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\ INDUSTRIAL A N D ENGINEERING CHEMISTRY
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VOl. 19, No. 1
Direct Titrometric Methods for Magnesium, Calcium, and Sulfate Ions and Their Amlication in Water Analysis’ a *
By E. P. Schoch UNIVERSITY OF TEXAS, AUSTIE;,TEXAS
W
ITH the aid of thymolphthalein as a n indicator it is
possible, by starting with a neutral solution containing magnesium, calcium, and sulfate ions, together with such non-interfering ions as sodium, potassium, and chloride, to determine (1) the magnesium ion by titration with a standard lime solution, (2) t h e calcium ion by titration with a standard sodium carbonate solution, and (3) the sulfate ion, by adding a standard barium chloride solution in excess of the sulfate ion present, and then titrating this excess with standard sodium carbonate solution. All of these titrations are performed with the mixtures a t temperatures between 90” and 100’ C. These titrations become possible through the fact that in solutions a t 90” t o 100’ C. thymolphthalein begins to turn blue with an alkalinity corresponding to a p H value of 10.23 and the titration of magnesium is completed with an alkalinity slightly less than that with which the blue color appears. Note-The final mixture obtained on titration of magnesium is a saturated solution of magnesium hydroxide in solutions of other salts. The latter generally do not exceed 70 gram-equivalents per kiloliter (about 4100 p. p. m.), and such solutions turn thymolphthalein a deep blue; but when a finely divided solid-e. g., precipitated calcium carbonate-is suspended in the liquid, the colloidal magnesium hydroxide is adsorbed and hence its solubility so reduced that thymolphthalein barely turns colorless. The calcium carbonate used in these titrations does not turn thymolphthalein blue because the calcium ion in the solutions reduces the solubility of calcium carbonate sufficiently to make the pH value due to it less than 9. Thls adsorption requires several minutes and hence i t is impracticable to reach the end point of this titration except by first addmg an excess of calcium hydroxide and then titrating this excess with a standard acid.
The precipitation of calcium carbonate is not completed until a definite blue color is obtained-about half as deep as the standard color. Since the hydroxyl ion produced by the hydrolysis of the carbonate ion is the guide in this titration, i t is essential that there be direct proportionality between the carbonate ion and its hydrolytic products. Hence hydroxyl ion introduced from any other source-particularly that introduced in the titration of magnesium and which corresponds to the difference between neutrality and pH = 10.23-must be removed by neutralization. Boiling reduces the dissolved carbon dioxide to a practically definite minimum as is shown by the reproducibility of hot saturated solutions of calcium carbonate-within the limits of error of these titrations. At the end of this calcium titration the pH value must be that of sodium salt solutions saturated with calcium carbonate. The range of salt concentrations in samples to be tested will extend approximately from 10 to 100 gram-equivalents per kiloliter and such solutions saturated hot with “precipitated” calcium carbonate (which is relatively coarsely crystalline) will show a thymoiphthalein color range extending from barely no color to the standard tint; a total of 0.3 gram-equivalent per kiloliter of sodium carbonate or calcium chloride will change the tint of either extreme concentration to that of the other. Hence a tint half way between these-or half of the standard tint-has been chosen as the end point. TTTiththis, the maximum error is 0.15 gram-equivalent per kiloliter, or 1.5 per cent if the sample titrated contains 10 gramequivalents per kiloliter of calcium ion. 1
Received April 9, 1926.
Resubmitted September 29, 1926.
The titration of thn excess of barium chloride used in the sulfate determination is complete only when the color has reappeared and passed the standard color. The barium carbonate precipitated in this titration tends to float on the liquid, and thus be lifted up in bubble films by the steam and deposited on the sides of the flask. If these are hotter than 100’ C., this material will be hydrolyzed with the consequent loss of carbon dioxide. Hence the flask should be heated with a moderate flame and shaken continuously. I n hot solutions barium carbonate is more soluble than calcium carbonate, and hence the final p H value for the titration of barium carbonate is greater than for calcium carbonate. If barium carbonate is suspended in solutions containing from 10 to 100 gram-equivalents per kiloliter of sodium chloride, plus 1 gram-equivalent per kiloliter of both soda and hydrochloric acid (to prevent any deficiency of carbon dioxide), and the mixtures boiled carefully, the thymolphthalein color range shown by them extends from a tint lighter than the standard to a tint heavier +an the standard; and 0.5 gram-equivalent per kiloliter of soda or barium chloride will change the tint of one extreme concentration to the other. Hence a tint equal to or slightly deeper than the standard is taken as the end point, and the error with the two extreme concentrations above is 0.25 gram-equivalent per kiloliter and for other concentrations i t is less or even zero. Since precipitated barium carbonate is isomorphous with “precipitated” calcium carbonate, the presence in the titrated samples of some of the latter-which is relatively coarsely crystalline-prevents the supersaturations otherwise due to the fine precipitate. Interfering ions are readily removed and, a t the same time, the solutions are prepared for the above titrations by adding as much calcium hydroxide to the hot mixture as is necessary to bring its pH value to 8.3-that is, the point a t which phenolphthalein just begins to turn red. With natural waters the precipitated substances are usually iron oxide, alumina, or small amounts of silica. I n the absence of alumina these precipitates need not be filtered off, because they do not interfere with the rest of the determinations. Alumina, however, dissolves a t slightly higher p H T-alues, and hence must be removed before magnesium ion can be titrated. Tests of Methods
Tests of these methods were made by three different operators. I n each case the volume of the sample titrated was 100 cc.; thus the number of cubic centimeters of 0.1 N reagent used is the same as the number of gram-equivalents per kiloliter. The concentrations of magnesium in the samples ranged from 3 to 20 gram-equivalents per kiloliter and the errors in thirty determinations were about equally positive and negative, ranging from 0 to less than 0.2 gram-equivalent per kiloliter. The concentrations of calcium ranged from 2.2 to 21.2 gram-equivalents per kiloliter. Twenty trials were made with calcium salts alone, and thirty trials with samples in which magnesium had previously been titrated by this method. The errors, both positive and negative, ranged from 0 to 0.2 gram-equivalent per kiloliter.
January, 1927
INDUSTRIAL AND ENGINEERING CHEMISTRY
Solutions containing potassium, magnesium, and calcium sulfates, either alone or mixed with the total sulfate concentration ranging from 10 to 20 gram-equivalents per kiloliter, were titrated, with errors, positive and negative, ranging from zero to 0.25 gram-equivalent per kiloliter. Materials a n d Equipment ( a ) 0 . 1 N Hydrochloric Acid. Accurately known titer. ( b ) Lime Solution. Kearly saturated (about 15.04 N), the titer of which is to be established from time t o time by means of the hydrochloric acid ( a ) and phenolphthalein as indicator. ( c ) 0.1 N Sodium Carbonate. To be titrated occasionally against a neutral calcium chloride solution prepared from solutions ( a ) and ( b ) , the titration t o be made hot, with thymol-
phthalein.
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natural waters are too small to make accurate titrations possible. The least quantity of one of these constituents should require 5 cc. of 0.1 iV reagent for its titration, and the best range of concentrations is such as t o require from 10 to 20 cc. of 0.1 N reagent in approximately 100 cc. total volume. Calcium or sulfate ion should never be in concentrations greater than that precipitable by 25 cc. of 0.1 N reagent per 100 cc., because quantities requiring 31 cc. of 0.1 N reagent per 100 cc. are a t the limit of solubility of calcium sulfate. I n general, waters will have t o be concentrated from 500 or 1000 cc. t o approximately 80 or 100 cc. Before evaporation, waters containing carbonates must be slightly acidified by adding an amount of 0.1 N or 0.5 iV hydrochloric acid which would be required for titration with methyl orange (determined on a separate portion). After concentrating a sample suitably, take out one-fourth by weight, and titrate hot with the standard lime solution, with phenolphthalein as an indicator, until a faint pink appears. Add three times as much of the same lime solution (to be recorded and marked A ) to the other three-fourths of the sample (but no phenolphthalein), and heat the mixture t o boiling. If even a slight white flocculent precipitate appears, filter the hot mixture through an ordinary 9-cm. filter paper, into another 350-cc. titration flask, and wash twice with about 10 cc. of hot water. Under these conditions, iron and aluminum hydroxide are completely precipitated.2 Aluminum hydroxide must be removed because it is completely redissolved in the range of alkalinity in which magnesium hydroxide is precipitated and hence it would vitiate the magnesium determination. Iron hydroxide and small amounts of silica do not interfere.
( d ) 0.1 N Calcium Chloride. If prepared from the anhydrous salt, as much hydrochloric acid must be added as would be required to turn phenolphthalein barely colorless. This solution is to be titrated with hot sodium carbonate ( c ) , with thymolphthalein. ( e ) 0.1 N B a r i u m Chloride. To be titrated with the 0.1 N sodium carbonate (c). (f) Thymolphthalein Solution. 2 grams per liter of 95 per cent ethanol. (g) Color Sharpening Device. A small shelf with a white surface (30 X 15 cm.) should be Dlaced near a window in such a position that the observer will ha;e his back to the window when viewing the sample on the shelf, and at such a height that he can look through the liquid horizontally into free space beyond the shelf. A piece of white glass (30 X 15 cm.) should be placed vertically on the shelf as a background t o the sample but without being fastened; when trying to observe the color of a sample, the operator should raise and lower this glass repeatedly. By this means he is enabled to distinguish between the superficial bluish appearance of a colorless liquid containing a very finely divided white precipitate in suspension and the same liquid with a blue color in it. As the glass is lowered into position the colorless liquids will appear to become lighter, while colored liquids become darker. A t night a “daylight” globe serves to make the Titration of Magnesium, Calcium, a n d Sulfate blue tints visible. The globe should be behind the observer and shaded so that the direct rays will not fall on the flasks. hfaomsiubf-Add 12 to 14 drops of thymolphthalein solu( h ) Color Standard. A flask of the same shape and size as , tion to the clear liquid obtained above, heat to boiling, and those in which the titrations are performed-. g., a 350-cc.’ add standard lime solution from a buret until the well-shaken, conical flask-is filled with as much of the standard color liquid as the average amount of liquid in the samples-. g., 100 to hot mixture shows a deep blue color. Keep a funnel in the 150 cc.-plus about 1 gram of precipitated calcium carbonate, mouth of the flask except when adding reagents. Add about and is tightly stoppered with a rubber stopper. The standard color liquid is prepared by mixing 6.5 cc. of 0.1 N copper sulfate, 1 gram of precipitated calcium carbonate, and boil the mix0.5 cc. of 0.01 N potassium chromate, and about 10 cc. of ordi- ture gently for 3 t o 5 minutes in order that the magnesium nary dilute ammonia with 1.500 cc. of distilled warer. This hydroxide may be thoroughly adsorbed by the calcium carflask is kept on the shelf described above, and is to be looked bonate. Then titrate the excess of lime with 0.1 N hydroat simultaneously with the samples. It should be shaken occasionally to put an amount of suspended particles in the liquid chloric acid, adding this a t first in 1-cc. portions, and reducing these gradually as the end point is approached. comparable with that in the samples. (2) PreLapitated Calcaum Carbonate, and a small porcelain After each addition shake the flask promptly, reheat the spoon. mixture to boiling, and allow it t o settle while the flask is The standard solutions should be mounted on a shelf and floating in a hot water bath-e. g., a 2-liter beaker. The end point is reached when the sample has become barely connected permanently with the burets below. I n order to minimize exposure to the air, the top of the burets should also colorless as compared with the standard color: 0.1 t o 0.15 cc. be connected to the tops of the supply bottles. The air vents (or 2 t o 3 drops) of 0.1 h‘ acid per 100 cc. of mixture changes of the latter should be small, and that of the lime solution the tint from the standard to this end point. Avoid adding should be connected with a soda-lime tube. Since duplicate an excess of acid, because the end point cannot be reached samples can be titrated simultaneously in practically the time accurately except by the procedure above-i. e., adding lime required for one, each bottle should be connected to two till the mixture is deep blue, etc. Put the flask in the water burets-all of the left-hand burets being used for one sample bath and read the burets, recording that of the lime as B and that of the acid as C. and the right-hand burets for the other. A heating appliance and also a water bath (preferably a 2CaLcIuJr-Add 0.1 N sodium carbonate from a buret to the liter beaker) should be a t hand-in duplicate when two sam- mixture left a t the end of the magnesium titration until ples are to be titrated simultaneously. The samples should the hot mixture shows a definite blue, but not necessarily a be placed in 350-cc. conical flasks, the necks of which should deep blue. Then add 0.12 cc of 0.1 N hydrochloric acid for be wrapped to admit of handling with bare fingers. Small every 100-cc. volume which the mixture had a t the end of the funnels are hung in the mouths of the flask to hinder the access magnesium titration. No account is to be taken of this acid of carbon dioxide. because its purpose is merely to neutralize the excess of hydroxyl ion left from the magnesium titration. Preparation of Samples Titrate the excess of soda with 0.1 N calcium chloride(To remove interfering substances and t o secure con- or add more soda-until a color about half as deep as the centrations suitable for accurate titrations.) The con’ J . A m . Chem. S O C , 38, 1282 (1916). THISJ O U R N . 4 L , 13, 420 (1921); centrations of magnesium, calcium, and sulfate icins in most 14, 1038 (1922).
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
standard is obtained, heating the mixture after each addition of reagent, and allowing it to settle in the boiling water bath. I n this case the end point can be reached directly from either side. When through, put the flask in the water bath, and read the burets, recording that of the soda as D and that of the calcium chloride as E. SULFATE-Refill the soda buret, add 2 cc. of soda to the mixture to giveit a deep blue color, and add 0.1 N barium chloride fairly rapidly and continuously until all the color is out of the solution. Heat the mixture with a moderate flame and shake it a t intervals to prevent overheating on the bottom and sides. To prevent excessive foaming, reduce the size of the flame as soon as boiling begins. Use these precautions in all subsequent heating of this mixture containing barium carbonate. Then add sodium carbonate in 1-cc. portions until after reheating, etc., the mixture again shows a blue color. Then, to replace a small amount of carbon dioxide that may have been lost through hydrolysis, as a result of which the end point would be insensitive, add 0.2 cc. 0.1 N hydrochloric acid and boil the well-shaken mixture gently for several minutes. Reduce the color by adding barium chloride (or increase by adding sodium carbonate) 0.2 or 0.3 cc. a t a time, with reheating, settling, etc., as usual, until a color equal to or slightly deeper than the standard color is obtained. Passing this tint two or three times in opposite senses will help to make sure of the right end point. Read and record the soda buret reading as F , that of the barium chloride buret as G, and the amount of acid used as H . Calculation of Results
If each reading is multiplied by the proper factor to reduce it to the corresponding volume of 0.1 N solution, and if V cc. is the original volume of the sample taken for analysis, the amounts of the three constituents determined, in gramequivalents per kiloliter, are obtained as follows: Magnesium = Calcium Sulfate
'$ ( B - C) 133
= V (D-A-B--E) 133
(G-F-H) V
Determination of Sulfate Ion Alone
If bicarbonates are present, boil the solution 5 to 10 minutes to decompose them. Add 12 drops of thymolphthalein and very dilute sodium carbonate (or 0.1 N ) solution until the solution is definitely blue and boil this mixture for 5 or more minutes to expel any liberated carbon dioxide. Add dilute calcium chloride (or 0.1 N ) solution to the mixture until, after the usual boiling and settling in a water bath, the liquid has about one-half of the standard color. It is unnecessary to add precipitated calcium carbonate if enough is formed by reaction in the mixture. Under these conditions iron, magnesium, and calcium ions are precipitated and aluminum is dissolved as aluminate. The mixture is now ready for the sulfate determination-this to be carried out as directed above. As an illustration, a mixture containing 14 gram-equivalents per kiloliter sodium chloride, 7 gram-equivalents per kiloliter calcium sulfate, and 3 gram-equivalents per kiloliter magnesium sulfate was treated according to the above directions and the amount of sulfate found in two trials was 9.97 and 10.02 gram-equivalents per kiloliter, respectively. Determination of Permanent Hardness or of Sodium Carbonates in Natural Waters
Boil 100 cc. of a water for 10 to 15 minutes, replenishing the evaporated water from time to time. Then add 12 to 15
Vol. 19, No. 1
drops of thymolphthalein solution and 1 gram of calcium carbonate. If the solution is blue, the permanent hardness is zero, and the solution should be titrated with calcium chloride in the usual manner t o determine the amount of sodium carbonate present, the end point being a tint half as heavy as the standard tint. If after adding thymolphthalein the solution remains colorless, i t should be titrated for magnesium and calcium ion as per directions above. The sum of these ions gives the permanent hardness. Determination of Amounts of Lime and Soda Required t o Soften a Water
T o a 100-cc. sample of the water in a conical flask add 12 to 15 drops of thymolphthalein and if the sample contains sodium carbonate as determined above, add calcium chloride in equivalent amount. Fill the lime buret and, without heating the mixture, add lime until the mixture shows a blue color. Then pIace it over a flame and as the color vanishes add more lime to keep i t deep blue until it reaches the boiling point. I n general, calcium carbonate need not be added to this mixture because the amount of this obtained by precipitation is usually large compared with the amount of magnesium hydroxide. Boil the mixture for several minutes, and titrate the excess of lime with 0.1 N hydrochloric acid as in the magnesium ion determination. Continue by the same directions to determine the calcium ion. If the buret readings are designed by the letters used above, and multiplied by the proper factors to convert them to the corresponding volumes of 0.1 N solutions, then the amounts of pure lime and soda required with 100 per cent efficient action are: Pounds CaO per million gallons = 233(B- C) Pounds soda ash per million gallons = 442(D - C - E ) If, as usual, 0.25 pound excess of soda is t o be used per 1000 gallons, use (D 0.57) in place of D above, and divide by per cent efficiency.
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To determine whether or not a softened water is over- or under-treated, a boiler-room operator should be furnished with a saturated solution of lime, some 0.04 N sodium carbonate, and some 0.04 N acid (either hydrochloric or sulfuric) together with three small burets (10 or 25 cc.), a conical flask, and thymolphthalein and phenolphthalein solutions. To avoid undue exposure to the air in filling the buret, the lime bottle should be fitted with a siphon or cock a t the bottom to draw off the liquid, and with an air-vent pipe containing soda lime. For convenience, the other solution bottle may be fitted similarly except that soda lime should not be used. An electric heater or a gas burner, tripod, and wire gauze should be supplied for heating the mixture, but the water bath may be omitted. Add eight cubic centimeters of the lime solution to 100 cc. of distilled water and titrate cold with the acid using phenolphthalein as indicator, and record this as &I. Add 8 cc. of the lime solution to a 100-cc. sample of the treated water, add 12 drops of thymolphthalein, heat the mixture to boiling, and add acid gradually while keeping the mixture a t the boiling point until the blue color has barely vanished out of the settled liquid. Read the buret and mark this reading 0. Immediately add 0.3 cc. more of the acid without taking account of it. Then reheat the mixture to the boiling point, and add sodium carbonate while keeping the mixture hot until a faint definite color is observed in the settled liquid. Record the amount of soda used as P. If M > O , CaO is lacking in amount equal to 93.4(M-O) pounds per million gallons If M O , soda ash is lacking in amount equal to 177.0(P-O) pounds per million gallons If P
