584
INDUSTRIAL AND ENGINEERING CHEMISTRY
version unless sufficient heat conversion has previously been accomplisled. In the case of these same glycerides and of other esters of the drying-oil acids, we have also, on the other hand, acids which have the potential functionality of their unsaturation, ranging from 4 to 6; but ordinarily this unsaturation is apparently developed only to the extent of 1. If by suitable modification of the reaction conditions we can bring more of this potential functionality into play, it should then be possible to advance the degree of polymerization. The observed heat conversion of the mono- and diglycerides as well as the ultimate conversion of certain of the nonairdrying resins of this experiment under the combined action of heat and of oxygen are believed to be due to just such factors. Further and more conclusive evidence relating to the characteristic effect of this potential but delayed functionality will be forthcoming. Perhaps one of the more important observations that may be made as a result of the present work relates to the observed mutual and apparently equivalent effect of various functional groups in determining the degree of the polymeric state. Thus, active carbon-to-carbon double bonds may serve fully as well as carboxyl and hydroxyl groups in those “hybrid” systems in which both may become operative. Nature thus s e e m to regard addition and condensation mechanisms as but two means of attaining the polymeric state. It is just such evidence which favors a general revision of the definition of polymerization such as has been advocated by Carothers ( 3 ) .
VOL. 29. h-0.5
Prevention of
Calcium Deposits in
Process Waters Relative Value of Sodium Metaphosphate and Pyrophosphate
Acknowledgment The writer gratefully acknowledges the invaluable assistance of his associates, L. P. Moore, R. T. Dean, W. C. Norris, V. Bishop, and W. B. Johnston, in connection with the preparation and analysis of a number of the compositions here described. Particular credit is also due W. M. Grosvenor and G. M. J. Mackay for valuable help and criticism and t o the American Cyanamid Company for its support of this work and the permission to publish it.
Literature Cited (1) Bradlev. T. F..IND.ENG.CHEM..29. 440-5 (1937). (25 Carotgers, W. H., J . Am. Chem. Soc.,‘ 51, 2548-59 (1929); Chem. Rev., 8, 353-426 (1931). (3) Carothers, W. H., Trans. Faraday Soc., 32, 43-9 (1936). (4) Drinberg, A. Y., and Blagonravova, A. A., Am. Paint Varnish Mfgrs.’ Assoo.. Sci. Circ. 501, 21-30 (1936): J . Gen. Chem. (U.S. S.R.),5, 1226-32 (1935). (5) Fonrobert, E . , and Pallauf, F., Chem. Umschau, 33, 44 (1926). (6) Kienle, R. H., U. S. Patent 1,893,873(Jan. 10, 1933). (7) Kienle, R.H., and Ferguson, C. S., IND.EXG.CHEM.,21, 349-52 (1929); Kienle, R.H., Ibid., 22,590-4 (1930). (8) Kienle, R.H., and Winslow, E . H., paper presented before Paint and Varnish Div. at 89th Meeting of A. C. S., New York, April 22 to 26, 1935. (9) Long, J. S., Kittelberger, W. W., Scott, L. K., and Egge, W. S., IND. ENG.CHEM.,21, 952-4 (1929).
BERNARD H. GILMORE Mellon Institute of Industrial Research, Pittsburgh, Pa.
Sodium metaphosphate and sodium pyrophosphate are compared on the basis of their relative effectiveness in preventing the precipitation of calcium orthophosphate, calcium carbonate, and calcium soaps. Sodium metaphosphate is more effective than sodium pyrophosphate both on the basis of the relative quantities necessary to prevent precipitation and of their relative tolerance for calcium ion in the presence of the precipitants studied.
R
ECEKT investigations in the science of water-conditioning which have culminated in the development of a new product, sodium metaphosphate (glassy), and a new process (4) for the softening of water, have sugRECEIVED September 14, 1936. Presented before the Division of Paint and Varnish Chemistry at the 92nd Meeting of the Ameriom Chemical gested the approach to the study of the prevention of calcium Society, Pittsburgh, Pa., September 7 t o 11, 1936. deposits in process waters described in this paper. It deals with the precipitation that takes place in industrial cleaning operations when alkaline salts are employed as cleaning agents in untreated waters. I n practically every field of Correction industrial cleaning, especially in the food industries which include the dairy, bottling, and beverage fields, the preferred An error has been brought t o my attention which occurred in m y article on “Fluid Flow Design Methods” [INDUSTRIAL cleaning agents are the alkaline salts, trisodium phosphate, sodium metasilicate, and sodium carbonate. These salts AND ENffINEERING CHEMISTRY, 29, 385-8 (1937)]. In t h e table are used for reasons of efficiency, economy, and adaptability of nomenclature on page 388 t h e symbol G is given as mass veto commercial equipment. However, in spite of their recoglocity, lb./(hr.) (sq. ft.); the time element should be seconds nized efficacy as detergents, they have one failing that is instead of hours. common to all. Because of the insolubility of their alkaline R. P. GENEREAUX
INDUSTRIAL AND ENGINEERING CHEMISTRY
586
the titration values subsequently determined were translated into the appropriate calcium concentration by reference to the curve. The effect of sodium metaphosphate on the concentration of calcium ion was next ascertained by adding progressively increasing quantities of sodium metaphosphate to a solution having an initial calcium-ion concentration of 40 p. p. m. and determining the calcium-ion content by the soap titration. I
I
I
I
I
I
I
I
FIQURE 1. CALIBRATION CURVEOF ANALYTICAL SOAP SOLUTION The solution of sodium metaphosphate used contained 10.20 grams per liter of unbuffered commercial flake metaphosphate, having the composition 93 per cent sodium metaphosphate, 7 per cent sodium pyrophosphate. One milliliter of the standard calcium solution was transferred to a 100-ml. volumetric flask, the appropriate amount of the metaphosphate solution was added, and the solution was made up to volume. It was then transferred to an oil sample bottle, the pH adjusted as in the calibration, and the titration conducted according to the standard method. The gpparent calcium-ion concentration was determined by referring the titration value to the calibration curve, and the results were plotted (Figure 3). Because of the effects of hydrogen ion, the soap titrations both during the calibration and the subsequent experiments with solutions of sodium metaphosphate were conducted after first neutralizing the solutions under test to the phenolphthalein end point (approximately pH 8.6) as suggested by Kean and Gustafson ( 5 ) . This was especially indicated because unbuffered flake sodium metaphosphate is slightly acid. However, in order to compare the relative effectiveness of sodium metaphosphate and sodium pyrophosphate a t the same pH, several values were obtained by titration after first adjusting the solutions to pH 10.0. I n determining the effect of sodium pyrophosphate on the apparent concentration of calcium ion, progressively increasing concentrations of this salt were added to solutions with an initial calcium-ion concentration of 40 p. p. m. The standard solution of sodium pyrophosphate used was a p proximately 0.1 M by weight, containing 44.61 grams per liter of commercial decahydrate, Na4Pz07.10Ht0. However, experimental results representing concentrations of sodium pyrophosphate have been expressed on an anhydrous basis in all cases. The soap titrations were conducted a t the characteristic pH of sodium pyrophosphate solutions (approximately 10.0) because the lather factor is not affected appreciably by the difference in pH between 8.6 and 10.0. The results which were plotted as shown in Figure 3 will be discussed in the next section.
Nephelometric Comparison The fatty acid soaps, which are among the most insoluble of
all calcium compounds, were used for the nephelometric determination (7) of small quantities of calcium. It seemed reasonable that some proportionality might be found to exist between the available calcium ion in equilibrium with com-
VOL. 29, NO. 5
positions of calcium and sodium metaphosphate or sodium pyrophosphate and the turbidity produced by the addition of a suitable precipitant to such combinations. The adoption of such a method would involve the assumption that the calcium-ion concentration corresponding to the turbidity produced by a given composition of calcium and sodium metaphosphate or sodium pyrophosphate in the presence of soap would approximate the concentration of calcium ion producing the same turbidity in the absence of sodium metaphosphate or sodium pyrophosphate. This assumption tacitly implies that, aside from their specific action toward calcium ion, metaphosphate and pyrophosphate ion must have a negligible or an equivalent effect on the state of dispersion of the precipitated calcium soap. Then there would be required only the construction of a calibration curve expressing the relation between the concentration of calcium ion and the degree of turbidity produced by the addition of a suitable precipitant. Rona and Kleinmann (7) determined nephelometrically concentrations of calcium between 1.6 and 16 p. p. m., using as precipitant sodium sulforicinoleate approximately 0.9 N as to sodium hydroxide. The high alkalinity of this reagent rendered its use inadmissible, inasmuch as it was desired to determine the calcium-ion concentration a t pH values characteristic of the phosphates, 8.6 for buffered sodium metaphosphate and 10.0 for sodium pyrophosphate. After considerable experimentation, a suitable reagent was found in a solution that was 0.1 M with respect to potassium laurate (prepared from Eastman's lauric acid) and 0.04 M with respect to sodium sesquicarbonate, which was present for its buffer effect against carbon dioxide. This reagent gave turbidities of excellent stability and the desired proportionality between turbidity and concentration between 1and 20 D.P. m. calcium. As the nephelometric standard a dilute suspension of b e n t o n i t e w a s used. A calibration curve was constructed presenting the relation between turbidity and the concentration of calcium ion a t 1, 2, 5, 10, and 20 p. p.m. of calcium (Figure 2). Solutions for examination were prepared as in the tit r a t i o n method, varying the quantities of sodium metaphosphate and pyrophosphate added to a fixed concentration of calcium ion, initially 40 p. p. m. One ml. of the reagent was added to a 25-ml. volumetric flask and the solution under investigation was O' i b cn - P P M 7.0 added up to the mark. After FIGURE 2. NEPHELO- shaking and allowing to stand 5 METRIC CALIBRATIONm i n u t e s , the solution was read against the standard in a 50-mm. CURVE Bausch & Lomb colorimeter that had been adapted for the purpose by substituting tube vials and light shields for the plungers and cups, respectively, and by providing a suitable light source. Readings were made and referred to the calibration curve. For purposes of comparison, the results were plotted on the same diagram as those obtained by the soap titration (Figure 3). The graphical results show that there is a great difference between the effects of sodium metaphosphate and sodium pyrophosphate in reducing the concentration of calcium ion. Sodium metaphosphate has a much more profound effect and reveals a more or less linear relation between concentration and reduction of ionization, reaching a calcium-ion concentration of less than 1 p.p.m. The results for the nephelometric and the soap titration methods are fairly concordant,
A L.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
earth salts, precipitation occurs when these compounds are used in waters containing more than traces of calcium and magnesium. Owing to their adherent character these insoluble compounds are deposited as films on the article being washed and also on the washing equipment. These deposits are not only very objectionable from the esthetic point of view, but they contribute to the difficulty of maintaining proper sanitation. A cleaning process that takes on industrial stature, when the multitude of small units is considered, is the daily mechanical washing of dishes and utensils in hotels, restaurants, and institutions. Millions of pounds of alkaline salts are consumed annually in this process. The mechanism of film formation and its prevention in mechanical dishwashing have been discussed by Schwarta and Gilmore (8). The introduction of glassy sodium metaphosphate as an ingredient of an alkaline detergent for mechanical dishwashing has led to generally improved results and to complete freedom from deposits of both the dishes and the equipment. Sodium metaphosphate, although not a detergent, by forming soluble complex ions with the ions of calcium and magnesium in alkaline solution prevents completely the formation of insoluble alkaline earth salts and soaps that constitute the deposits forming in ordinary process waters. Sodium pyrophosphate has also been recently introduced into dishwashing compositions for the same purpose as sodium metaphosphate-namely, to prevent the precipitation of adherent, insoluble, alkaline earth salts and soaps responsible for the formation of the deposits that result when alkaline salts are used in ordinary waters. I n view of the scarcity of published information available concerning the properties of these two compounds, it was considered desirable to make a comparative study of their behavior with respect to calcium ion, which is, on the average, four times as abundant as magnesium in water supplies. Three different approaches have been employed in this study of the relative effects of sodium metaphosphate and sodium pyrophosphate: (1) The determination of their relative effects on the concentration of calcium ions in the presence of soap by (a) a modified Clark soap titration or ( b ) a nephelometric method. (2) The determination of their relative effectiveness in dissolving freshly precipitated calcium orthophosphate. (3) The determination of their relative effectiveness in preventing the precipitation of typical insoluble calcium compounds. Calcium orthophosphate and calcium carbonate were adopted for study because they are typical of the precipitate formed in mechanical dishwashing, inasmuch as most dishwashing compositions contain either trisodium phosphate or sodium carbonate, or both. These determinations were made at room temperature (28’ C. average), and at 65”, which was chosen as a representative temperature in mechanical dishwashing practice.
Some consideration was given to the possibilities of the calcium electrode as a tool for the study of the relative behavior of sodium metaphosphate and sodium pyrophosphate toward calcium ion. A study of the literature was not encouraging. Fosbinder (3) reported that the potential of the calcium electrode was lowered by the presence of other cations, including sodium. Velisek and Vasicek (9) after intensively investigating the calcium electrode of the third order, described by Luther (6), were unable to find a satisfactory combination. The idea of using a calcium electrode was therefore abandoned in favor of more practical measurements. The methods adopted in this study for determining the relative effects of sodium metaphosphate and sodium pyrophosphate on the concentration of calcium ion necessitate the addition of soap, as an indicator in the case of the Clark soap titration, and as a precipitant in the case of the nephelometric method. The results obtained from the application of these methods are admittedly an approximation
585
of the truth, and they are designated as “apparent” calciumion concentrations. When soap is added to a system representing an equilibrium between calcium ion in solution and calcium in complex form, the existing equilibrium is disturbed because of the precipitation of calcium soap, and a new set of conditions corresponding to a somewhat different value of calcium ion is established. However, the conclusions drawn as to the relative effects of sodium metaphosphate and sodium pyrophosphate on the concentration of calcium ion are valid as long as they are confined to conditions where soap is present. Although the results so obtained may not be rigorously true calcium-ion concentrations, the methods at least permit a comparison of the relative value of the two phosphates under the same conditions.
Application of the Clark Soap Titration If a solution of a definite calcium-ion concentration is titrated with analytical soap solution, a definite quantity of soap solution will be required to obtain a &minute suds and, within limits, this amount of soap solution will be proportional to the quantity of calcium present. If a quantity of sodium metaphosphate or sodium pyrophosphate is added to a solution of known initial calcium-ion concentratiohand the resulting solution is then titrated with analytical soap solution, a lower titration value corresponding to a smaller value for calcium ion than that initially obtained will be found. This fact indicates that both sodium metaphosphate and sodium pyrophosphate affect the concentration of calcium ion. If varying amounts of sodium metaphosphate or sodium pyrophosphate are added to solutions of fixed initial calcium-ion concentration, the resulting solutions might be considered as of unknown calcium-ion concentration and would be titratable with analytical soap solution. Thus, by adding progressively increasing quantities of sodium metsphosphate or sodium pyrophosphate to a known initial concentration of calcium, titrating the resulting solutions, and referring the results to a calibration curve, a measurement might be made of the comparative effects of sodium metaphosphate and sodium pyrophosphate on the concentration of calcium ion. This procedure would involve the assumption that the calcium-ion concentration characteristic of the given calcium metaphosphate or calcium pyrophosphate composition in the presence of soap would approximate that of a solution of known calcium-ion concentration that had the same soap titration. The only source of uncertainty here would be the relative effects of the two phosphates on the surface activity of the soap. Such a procedure necessitates the calibration of the analytical soap solution against known concentrations of a standard calcium solution. The standard soap solution had been prepared according to the standard methods for water analysis of the American Public Health Association. The standard calcium solution was prepared by dissolving 10.00 grams of c. P. calcium carbonate in hydrochloric acid, evaporating to dryness several times to expel free acid, and finally diluting the solution to a volume of one liter. The soap solution was standardized against the standard calcium solution as follows: At hardness corresponding to 0.5, 1.0, 10, and 40 p.p.m. of calcium, the appropriate amount of standard calcium solution was added to a 100-ml. volumetric flask and diluted to the indicated volume. The solution was then transferred to an 8-ounce oil sample bottle and adjusted with dilute alkali to a faint pink coloration with phenolphthalein, corresponding to a pH of 8.8 to 9.0. The titration was conducted according to the standard procedure. The end point adopted was an unbroken foam persisting for 5 minutes and not broken by a subsequent small addition of the soap solution. The results were plotted (Figure 1) and
MAY, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
the deviation amounting to 4 p. p. m. on the average. At a sodium metaphosphate concentration of 540 p. p. m., the calcium-ion concentration is identical by the two methods and equals 6 p. p. m. On plotting the results obtained for sodium metaphosphate by the titration method a t p H 10.0, it was found that these values were nearly coincident with those obtained by the nephelometric method a t p H 8.6 below the intersection of the two curves. The results indicate that approximately 50 per cent more sodium metaphosphate is required to reduce the calcium-ion concentration below 1 p. p. m. a t p H 10.0 than a t pH 8.6. Sodium pyrophosphate is less effective than sodium metaphosphate in suppressing the calcium ion in the presence of soap. The comparative results for the two methods are not in such good agreement as in the case of sodium metaphosphate. The two methods give an identical result, 7.5 p. p. m. calcium, a t a sodium pyrophosphate concentration of 5150 p. p, m. At the minimum calcium value reached (2.5 p. p. m. by the soap titration) the nephelometric method yields a value of 6.5 p. p. m. The deviation is greatest where the concentration of oleate soap is greatest and diminishes as the concentration of oleate diminishes. The concentration of laurate soap is constant throughout and is four times as great as the concentration of oleate where it is a t a maximum and forty times as great where it is a t a minimum. If the relative effects of sodium metaphosphate and sodium pyrophosphate are compared a t pH 10.0 on the basis of the amounts necessary to yield an apparent calcium concentration of 2.5 p. p. m. (the lowest attained by sodium pyrophosphate) the quantities stand in the approximate weight ratio of 1 to 9 for the Clark method. The assumption seems fully warranted that the relative effects of sodium metaphosphate and sodium pyrophosphate on the concentration of calcium ion in the presence of soap would also be correlative with their relative effectiveness in preventing the precipitation of calcium soaps. Although soap is not ordinarily an ingredient of commercial dishwashing compounds, nevertheless calcium soaps constitute a significant proportion of the deposits forming in mechanical dishwashing. The soap originates from the saponifying action of the alkaline detergent on the fatty matter usually present on soiled dishes and utensils, and it is effective until precipitated by the hardness in the water supply*
Relative Behavior toward Calcium Orthophosphate If a dilute solution of sodium metaphosphate is added carefully to a dilute solution of a calcium salt, a precipitate is first formed which becomes soluble in an excess of sodium metaphosphate. Similar behavior is noted when a solution of sodium pyrophosphate is added in the same manner. This deportment suggests formation of a complex with the possibility of subsequent precipitation of double salts. Qualitatively it was noted that freshly precipitated calcium orthophosphate produced by the addition of a dilute calcium solution to a solution of trisodium phosphate was also dissolved by solutions of both sodium metaphosphate and sodium pyrophosphate. Similarly, freshly precipitated calcium carbonate was also dissolved by both solutions. An attempt was made to apply this behavior in the quantitative measurement of the comparative effects of sodium metaphosphate and sodium pyrophosphate. It was observed that the disappearance of the precipitate could be observed more accurately when the solution under examination was placed in the path of a brilliant light source, so that the Tyndall effect was visible. The light source consisted of an inexpensive motion picture projector composed of a 6-volt, 32-candle-power automobile headlight bulb and a condensing system made up of two 2.5-inch (6.4-cm.) convex lenses.
587
When a solution containing a dispersed phase was placed a t the focus of the light, the path of the Tyndall beam through the solution could be clearly discerned. Observation was further facilitated by placing the vessel containing the solution on a sheet of black lacquered metal, so that the beam was intensified against this background. The method, as applied, was in effect a Tyndallmetric titration, depending for the end point on the attenuation of the Tyndall beam projected through the solution. The procedure consisted in adding a calcium solution to a solution of 0.25 per cent trisodium phosphate and immediately titrating the precipitated calcium phosphate by adding from a buret standard sodium meta-
FIGURE3. COMPARATIVE EFFECTSOF SODIUM AND SODIUM PYROPHOSPHATE ON METAPHOSPHATE CALCIUM-ION CONCENTRATION
phosphate or sodium pyrophosphate, stirring vigorously and continuing the addition of reagent until the Tyndall beam attained a constant minimal intensity. The titration was conducted as rapidly on the whole as an acidimetric or alkalimetric titration. The rate of solution of precipitated calcium phosphate by sodium metaphosphate and sodium pyrophosphate was observed to be influenced by the concentration of the reagent and by the age of the precipitate, the rate of solution increasing with increasing concentration of solvent and being more rapid in the case of fresh precipitates than in the case of those which had aged. X-ray examination showed that freshly precipitated calcium phosphate had a diffuse amorphous structure and that a precipitate which had aged had a definite crystalline structure although the precipitate was still in a flocculent condition. Although the age of the precipitate does affect the rate a t which calcium ion is produced in solution, it does not affect the relative capacity of sodium metaphosphate and sodium pyrophosphate for removing calcium ion in complex form. Freshly precipitated calcium phosphate was used in the experiments for purposes of reproducibility and rapidity of solution. The accuracy of the titration method was checked by studying the relative reversibility of the processes of the solution of a fresh precipitate and of the prevention of precipitation. The procedure adopted consisted in first determining the minimal quantities of sodium metaphosphate and sodium pyrophosphate, respectively, necessary to prevent the precipitation of calcium phosphate for a given concentration of calcium ion, and adding the necessary quantities of these reagents to the corresponding amounts of freshly precipitated calcium phosphate. The solutions were then observed in the path of the light source in order to determine the length of time necessary to dissolve the precipitate completely, as determined by the attenuation of the Tyndall effect. At a calcium-ion concentration of 20 p. p. m. it was observed that the same concentration of sodium metaphosphate that would prevent precipitation-namely, 125 p. p. m.-would also completely dissolve the precipitate, although a period of 20 minutes was required. Although the minimal concen-
588
INDUSTRIAL AND ENGINEERING CHEMISTRY
tration of sodium pyrophosphate necessary to prevent precipitation at the calcium-ion concentration of 20 p. p. m,namely, 800 p. p. m.-was insufficient to dissolve the precipitate completely in 60 minutes, the results might have been reversible had observations extended over a longer time period. On the other hand, the results obtained by the
FIGURE 4. RELATIVEMINIMALQUANTITIES OF SODIUM METAPHOSPHATE AND SODIUM PYROPHOSPHATE NECESSARY TO PREVENT PRECIPITATION IN 0.25 PERCENTNasP04.12H20 SOLUTION Tyndallmetric titration a t a calcium-ion concentration of 20 p. p. m. gave values of 300 p. p. m. and 1600 p. p. m. for sodium metaphosphate and sodium pyrophosphate, respectively. Although the titration method as practiced required 100 per cent more reagent than the method of preventing precipitation, the relative effectiveness of sodium metaphosphate and sodium pyrophosphate remained approximately the same, which indicates that the principal source of error in the titration method is the excess of reagent necessary to effect the rapid solution of the precipitated calcium phosphate. Hence, while the absolute values obtained by the titration method may be subject to question, the comparative results should be acceptable as an approximation. On this basis the relative weights of sodium metaphosphate and sodium pyrophosphate necessary to dissolve the same amount of precipitated calcium phosphate are as 1to 5 . In order to secure more accurate absolute results, subsequent comparisons were made by the method of the prevention of precipitation.
Prevention of Precipitation The following technic was adopted for determining the minimal quantities of sodium metaphosphate and sodium pyrophosphate necessary to prevent the precipitation of calcium phosphate. For convenience the standard calcium solution was prepared by weighing 4.40 grams of c. P. calcium acetate monohydrate and diluting to 1 liter, so that each milliliter contained 1 mg. of calcium. The solutions of sodium metaphosphate and sodium pyrophosphate were 1.02 and 4.46 per cent by weight, respectively. The trisodium phosphate solution was the c. P. dodecahydrate and was 2.53 per cent by weight. A fixed concentration of trisodium phosphate was used throughout the tests-0.25 per cent-which corresponds to the concentration commonly used in mechanical dishwashing. The order of procedure was as follows: Five milliliters of the solution of trisodium phosphate were added to a 50-ml. volumetric flask; the sodium metaphosphate or sodium pyrophosphate solution was added, the solution was di-
VOL. 29, NO. 5
luted nearly t o volume, the appropriate amount of calcium was added, and the solution was made up to 50-ml. volume with distilled water. This solution was then filtered through a No. 40 filter into a clean 125-ml. Erlenmeyer flask, which was then stoppered. After standing for 10 minutes, the flask was placed in the path of the light and the Tyndall effect was observed. Because of the fact that the distilled water and the reagent solutions were not Tyndall-free, it was necessary t o adopt an arbitrary solution of minimum turbidity as a standard. For this purpose a solution with a turbidity corresponding to 2 p. p. m. of calcium in 0.28 per cent trisodium phosphate solution was prepared as a reference standard. A solution with a turbidity less than that of the standard was considered clear; one with a turbidity equal to or greater than that of the standard was regarded as indicating precipitation. The preferred procedure was to begin with quantities of sodium metaphosphate and sodium pyrophosphate that were insufficient to produce clear solutions and gradually to increase the concentration, starting from the beginning each time until a clear solution was obtained. The solutions were set aside at room temperature or held at 65" C. in an air thermostat constant to 11.0". They were then observed for turbidity or precipitation at 30-minute intervals over a period of 8 hours. After determining the minimal quantities of sodium metaphosphate and sodium pyrophosphate, respectively, that were necessary to produce clear solutions (Table I), a plot was made of the results (Figure 4). Corresponding to each calcium concentration (within limits) is a point representing the minimal concentration of sodium metaphosphate or sodium pyrophosphate that will prevent precipitation. When these points are connected by drawing a line, the result is a curve expressing the solubility of calcium phosphate in sodium metaphosphate or sodium pyrophosphate. Any initial mixture represented by a point above the curve corresponds to a condition of precipitation; any initial mixture represented by a point below the curve indicates the composition of a clear solution. The curves for 65" C . are coincident with those for room temperature, which averaged 28" during the experiments. TABLEI . MINIMALQUANTITIES OF SODIUM METAPHOSPHATE AND SODIUM PYROPHOSPHATE NECESSARY TO PREVENT PRECIPITATION SOLUTION ,---In
0.25% NaaPOa.12HrONarPzOi Nap08 (anhydrous) P.p.m. P.p.m. 50 330 125 800 2120 250 2660 320 350 3200 500 820 1630
Ca P.p.m.
10 20 40 50 55 80
120 240
-In
Ca P.p.m.
10 20 40 80 100 120 240
0.10% NazCOaNatPlOi NaPOs (anhydrous) P . P . ~ .P . p . m .
10 50 200 600 600 720 1630
25 130 400 1060 1600 2400
..
With sodium pyrophosphate a t 28" C . a limiting calcium concentration was found to exist above which it was impossible to prevent precipitation, regardless of how great an excess of sodium pyrophosphate was present. This upper limit occurs between 55 and 60 p, p. m. of calcium. It was noteworthy that a t this concentration the solution did not become turbid but a flocculent precipitate formed. This precipitate was not calcium orthophosphate and, after standing several days, it assumed a crystalline character. At a calcium concentration of 240 p. p. m. which approximates the highest value for a natural water recorded by Collins et al. (f.?), sodium metaphosphate was still effective in preventing the precipitation of calcium orthophosphate. Solutions of both sodium metaphosphate and sodium pyrophosphate that were initially clear at room temperature remained unchanged for days. At the same calcium concentration the weight ratio of the relative amounts of sodium metaphosphate and sodium pyrophosphate necessary to prevent the precipitation of calcium orthophosphate is approximately 1to 6.
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INDUSTRIAL AiVD ENGINEERING CHEMISTRY
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The results obtained a t 65" C. are of particular interest, since they show characteristic differences in the behavior of sodium metaphosphate and sodium pyrophosphate. I n the case of sodium metaphosphate the amount of reagent capable of producing a clear solution at 28" C . was not capable of preventing precipitation indefinitely a t 65". After standing some time, these solutions became turbid and later flocculated although considerable time elapsed between the onset of turbidity greater than 2 p. p. m. of calcium as phosphate, and. actual flocculation. It was found that larger quantities of sodium metaphosphate than the minimal quantity prolonged the duration of the clear solution'and delayed precipitation. 8C 0 CT
40
f a 02
IO 1
2
3
4
5
6
7
8
TIME -HOURS FIGURE5 . RELATION OF CONCENTRATION OF SODIUM METAPHOSPHATE TO TIME OF PRECIPITATION A graph was made for several concentrations of calcium, showing the relation between the amount of metaphosphate present and the time required for the appearance of turbidity and of flocculation (Figure 5). The development of turbidity and the subsequent precipitation are caused by the rehydration of sodium metaphosphate to sodium pyrophosphate. The behavior noted in the solutions containing sodium pyrophosphate was quite different. Initially clear solutions of sodium pyrophosphate on long standing did not become turbid but slowly deposited a crystalline precipitate. This process of crystallization was extremely slow, and it was difficult to determine when it began because the crystals tended to form a transparent incrustation on the bottom of the flask. Only after about 6 hours of standing a t 65" C. could the presence of these crystals be definitely ascertained. If allowed to stand overnight in the thermostat, a pronounced incrustation on the bottom of the flask could be noted on the following morning. When calcium ion is added to solutions containing trisodium phosphate and sodium pyrophosphate in sufficient quantity to cause precipitation, the solid phase which forms is flocculent but becomes crystalline on standing. The precipitate was found to contain sodium, and its composition varied according to the concentration of sodium pyrophosphate; the calcium content diminished and the sodium content increased with increasing concentration of sodium pyrophosphate. When the composition of the precipitate indicated equimolecular proportions of calcium oxide and sodium oxide, no further change in composition occurred with increasing concentration of sodium pyrophosphate. This salt, NazCaPZO7.4Hz0, which may also be written CazP~O7.Na~P~O7.8H2O1 was described by Baer (1). The solid phase found in these experiments at concentrations of calcium ion up to 60 p. p. m. and a t the corresponding minimal concentrations of sodium pyrophosphate approached the composition 7Ca2Pz0~.4Na4P~0,.zHz0. A third solid phase was found whose analysis approximated that of the hydrate of a salt described by Wallroth (IO) as 5CazPz07.4NarPz07.
I
LIMIT FOR.
')
2
E2000 I
Na4P207
a t 28°C.
/
FIGURE6. RELATIVE MINIMALQUANTITIES OF SODIUM METAPHOSPHATE AND SODIUM PYROPHOSPHATE NECESSARY TO PREVENT PRECIPITATION IN 0.10 PER CENTSODIUM CARBONATE SOLUTION
At 65" C. the same differences in behavior occurred which characterized the experiments with calcium phosphate. Solutions containing sodium metaphosphate slowly became turbid on standing and finally deposited flocculent precipitates. I n solutions containing sodium pyrophosphate a t 65" C., crystallization was particularly pronounced, and the start of crystallization could be detected much more easily than in the preceding study. The crystals were flaky and rather
INDUSTRIAL AND ENGINEERING CHEMISTRY
590
large, and did not tend to form incrustations. At a calcium concentration of 40 p. p. m. crystallization set in within 30 minutes to 3.5 hours, depending on the concentration of sodium pyrophosphate. At 30 p. p. m. of calcium, crystallization also set in within 1 to 2 hours. The composition of the precipitate forming in solutions of sodium carbonate a t the minimal concentration of sodium pyrophosphate was found to be CazPzO7.5H20. The crystals separating from solutions with a concentration of sodium pyrophosphate greater than the minimal quantity proved to be double salts which exhibited the same relation between composition and concentration of sodium pyrophosphate that was noted in the previous series of experiments.
Mechanism of Action The necessary condition for the disappearance of a precipitate in contact with a solution is that the product of the concentrations of its ions must be smaller than the solubility product constant. Thus, the solution is unsaturated in regard to the solid, and it will go into solution. The solution of a precipitate is favored if one or both of its ions are s u p pressed. The experimental results obtained with the soap titration and with the nephelometric procedure indicate that both sodium metaphosphate and sodium pyrophosphate exercise a suppressive effect on calcium ion. The presumption is that a soluble complex is formed between calcium ion and sodium metaphosphate and calcium ion and sodium pyrophosphate. I n view of the results obtained by the different avenues of approach to the problem, there can be no doubt that the calcium metaphosphate complex corresponds to a much lower calcium-ion concentration in solution than the calcium pyrophosphate complex. This is particularly evident in the presence of the soaps that form the least soluble calcium salts of all those included in this study. The mechanisms whereby initially clear solutions of sodium metaphosphate and sodium pyrophosphate in the presence of calcium ion and inorganic precipitants deposit insoluble solids on standing a t an elevated temperature are of considerable interest. The flocculent precipitate that eventually separat,es from solutions containing sodium metaphosphate in the presence of calcium ion and trisodium phosphate or sodium carbonate has been identified tentatively as calcium pyrophosphate. I n aqueous solution sodium metaphosphate slowly rehydrates to pyrophosphate and orthophosphate, a reaction that is accompanied by a fall in pH; during a typical experiment in which trisodium phosphate was present in 0.25 per cent concentration, the pH fell from 11.6 to 10.6. This increase in hydrogen-ion concentration is accounted for on the basis of the rehydration of sodium metaphosphate, the reversal of the process by which it is produced : 2NaPOa
+ H20
+
Na2HzPz07
Rehydration is a relatively slow process. At a calcium-ion concentration of 40 p. p. m. the minimal quantity of sodium metaphosphate necessary to produce a clear solution a t 28' (250 p. p. m.) was sufficient to keep the solution clear for 2.5 hours, Twice this amount (500 p. p. m.) sufficed to keep the solution clear for 8 hours (Figure 5 ) . Obviously it is possible to offset completely the effect of rehydration by providing a sufficient excess of sodium metaphosphate. The solid phases separating from solutions containing calcium ion and sodium pyrophosphate in the presence of the inorganic precipitants here studied have been found to be either calcium pyrophosphate or double salts of sodium and calcium pyrophosphate. This indicates that calcium pyrophosphate is less soluble than calcium carbonate and calcium orthophosphate and that the double salts are still less soluble.
VOL. 29, NO. 5
Conclusions Sodium metaphosphate and s'odium pyrophosphate are compared as to their effects in preventing the precipitation of calcium orthophosphate, calcium carbonate, and calcium soaps under conditions of temperature and concentration approximating commercial mechanical dishwashing practice. I n the presence of soaps the quantities of sodium metaphosphate and pyrophosphate necessary to produce the same reduction in calcium-ion concentration a t pH 10.0 bear the ratio 1 to 9 by the Clark method. It is believed that the relative effectiveness in preventing precipitation of calcium soaps would be expressed by approximately the same ratio. The effectiveness of sodium metaphosphate and pyrophosphate in dissolving precipitated calcium phosphate is jindependent of the age of the precipitate. The amounts of sodium metaphosphate and pyrophosphate which dissolve the same quantity of calcium phosphate are as 1 to 5. In the prevention of the precipitation of calcium orthophosphate, an upper limit to the effectiveness of sodium pyrophosphate was found at a calcium-ion concentration near 60 p. p. m. No such limit was found for sodium metaphosphate, even a t a concentration of 240 p. p. m. calcium, the highest reported for a natural water supply in the United States. The weight ratio of the relative amounts of sodium metaphosphate and sodium pyrophosphate necessary to prevent the precipitation of calcium orthophosphate is 1 to 6. In the prevention of the precipitation of calcium carbonate, an upper limit to the effectiveness of sodium pyrophosphate was found a t a calcium concentration near 120 p. p. m. a t 28" C. and near 40 p. p. m. a t 65'. No such limit was found for sodium metaphosphate a t a calcium concentration of 240 p. p. m. The weight ratio of the relative quantities of sodium metaphosphate and sodium pyrophosphate necessary to prevent the precipitation of calcium carbonate is 1 to 2.5. The effectiveness of sodium pyrophosphate in preventing the precipitation of insoluble calcium salts is limited by the insolubility of the double salts which it tends to form. Three double salts were identified during this investigation; one is identical with the double pyrophosphate Na4Pz0,-Ca2P207~8Hz0, first described by Baer ; the second approximates the composition 7Ca2P207.4Na4P207*zH20;the third is believed to be a mixture of the other two. The effectiveness of sodium metaphosphate in preventing the precipitation of insoluble calcium salts is limited only by the slow breakdown of the complex caused by a partial rehydration of the metaphosphate. An appropriate excess of sodium metaphosphate will prevent precipitation. All the experimental evidence indicates that calcium ion is more tenaciously held in complex form by sodium metaphosphate than by sodium pyrophosphate. The greater effectiveness of sodium metaphosphate in the prevention of calcium deposits is accounted for on this basis.
Literature Cited (1) Baer, Pogg. Ann., 75, 161 (1847). (2) Collins, Lamar, and Lohr, U. 5. Geol. Survey, Water Supply Paper 658 (1934). (3) Fosbinder, J . Am. Chem. SOC.,51, 1345 (1929). (4) Hall, U.S. Patent 1,956,515(April 24,1934). (5) Kean and Gustafson, IND.ENQ.C H ~ MAnal. . , Ed., 3, 355 (1931). (6) Luther, 2. physik. Chem., 27, 364 (1897). (7) Rona and Kleinmann, Biochem. Z . , 137, 157 (1923). (8) Schwartz and Gilmore, IND.ENQ.CHEM.,26, 998 (1934). (9) Velisek and Vasicek, Collection Czeclcoslou. C h m . Commun., 5, 10 (1933). (IO) Wallroth, Bull. SOC. chim., [ 2 ] 39,318 (1883). RECEIVH~D November 18, 1936. Presented before the Division of Water, Sewage, and Sanitation Chemistry at the 9Znd Meeting of the American Chemical Society, Pittsburgh, Pa.,September 7 to 11, 1936. Contribution from the Multiple Fellowship on Calgonizing at Mellon Institute.
