the inhibiting actiok of minute amounts of sodium hexametaphosphate

T H E INHIBITING ACTIOK O F MINUTE AMOUNTS OF SODIUM HEXAMETAPHOSPHATE OK T H E PRECIPITATION O F CALCIUM CARBONATE FROM AMMONIACAL SOLUTIONS. I1f* QUANTITATIVE STUDIES OF

THE

INHIBITION PROCESS

R. F. REITEMEIER' A N D T. F. BUEHRER

Department of Agricultural Chemistry and Soils, University of Arizona, Tucson, Arizona Received August 84, 1939

The fact that polymerized metaphosphates can inhibit as well as entirely prevent the precipitation of calcium carbonate has been applied on an industrial scale in the conditioning of boiler water, in flotation, in the removal of calcareous incrustations, in the stabilization of sols, and in other processes. These applications have, in the main, been developed by Hall (4). In those processes in which the water is completely softened, Hall4 states that the metaphosphate must be added in the proportion of ". . . 4 formula-weights of sodium metaphosphate to 1 formula-weight of calcium a t a pH of 8.5, but 7 formula-weights of sodium metaphosphate were required for this amount of calcium when the pH value was 10." The calcium is assumed to be present in the form of a stable complex from which it cannot be precipitated either as the carbonate or as an insoluble soap. The exact composition of this complex has not yet been definitely established. A more recent application of sodium metaphosphate has been made by Rosenstein (11) for the prevention of calcium carbonate deposition from irrigation water when anhydrous ammonia is introduced as a source of nitrogen for growing crops. It was discovered that the precipitation of calcium carbonate can be prevented by an extraordinarily minute conThis paper is part of a dissertation submitted by R . F. Reitemeier t o the Graduate College of the University of Arizona in partial fulfillment of the requirements for the degree of Doctor of Philosophy, which was conferred June 1, 1938. A more detailed account of this investigation, including the results of later work, is soon t o appear in a Technical Bulletin of the Arizona Agricultural Experiment Station. * Presented before the Division of Physical and Inorganic Chemistry a t the Ninety-sixth Meeting of the American Chemical Society, held in Milwaukee, Wisconsin, September, 1938. a Holder of the Shell Chemical Company Fellowship, 1936-38. Present address: U. S. Regional Salinity Laboratory, Riverside, California. ' Hall: U. S. patent 1,956,515(April 24, 1934), page 5. 536

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R. F. REITEMEIER AND T. F. BUEHRER

centration of metaphosphate, of the order of one part per million by weight. Rosenstein (11) states, for example, that when ammonia in an amount sufficient to produce a concentration of 75 p.p.m. NHa was added to a water containing 120 p.p.m. of calcium, the precipitation of calcium carbonate could be prevented entirely by the addition of sodium metaphosphate in an amount sufficient to bring its concentration to 0.75 p.p.m. In this case the water is not softened, as it is in the instances cited above, since the metaphosphate concentration represents only an extremely small fraction of the amount necessary to convert the calcium into the complex ion.6 The most striking characteristic of this phenomenon is the minuteness of the concentration of metaphosphate which is capable of preventing precipitation. No attempt has apparently been made to explain the mechanism of this process. A quantitative study of the factors affecting it wm therefore made, the results of which will be presented in this paper. THEORETICAL CONSIDERATIONS

The precipitation of calcium carbonate can be represented by the equation: Ca*

+ HCOB + OH- = CaCOs + HtO

This reaction is actually a summation of three simpler reactions, and its equilibrium constant is of the order of 1012. Hence the precipitation would be expected to approach completion even in the presence of a minute concentration of metaphosphate, since under these conditions the conversion of the calcium into the stable ionic complex, as is the case in water softening, must be negligibly small. The effect of the metaphosphate therefore appears to be largely an indirect one. Theoretically, however, it should be possible to introduce into the above equation the metaphosphate ion or molecule involved in the mechanism so it may adequately represent the inhibition process. This intriguing phenomenon at once suggests a number of problems for investigation: Does the metaphosphate affect the eztent to which the reaction proceeds or primarily its rate? What is the magnitude of the effect at different concentrations of the reactants? Do other phosphates, genetically related to the metaphosphates, as well as other similar inorganic salts, exhibit a similar property? Does the presence of neutral salts influence the effectiveness of metaphosphate M an inhibiting agent? Is

'

Since this manuscript waa first written, the use of hexametaphosphate in these low concentrations for the prevention of calcium carbonate deposits from industrial waters, aa well aa their disintegration after having been deposited, haa been described (see Hatch and Rice: Ind. Eng. Chem. 81, 61-7 (1939); Rice and Partridge: Ind. Eng. Chem. 81, 6883 (1939)).

PRECIPITATION OF CALCIUM CARBONATE.

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the high specificity of the metaphosphate related to the chemical nature of the reactants or to that of the reaction products? The quantitative evidence, both analytical and physicochemical, which was obtained in carefully controlled experiments in which these factors were systematically varied, will now be presented. The results of these studies are of interest not only from an academic but also from an industrial point of view. PREPARATION OF MATERIALS

The calcium bicarbonate stock solution was prepared by the action of carbon dioxide on a suspension of pure calcium carbonate, the latter having been precipitated from solutions containing recrystallized calcium chloride and ammonium carbonate, respectively. This solution was diluted with distilled water, before addition of the desired reagents, to give the calcium bicarbonate concentration desired. Standard ammonia solutions, of approximately 1 N concentration, were prepared by dilution of reagent grade ammonium hydroxide, and standardized against 0.1 N hydrochloric acid. Sodium metaphosphate may be obtained in various crystalline forms or as an amorphous glass, according to the conditions of preparation. All of these forms exhibit more or less polymerization when dissolved in water, the extent depending upon the method of preparation. On this account it seemed desirable to use both the amorphous and one of the definite crystalline forms for purposes of comparison. The so-called hexametaphosphate, known as “Graham’s salt” (3), was prepared as follows: Twice-recrystallized monosodium dihydrogen orthophosphate was fused in a platinum dish a t about MOOC., and held a t that temperature for 15 min. It was then quickly chilled, ground, and used without further purification. The crystalline form was prepared according to the method of Beans and Kiehl (1) and twice recrystallized from aqueous solution by the addition of ethanol, according to the method used by Hill (7). In this paper we shall use the term “hexametaphosphate” in referring to the amorphous form of sodium metaphosphate, realizing that the extent of its polymerization is variable and that the term probably does not represent the exact nature of this salt. Because of a similar uncertainty in regard to the crystalline modification, we shall designate this form as “soluble crystalline sodium metaphosphate.” Tetrasodium pyrophosphate wm prepared by the ignition of recrystallized disodium monohydrogen orthophosphate, in a platinum dish, after which it was recrystallized from the aqueous solution by means of ethanol. The other inorganic salts which were tested for their effectiveness as in-

538


hibiting agents, such as certain borates, nitrates, and vanadates, were of reagent grade and not further purified. EXPERIMENTAL RESULTS

A . Efect of varying the ammolaia concentration Since ammonia provides the alkalinity for the precipitation of calcium carbonate, it seemed of interest to carry out a series of experiments in which its concentration was varied, in order to determine ( 1 ) its effect on the actual amount of calcium carbonate precipitated, and (2) its effect on the concentration of metaphosphate required for complete prevention of precipitation. In the first series, the re,action mixtures were prepared by adding different volumes of the stock ammonia solution to a constant volume of stock calcium bicarbonate solution and diluting the mixtures in all cases to a final volume of 100 cc. One such series was allowed to stand 1 hr.; the other, 6 days. After standing the desired length of time, the carbonate precipitates formed were collected on filter paper and analyzed for total calcium. The results are shown in figure 1(A). The two curves show that the amount of calcium carbonate formed varies as the ammonia concentration, but not linearly. The time factor is evidently of great importance. After 6 days, the amount of calcium carbonate precipitated was about double the amount which formed during the first hour. The 6-day curve approaches a limiting value, which approximates the solubility of calcium carbonate under these conditions. The amount of calcium carbonate precipitated after standing 1 hr. was found to be a linear function of the pH of the solution. As the ammonia concentration WBS increased from 200 to 2000 p.p.m., the pH increased from 9.8 to 10.8. The fact that time is so important in this process and that precipitation is relatively slow suggests that supersaturation may occur in the precipitation of calcium carbonate even from such dilute solutions. In the second experiment sodium hexametaphosphate was added to the reaction mixtures before the addition of ammonia. Four series of mixtures were prepared, each with the same amount of calcium bicarbonate but with a concentration of ammonia which, though increasing from series to series, was held constant within each series. After 1 hr. the precipitates were analyzed for calcium with results shown in figure 1(B). Several significant conclusions can be drawn from these graphs. First, the amount of calcium carbonate precipitated in the absence of the metaphosphate increases only about 95 per cent as the ammonia concentration is increased 1100 per cent. This result agrees with those presented in figure 1(A) and is consistent with the low degree of dissociation of ammonium hydroxide. Secondly, an increase in metaphosphate concentra-


I

539

tion results in a gradual diminution of the amount precipitated, up to the concentration beyond which no calcium carbonate formation can be detected. This limiting concentration we have chosen to call the “threshold inhibition value.” Near this point the amount of carbonate precipitate formed is very small and therefore the analytical determination of calcium is relatively inaccurate. The actual threshold value is obtained by prolonging the curve until it intersects the z-axis. Thirdly, while the threshold inhibition value increases with the ammonia concentre tion, the increase is very slight. Thus, an increase of 1100 per cent in the ammonia concentration caused an increase in the threshold concentration of only 53 per cent. In the presence of the metaphosphate the relative efficiency of ammonia in effecting precipitation by virtue of its alkalinity decreases with increasing concentration of metaphosphate.

B. Efect of varying the calcium Concentration In this case the procedure was the same as in the foregoing experiment, except that the ammonia concentration was held constant and the calcium bicarbonate and metaphosphate concentrations were varied. The results are shown in figure 1(C). The curves show that calcium ion, accompanied by an equivalent amount of bicarbonate ion, exerts a pronounced effect on the reaction. In the absence of metaphosphate, a fourfold increase in calcium concentration brings about a tenfold increase in the amount of calcium carbonate precipitated. The effect of varying the calcium coiicentration on the threshold inhibition value is even more pronounced. A fourfold increase in the former, from 25 to 100 p.p.m., increases the metaphosphate requirement from 0.1 to 12.0 p.p.m., a one hundred and twentyfold increase. The present investigation indicates that the calcium bicarbonate concentration may be the most important single factor which affects the inhibition of calcium carbonate precipitation. C. Efect of ammonium ion

Since ammonium ion reduces the dissociation of ammonium hydroxide, an increase in the initial concentration of ammonium ion should cause a marked decrease in the threshold metaphosphate requirement, if the reaction as shown by the equation proceeds as written in the presence of metaphosphate. Using the same procedure as before, but keeping the calcium bicarbonate and ammonia concentrations as well as the ammonium-ion concentration in each of four series of determinations constant, the amount of calcium carbonate precipitated in the presence of increasing concentrations of metaphosphate was determined. The results are shown in figure 1(D). With no ammonium chloride present, the threshold metaphosphate

540

R. F. REITEMEIER AND T. F. BUEFfRER

FIQ.1. Effect of varying the concnntratiom of the individual reacting componenta on the precipitation of calcium carbonate.

541

542


requirement was 1.17 p.p.m., while in the presence of 2140 p.p.m. ammonium chloride (0.04 N ) , it was reduced to 0.05 p.p.m., that is, about twenty-threefold. The fact that calcium carbonate is appreciably soluble in ammonium chloride solution is well known; however, in this case free ammonia present to the extent of 550 p.p.m. would reduce such solvent action considerably. The increase in effectiveness of metaphosphate may be attributed in part to a reversal of the precipitation equilibrium as * a result of a reduction in the dissociation of the ammonia.

D. Effect of neutral salts When hexametaphosphate is used to inhibit precipitation of calcium carbonate from natural waters, it has been found that a lower concentration of metaphosphate is required than in the case of pure calcium bicarbonate solution, which suggests that the neutral salts usually present in the water may affect the process. To investigate this effect, sodium chloride and sodium sulfate were chosen as typical of the neutral salts present in such waters. Four series of determinations were made. The calcium bicarbonate and ammonia concentrations were the same as used in previous experiments, but sodium chloride and sodium sulfate were used in progressively increasing amounts so that the concentrations of C1- and SO4-- ranged from 0 to 0.04 N . After 1 hr. the calcium in the precipitates was determined. The results are shown in figure 1(E, F). Increasing the salt concentration causes a comparatively slight decrease in the amount of calcium carbonate precipitated, but a much greater decrease in the concentration of metaphosphate required for complete prevention of precipitation. For example, addition of sodium sulfate to produce a concentration of 0.04 N reduces the threshold requirement of hexametaphosphate from 1.25 p.p.m. to about 0.3 p.p.m., whereas sodium chloride a t a concentration of 0.04 N reduces it to about 0.65 p.p.m. The nature of this neutral-salt effect is not entirely obvious. These salts may conceivably affect the activity of the ammonia so as to lower the pH perceptibly and thus reduce the precipitation. To secure evidence on this point, pH measurements were made on ammonia solutions which contained 560 p.p.m. ammonia and to which the respective neutral salts had been added to give chloride and sulfate concentrations of the same range as used in the foregoing experiments. The data obtained with the glass electrode are assembled in table 1. The above data show that instead of the pH being lowered by the presence of neutral salts, a definite increase occurs, the effect of sulfate being more pronounced than that of chloride ion. Since the differences in pH are scarcely significant, however, so far as the precipitation of calcium carbonate is concerned, the neutral salts must in some manner influence the activity or behavior of the calcium or bicarbonate ions. The effect


543

I

may consist in a reduced activity of one or both of them, or, on the other hand, an increase in the solubility of calcium carbonate under these conditions. That sodium chloride and sodium sulfate increase the solubility of calcium carbonate is well known. Kohlschutter and Egg (8) found that the presence of potassium chloride and sodium chloride caused fewer crystals to form when a current of air was passed through a calcium bicarbonate solution. Stumper (12) found that sodium chloride and sodium sulfate inhibit the thermal decomposition of calcium bicarbonate, the effect of the sulfate being about three times that of the chloride. While these evidences point to a possible reduction in activity of calcium and/or bicarbonate ions, these neutral salts may affect the dissociation or depolymerization of the hexametaphosphate, favoring the formation of an ionic or molecular species which is highly specific in its inhibiting action. TABLE 1 Effect of neutral salts on the alkalinity of ammonia solutions Ammonia concentration = 560 p.p.m. NHs CONCENTRATION OF

N

N&SO.

I

CONCENTRATION OF

0.0209

N 0 0 0

0.0417 0 0 0

0.0141 0.0282 0.0564

0 0.0104

0

NaCl

I

PH

10.16 10.55 10.79 10.86 10.55 10.61 10.62

E. Variation of the threshold value with the time The importance of the time factor in the precipitation of calcium carbonate was illustrated in figure 1(A). Since the precipitation experiments presented in figure 1(B to F) involved a reaction period of 1 hr., several experiments were designed to determine the specific effect of time on the threshold inhibition value. For this purpose the conductance method offered the most sensitive means of following the precipitation process over an extended period of time, since precipitation of calcium carbonate would result in a diminution of the conductance due to the removal of ions. A glass-stoppered conductivity cell of 63-ml. capacity was employed. Appropriate volumes of calcium bicarbonate, sodium hexametaphosphate, and ammonia solutions, all at 25"C., were pipetted into the cell in the order just mentioned and diluted in each case to 63 ml. with conductivity water. The results of a series of studies with different concentrations of the metaphosphate are presented in figure 2. These curves show clearly that precipitation proceeds through three

544

R. F. REITEYESER AND E ' . F. BUl4XIUiiB

stages, which are modified somewhat according to the metaphosphate concentration. First, there is a sharp increase in the conductance; secondly, the conductance reaches a maximum value a t which it may remain for a considerable length of time; and thirdly, there is a decrease in the conductance, the rate of which depends on the metaphosphate concentration. The initial rise in the conductance curves was found, from studies on two-component systems, to be due to the reaction between ammonium hydroxide and carbonic acid. I n relation to the precipitation reaction

Fro. 2. Variation of the threshold value with the time

it evidently represents the time required for a sufficient concentration of carbonate ions to be formed to saturate the solution with calcium carbonate and thus allow precipitation to proceed. The comparative slowness of this process is apparently due to the slow rate of hydration and neutralization of carbon dioxide (9). When sodium hydroxide is used instead of ammonia in the precipitation reaction, no such initial rise in the conductance curve occurs. This result is in accord with the interpretation given by Hammett ( 5 ) . When ammonia is used, the reaction with carbon dioxide produces primarily bicarbonate ion, whereas with sodium hy-


I

545

droxide, carbonate ion is formed. The rate of the latter reaction being proportional to the square of the hydroxyl-ion concentration, it is evident that the formation of carbonate ion is much more rapid in the presence of sodium hydroxide than of ammonia. Another significant fact is shown by the curves of figure 2, namely, that an increase in metaphosphate concentration prolongs the time during which the conductance remains a t its maximum value. This effect is shown in table 2. As the concentration of metaphosphate increases, a longer period of time is required for the system to reach the point where calcium carbonate begins to precipitate from the solution. After precipitation has begun, its rate is fairly constant, depending upon the metaphosphate concentration. TABLE 2 Effect of sodium hexametaphosphate on the period during which the conductance was at a maximum CONCENTRATION OF

p.p.m.

0 0.3 0.9 1.2 2.0 4.0

(NaPO,),

I

EBTIYATED PERIOD OF Y A X I Y V Y CONDUCTANCE

minulca

2 6 25 40 55 >150

These results indicate three significant aspects of the inhibition process: first, the presence of the metaphosphate lengthens the time required for precipitation to commence; secondly, after precipitation has begun, the metaphosphate diminishes the rate of the reaction ; and thirdly, if sufficient metaphosphate is present, precipitation is entirely prevented. Reference has already been made to the fact that the inhibition mechanism might involve an effect of metaphosphate on the conversion of bicarbonate to carbonate ion. Evidently, if the metaphosphate exhibits the same order of effectiveness in systems containing carbonate ions initially, this step in the mechanism might be definitely eliminated. A conductance study similar to the above was accordingly made, in which calcium chloride and sodium carbonate were substituted for calcium bicarbonate and ammonia. The results are shown graphically in figure 3. Except for the absence of the initial rise in conductance, the same general effects were observed. Sodium hexametaphosphate appears to be as effective in systems containing a high initial concentration of carbonate ions rn in bicarbonate solutions having a high potential supply of car-

546


bonate ions. It is therefore concluded that the inhibiting action of metaphosphate does not consist of a retardation of the conversion of bicarbonate to carbonate ions.

>

f

5.0

-

8

./*

0

I

I

I

10

40

I

eo

nuE

I

ea

YII

FIG. 3. Effect of sodium hexametaphosphate on the precipitation of calcium carbonate from calcium chloride-sodium carbonate mixtures.

I

H G. 5

,

STANDING 31

Mn

C Q K C ~ T I O Nor [ W q ,

FIG.4. Effect of time on the precipitation of calcium carbonate in the presence of sodium hexametaphosphate.

The effect of time on the inhibition process was likewise studied by way of precipitation experiments. Three series of mixtures containing constant amounts of the reactants were prepared and allowed to stand for different periods of time: namely, 1 hr., 1 day, and 32 days, all a t room


I

547

temperature. The precipitates were analyzed for calcium as before, with the results shown in figure 4. The maximum amount of calcium that could be precipitated in the absence of metaphosphate was 5 mg. It will be noted that about 4.8 mg. of calcium, representing 96 per cent of the total calcium present, had been precipitated both a t the end of 1 day and after 32 days; the amount thus remaining in solution is equivalent to the solubility of calcium carbonate under these conditions. As the time of standing increases, the efficiency of metaphosphate decreases; thus, in l hr. l l per cent of the total calcium precipitated in the presence of 0.6 p.p.m. of metaphosphate, while a t the end of 1 day and 32 days, respectively, this concentration had permitted 2.7 mg. (54 per cent) and 4.6 mg. (92 per cent) of the calcium to precipitate. The threshold requirement does not, however, show so marked an increase; for the three periods of time, the threshold values were 1.1, 1.35, and 1.5 p.p.m. of metaphosphate, respectively. It will be noted that as the time allowed for precipitation was increased, the shape of the area under the curve approached that of a rectangle. The respective areas represent a summation of the amounts of precipitate formed in the mixtures to which had been added varying amounts of metaphosphate from zero to the threshold concentration, and during the period of time chosen for the respective experiments. The magnitudes of the areas give a quantitative indication of the extent to which the time element enters into the process and the extent to which the inhibition is permanent, When measured with a planimeter, the three areas, when referred to the area under the I-hr, curve as unity, are to each other as 1:3.47:7.25. An increase of only 36 per cent in the threshold value thus increased the total precipitation, as determined from the areas, by 625 per cent. Only 3.8 per cent of the total precipitation occurring during the 31-day interval occurred a t metaphosphate concentrations higher than the threshold inhibition value required for complete prevention of precipitation on 1 day’s standing. These facts indicate that the inhibition is a permanent effect under the conditions in these experiments. For the system under consideration, it appears safe to assume that there is a permanent threshold value of approximately 1.5 p.p.m. of metaphosphate, beyond which no calcium carbonate will precipitate regardless of the length of time allowed for precipitation to take place.

F . Comparison o j various phosphates as inhibiting agents The close genetic relationship between the ortho-, meta-, and pyrophosphates suggested that the structure, polymerization, and chemical nature of the phosphate molecule might determine in large measure its

548


ability to inhibit calcium carbonate precipitation. It was therefore thought that a knowledge of their relative efficiencies &s inhibiting agents under similar experimental conditions might afford evidence rn to the mechanism of the inhibition process. By means of the technique previously described, the relative effectiveness of hexametaphosphate, tetrasodium pyrophosphate, and primary sodium orthophosphate was determined. The precipitates were analyzed for calcium after standing for 2 days. The data are plotted in figure 5, the “concentrations of phosphate” along the axis of abscissae having been calculated to indicate the actual concentration of the respective phosphates on a weight basis.

FIQ.5. Inhibiting action of various phosphates

The meta- and pyro-phosphate curves exhibit a striking similarity in shape and in magnitude of the threshold inhibition value a t the calcium concentration here used, namely, 60 p.p.m. These results are in agreement with those of Hartung (6), who worked with calcium solutions at ordinary concentrations. He concluded that the meta- and pyro-phosphates were about equally effective in the stabilization of a water softened by the limesoda process. Quoting Hartung’s statement: “. . . waters with high calcium concentrations are not so easily (if at all)6 stabilized by tetra-sodium pyrophosphate.” The orthophosphate curve is a straight line of gentle slope with no evidence of any definite threshold value. In the latter case it was observed that a precipitate of calcium phosphate was formed at the higher phosphate concentrations, which appears to ofiset the inhibiting effect of 6

The parentheses are Hartung’s.


I

549

the orthophosphate. Nevertheless, the orthophosphate is capable of reducing the precipitation of calcium carbonate to a limited extent and within a limited range of conditions. Similar experiments with the crystalline sodium metaphosphate indicated that it was practically without effect. The close similarity in behavior of the pyrophosphate and hexametaphosphate suggests that the inhibition mechanism may be similar, if not identical, in the two cases. From a study of these two phosphates in connection with the softening of water, Gilmore (2) concluded that both are effective, but that the latter is superior to the former. Since the metaphosphate is genetically related to the pyrophosphate,-by way of NaSZP20, which forms as an intermediate product in the original decomposition of the primary phosphate,-and since it is also formed in the re-hydration of the metaphosphate, it is reasonable to assume that some intermediate form of pyrophosphate, or possibly a more complex phosphate, is responsible for the inhibition in the two cases. It has been found by Partridge (10) that the conditions of preparation of the glassy hexametaphosphate, such as the time and temperature at which the melt is held in the fused state, may modify the effectiveness of the product as an inhibiting agent. A metaphosphate glass, which he prepared “by dehydration and fusion of monosodium orthophosphate at 650°C., for 15 hours, followed by quenching between steel plates,” was found to possess an inhibiting power simiiar to that which we have reported for hexametaphosphate in this paper. However, a more active product was obtained by holding the melt a t 1000°C. for 2 hr. Hence he concludes that the temperature of preparation is of primary importance in the manufacture of an actively inhibiting product.

G . Behavior of other inorganic salts as inhibiting agents The action of the various phosphates illustrated in the foregoing experiments led to the question whether analogous salts of other elements possess a similar property. Inhibition experiments were accordingly carried out with the following salts: sodium tetraborate, ammonium metavanadate, sodium metavanadate, fused sodium metavanadate, sodium orthovanadate, fused sodium orthovanadate, sodium metaborate, dipotassium dihydrogen antimonate, sodium bismuthate, sodium arsenite, disodium orthoanenate, potassium nitrate, and fused potassium nitrate. (It is recognized that fusion of the vanadates and nitrates results in partial

’

A somewhat more reasonable possibility to account for this behavior is suggested by Terrey (13), quoting several authors who attribute it to tri- and tetrapolyphosphates which may be formed in the original fusion process, depending upon the conditions. The action of such phosphates was not, however, considered in the present investigation.

550


decomposition.) However, of the fourteen salts studied, none was found to inhibit measurably the precipitation of calcium carbonate. In nearly all cases the concentration of the particular salt was carried to a value as high as 20 p.p.m. without noticeable effect. Although most of these salts can form an analogous series of ortho, meta, and pyro salts, they do not possess the ability to inhibit calcium carbonate precipitation. Even the transformation to an amorphous or glassy form does not impart to them this peculiar property. It therefore appears to be a highly specific characteristic of the inorganic salts containing phosphorus in the pentavalent form. DISCUSSION

The data presented in this paper demonstrate conclusively that the molecularly hydrated phosphates,-sodium hexametaphosphate and sodium pyrophosphate,-in their ionic or polymerized molecular forms are able, in minute concentrations, not only to retard but also to prevent entirely the precipitation of calcium carbonate from ammoniacal solutions. The possibility of permanent prevention of such precipitation, demonstrated by way of long-time inhibition experiments, indicates the existence of an extraordinarily stable system. The evidence obtained by varying the concentration of individual reactants indicates that calcium bicarbonate is the most important constituent involved in the mechanism, and hence implies a close relationship between the phosphate and either calcium or bicarbonate ion. Since the use of metaphosphates in water softening involves the removal of calcium, it is very unlikely that bicarbonate ion plays an important part in the mechanism. In the light of these findings, the authors are inclined to postulate a mechanism involving first, the primary precipitation reaction and secondly, an inhibition reaction which involves the meta- or pyro-phosphate. Varying the concentrations of ammonia, ammonium ion, and of neutral salts like sodium chloride and sodium sulfate affects only the primary precipitation reaction in the conventional manner. Variations in calcium bicarbonate concentration, however, affect the inhibition process and thus have a more pronounced effect on the quantitative results. The mechanism is, however, complicated by certain physical factors which are concerned in the formation of the calcium carbonate crystal. These have been subjected to an intensive study, the results of which will be presented in a subsequent paper. SUMMARY

1, Glassy sodium hexametaphosphate, a t extremely low concentrations of the order of 1 p.p.m. molal), is found to prevent the precipitation of calcium carbonate from solutions containing 200 p.p.m. of calcium


I

,551

bicarbonate molal) and 550 p.p.m. of ammonia (0.03 molal). Crystalline sodium metaphosphate is relatively ineffective under similar conditions. At low calcium concentrations pyrophosphates are as effective as metaphosphates; orthophosphates exhibit a slight ability to inhibit precipitation, but it is limited by the precipitation of an orthophosphate of calcium. No other analogous inorganic salts, similar to the phosphates, possess this property. It is highly specific of the molecularly dehydrated phosphates only. 2. The threshold concentration of hexametaphosphate required to prevent precipitation increases with the ammonia concentration as well as with that of calcium bicarbonate, but to a much greater extent with the latter. Ammonium salts and neutral sodium salts increase the effectiveness of the metaphosphate; this indicates an indirect effect on the activities of calcium ion, bicarbonate ion, and ammonium hydroxide. 3. At such low concentrations the metaphosphate retards precipitation and, if present in sufficient amount, will prevent it permanently. The evidence points to the existence of a highly stable inhibition system, possibly involving calcium ions. We desire herewith to express our thanks to the Shell Chemical Company of San Francisco, California, for their support of this research by way of a fellowship; to Dr. Ludwig Rosenstein, who suggested the problem and manifested much interest in its progress; and to Mr. W. T. McGeorge, Chemist of the Arizona Agricultural Experiment Station, for his interest and support. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)

BEANS,H. T., AND KIEHL,S. J.: J. Am. Chem. SOC.49, 197&91 (1927). GILMORE, B. H.: Ind. Eng. Chem. 29, 584-90 (1937). GRAHAM, T . : Phil. Trans. 143, 253 (1833). HALL,R. E.: U. S. patents 1,956,515 (1934); 1,965,339 (1934); 2,035,652 (1936); 2,087,089 (1937); reissue 19,719 (1935). HAMMETT, L. P . : Solutions of Electrolytes, 2nd edition, pp. 91-2. McGrawHill Book Company, New York (1936). HARTUNG, H. 0.: Water Works & Sewerage 88, 56-60 (1939). HILL,T. M. : Dissertation, Columbia University, New York, 1925. KOHLSCHUTTER, V., and EQQ,C.: Helv. Chim. Acta 8, 470-90 (1925). MCBAIN,J. W.: Trans. Chem. SOC. (London) 1912, 814. PARTRIDGE, E. P. : Personal communication. ROSENSTEIN, L.: U. S. patents 2,038,316 (1936); reissue 20,360 (1937); reissue 20,751 (1938). STUMPER, R.: Z. anorg. allgem. Chem. 204, 367-77 (1932). TERREY, H . : Annual Reports on Progress in Chemistry 54, 121 (1938).

the inhibiting actiok of minute amounts of sodium hexametaphosphate

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