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────

This table does not provide for the use of so large volume of soap solution for a single determination as former ones because the end-point becomes somewhat obscured in the presence of magnesium if more than 7 cc. is used.

Second method of standardization.—Dilute 100 cc. of the stock soap solution to 1 liter with 70 per cent alcohol. This dilute solution should be of such strength that approximately 6.4 cc. of it will give a permanent lather when 20 cc. of standard calcium chloride solution diluted to 50 cc. with distilled water is titrated with it. Determine the amount of soap solution required to give a permanent lather with 50 cc. of distilled water and with 5, 10, 15, and 20 cc. of standard calcium chloride solution diluted to 50 cc. with distilled water. Finally plot on cross-section paper a curve showing the relation of various quantities of soap solution to corresponding quantities of standard calcium carbonate solution and therefore to parts per million of hardness.

Procedure.—Measure 50 cc. of the water into a 250 cc. bottle and add to it soap solution in small quantities in precisely the same manner as described under the standardization of the soap solution. From the number of cubic centimeters of soap solution used obtain from Table 6 or from the plotted curve the total hardness of the water in parts per million of calcium carbonate.

To avoid mistaking the false or magnesium end-point for the true one when adding the soap solution to waters containing magnesium salts, read the burette after the titration is apparently finished, and add about 0.5 cc. more of soap solution. If the end-point was due to magnesium the lather will disappear. Soap solution must then be added until the true end-point is reached. Usually the false lather persists for less than five minutes.

If more than 7 cc. of soap solution is required for 50 cc. of the water take less of the sample and dilute it to 50 cc. with distilled water which has been recently boiled and cooled. This step reduces somewhat the disturbing influence of magnesium, which consumes more soap than an equivalent weight of calcium.

At best the soap method is not a precise test on account of the different relative amounts of calcium and magnesium in different waters. For hard waters, especially in connection with processes for purification and softening, it is advised that this method be not exclusively used. If the same water is frequently analyzed it may be of assistance to standardize the soap solution against a mixture of calcium and magnesium salts, the relative proportions of which approximate those found in the water.

The strength of the soap solution should be determined from time to time, to make sure that it has not materially changed. Record all results in parts per million of calcium carbonate.

One English degree of hardness, Clark’s scale, is equivalent to 1 grain per Imperial gallon of calcium carbonate. One French degree of hardness is equivalent to 1 part per 100,000 of calcium carbonate. One German degree of hardness is equivalent to 1 part per 100,000 of calcium oxide, and multiplied by 17.9 gives parts per million of calcium carbonate. The relations of these various scales are indicated in Table 7.

Table 7.—CONVERSION TABLE FOR HARDNESS.

───────────────────────────┬─────────────────────────────────────────── Unit. │ Equivalent. ───────────────────────────┼──────────┬──────────┬──────────┬────────── │Parts per │ Clark │ French │ German │ million. │ degrees. │ degrees. │ degrees. ───────────────────────────┼──────────┼──────────┼──────────┼────────── One part per million │ 1.00│ 0.07│ 0.10│ 0.056 One Clark degree │ 14.3│ 1.00│ 1.43│ .80 One French degree │ 10.0│ .70│ 1.00│ .56 One German degree │ 17.9│ 1.24│ 1.78│ 1.00 ───────────────────────────┴──────────┴──────────┴──────────┴──────────

TOTAL HARDNESS BY SODA REAGENT METHOD.

Add standard sulfuric acid to 200 cc. of the sample until the alkalinity is neutralized. (See Procedure with methyl orange, p. 37.) Then apply the non-carbonate hardness method (pp. 34–35). This method gives fairly satisfactory estimates of total hardness of hard waters.

TEMPORARY HARDNESS BY TITRATION WITH ACID.

Determine the alkalinity in presence of methyl orange (see p. 37) in the original sample and also in the sample after boiling, cooling, restoring to the original volume with boiled distilled water, and filtering. The difference between the two, if any, is the temporary hardness. This is the most accurate method of determining the temporary hardness of ordinary waters. Iron bicarbonate is included as a part of the temporary hardness.

NON-CARBONATE HARDNESS BY SODA REAGENT METHOD.

The use of soda reagent does not avoid entirely the error due to solubility of the salts of calcium and magnesium; consequently, if much depends on the results, as in water softening, gravimetric determinations of the calcium and magnesium that remain in solution should be made and a correction should be applied for those amounts.

Reagent.—Prepare soda reagent from equal parts of sodium hydroxide and sodium carbonate. It should be approximately tenth normal.

Procedure.—Measure 200 cc. of the sample and 200 cc. of distilled water into 500 cc. Jena or similar glass Erlenmeyer flasks. Treat the contents of each flask in the following manner. Boil 15 minutes to expel free carbon dioxide. Add 25 cc. of soda reagent. Boil 10 minutes, cool, rinse into 200 cc. graduated flasks, and dilute to 200 cc. with boiled distilled water. Filter, rejecting the first 50 cc., and titrate 50 cc. of each filtrate with N/50 sulfuric acid in the presence of methyl orange or erythrosine indicator. The non-carbonate hardness in parts per million of calcium carbonate is equal to 20 times the difference between the number of cubic centimeters of sulfuric acid required for the soda reagent in distilled water and the number of cubic centimeters of N/50 sulfuric acid required for the soda reagent in the sample.

Water naturally containing bicarbonate and carbonate in excess of calcium and magnesium requires a larger amount of acid to neutralize the sample after it has been treated than is required to neutralize the volume of soda reagent originally added. (See p. 39.)

NON-CARBONATE HARDNESS BY SOAP METHOD.

Non-carbonate hardness may be calculated for waters which are soft or moderately hard in a fairly satisfactory manner by deducting the total alkalinity from the total hardness by the soap method (pp. 31–34). For waters that are very hard, and particularly those that contain much magnesium, this method is not advised.

ALKALINITY.

The alkalinity of a natural water represents its content of carbonate, bicarbonate, borate, silicate, phosphate, and hydroxide. Alkalinity is determined by neutralization with standard sulfuric acid or potassium bisulfate in the presence of phenolphthalein and either methyl orange, erythrosine, or lacmoid as indicators. Methyl orange may be used except in waters containing aluminium sulfate or iron sulfate. The relations between estimates in presence of these indicators and the carbonate, bicarbonate, and hydroxide radicles are indicated in Table 8. The alkalinity of carbonates in the presence of phenolphthalein is different from that in the presence of methyl orange, partly because of loss of carbon dioxide and partly because of defects in phenolphthalein as an indicator in such conditions.