HYDROLYTIC DEGRADATION OF SODIUM TRIPOLYPHOSPHATE IN CONCENTRATED SOLUTIONS AND IN PRESENCE OF FOREIGN IONS C. Y . SHEN AND D. R. D Y R O F F Inorganic Chemicals Diuision, Monsanto Co., St. Louis, M o .
The degradation rate of tripolyphosphate in concentrated solutions follows first-order kinetics and increases exponentially with concentration of solids or sodium ion. The reaction product, which contains a higher amount of pyrophosphate at higher concentrations, may be explained by the slow formation of an activated tripolyphosphate complex which undergoes various faster reactions. Traces of multivalent cations have only a small effect on the degradation rate in concentrated solutions.
ODICM
tripolyphosphate, NajP,Olo, is used for defloccula-
S tion, buffering, and sequestration of a variety of metal
ions. These properties all contribute to detergent building (70), which is its largest use. T h e tripolyphosphate anion tends to degrade in aqueous media to ortho- and pyrophosphates. T h e hydrolytic degradation rates in dilute solutions ( 7 , 72) and in certain detergent slurries (8) have been extensively studied. T h e kinetics appeared to be first order for the dilute solutions, but no mechanism was suggested in the detergent work. T h e available results were difficult to apply by extrapolation to various commercial formulations because in slurries or concentrated solutions the reaction is affected by variables which had not been adequately studied-for example, the catalytic effects of metal ions (72, 73),though well known, had not been quantitatively established. Also, in concentrated solutions, a mechanism different from that in dilute solution was indicated by the presence of larger amounts of pyrophosphate in the products (4, 7 7 ) The purposes of the present work were to determine the degradation rate of tripolyphosphate in concentrated solutions under conditions simulating those in detergent slurries, the effect of metal ions commonly present in water, and the effect of chelating agents. T h e latter had been claimed to reduce the degradation rate (2). Experimental
Chemicals. All chemicals were reagent grade except sodium tripolyphosphate, which was prepared in this laboratory by calcining recrystallized sodium tripolyphosphate hexah>drate at 400' C. for several hours. The high purity of the product, N a j P ~ O l o - I Iwas , established by ion exchange chromatography (6) and x-ray diffraction (7). T h e tripolyphosphate was poi\ dered and kept in sealed containers until used. Procedure and Conditions. All ingredients but the tripolyphosphate were mixed in stoppered 125-m1. Erlenmeyer flasks and preheated to 70' + 1' C. in a water bath. This temperature is close to that generally used for preparing detergent slurries for spray drying. The flasks were shaken continuously to ensure uniformity in temperature and to prevent formation of lumps as the NajPaOla was added. Reaction times were measured from the moment of addition of the dry tripolyphosphate. Samples were withdrawn a t intervals and quenched in a
chilled potassium acetate solution buffered a t p H 5.0 lvith acetic acid. The quenched samples held a t about 0 ' C. were stable for ueeks. The various phosphate species were promptly separated by ion exchange chromatography and measured colorimetrically. The rate of color development was used to correct for interference by silicate ( 9 ) . For runs a t temperatures above 100' C., test solutions were stored in identical 316 stainless steel capsules made from inch tubing. These capsules, containing about 2 ml. of solution, were dropped into an agitated hot oil bath controlled within i1' C. They were removed a t intervals and quenched in an acetone-dry ice mixture. The same analytical procedure was used for these samples. Results and Discussion
Na5P3Olo-Na2SO4-Na~Si0~-H2O Series. T h e test solutions of this series were intended to simulate the aqueous phase of a detergent slurry. The compositions are given in Table I. The most concentrated test solution has a composition close to that of the inoiganic portion of the aqueous phase of a typical anionic detergent slurry ( 7 0 ) . The solubility of sodium tripolyphosphate is decreased considerably by the high pH and by the high concentrations of other salts in the test solution. Identical solutions with 0.60 gram of disodium EDTA dihydrate (Na2C10H1408N2 2H20) added per gram of NajPaOlO were also studied. T h e presence of silicate kept the p H near 11.1 measured a t 1% solids concentration for all solutions. At 70' C., the EDTA reduced the degradation rate of tripolyphosphate noticeably only in the most concentrated solution, where about 1% more of the phosphate was present as Na5P,010after 1000 minutes. T h e rate of tripolyphosphate disappearance followed firstorder kinetics, as shown in Figure 1. T h e first-order rate constant increased exponentially with total solids concentra-
Table 1. Test Soh. *Vo.
1 2 3 4
Composition of Buffered Tripolyphosphate Solutions Total Solids, iVa,P,Olo, .Va2S0dr Ah Si 0 3, yc Wc % Wt yo 5 00 35 20 00 10 00 20 2 86 11 43 5 71 10 1 43 5 71 2 86 0 71 5 2 86 1 43
wt
wt.
VOL. 5
wt
NO. 2
JUNE 1966
97
tion, as shown in Figure 2. This suggests that the primary salt effect of Br6nsted ( 3 )is still observed in the concentrated solutions. T h e pyro- and orthophosphate content of the reaction product was not in accord with the generally accepted first-order reaction
+
P ~ O ~ J -HzO ~
+
+
H P z O ~ - ~HP04-'
+
(P3010-5 . H z O ) *
-f
(HP20i-3)*
+ HP04-*
( H P 7 0 7 + ) *+ H P E O ? - ~ (HP207-3)*
+ H20
+ HP04-2
f HsP04-
+ ~ H P ~ OfT -P20i-4 ~
2(P3OlO-5 . H20) '+ HQO
-
(1)
The mole ratio of pyro- to orthophosphate (Figure 3) was less than 1 for low solids concentrations and increased with solids or phosphate concentration to values higher than 1. Additional experiments showed that under the conditions used in this work orthophosphate cannot react to give pyrophosphate, and pyrophosphate degrades to orthophosphate a t a negligible rate (less than 270 in 10,000 minutes a t 70" C. and p H 11 with 3570 solids). Also, since a similar product distribution was observed in pure sodium tripolyphosphate solutions, complexes of tripolyphosphate with sulfate or silicate anions are not the cause of the deviation from a simple 1 to 1 ortho-pyro ratio. One possible way to account for the products while retaining first-order kinetics is to assume that an activated complex of tripolyphosphate is first formed (Equation 2). It is assumed that most of the activation energy required to break a P-OP bond is supplied to the tripolyphosphate anion in this step, with much smaller additional amounts being added in the subsequent processes by which the activated complex decays to the reaction products. Depending upon the deactivation step, which is not rate-controlling, different products can form (Equations 3 to 6).
P ~ O I O - H20 ~ + (P3010-5.H20)* (rate controlling)
40
(2)
.-c U
al
c 0
c
n v)
0 5 -a .-8
' c
4
1097 -
6r
4 51
3-
2-
I
1:
I .
2000
I
4000
6000
Time, min. Figure 1. Degradation of tripolyphosphate in solutions containing Na2SiOs and Na2S04 at 70" C.
(3)
(4) (5) (6)
T h e actual composition of the activated complex, (P3010-5 ,
H20)*,is uncertain, since it probably contains some water molecules and cations. At the start of the hydrolysis, especially for higher initial concentrations of tripolyphosphate, Reaction 6 is enhanced, and large amounts of pyrophosphate are formed. Equations 3 to 5 assume that a n activated pyrophosphate can be a n intermediate, leading to the formation of larger amounts of orthophosphate. T h e exact product distribution is a function of the rates of the Reactions 3 to 6, and these rates depend upon many factors in unknown ways which may be clarified by further theoretical and experimental work. The postulated reaction scheme is still oversimplified because such factors as interaction of phosphate polyanions and complexed alkali metal cations ( 5 ) are neglected. Yet it is certainly more adequate than Equation 1. For the most concentrated (350jo) solution, the kinetics remained first order a t 110' and 120" C. T h e slope of the Arrhenius plot in Figure 4 indicated a n activation energy of 19.8 kcal. per mole. This is lower than 22.8 kcal. per mole which was reported for a 1% solution (72), but this difference is consistent with studies on the hydrolysis of sodium trimetaphosphate in our laboratory. I n that reaction, the activation energy dropped from 17.5 kcal. per mole for a 5% solution to 15.3 kcal. per mole for a 20% solution. T h e difference, 2 to 3 kcal. per mole, is probably due to changes in the degree of hydration of polyphosphate anions as the concentrationchanges. T h e observed activation energy is within the expected range 98
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
60 50
I
I
I
I
I
I
5
10
IS
20
25
30
? i
Concetrotion of Solids, Wt. YO (YONoSP3Olo + %Na,SO, + %No2Si0,) Figure 2. Effect of solids concentration on STP degradation rate at 70" C.
. 0
0
20
I
I
30
40
Temp. 'C.
35 35
I20
70
20
70 70 70
IO 5
X
10
Solids Conc. (Weight %)
50
I
1
60
70
,
eo
1
I
Percent of Na,P3OlO Degraded Figure 3. Effect of solids concentration on amount of pyrophosphate in degradation products
rate due to variations in the p H and in the concentrations of species other than tripolyphosphate. The pseudo zero-order rate constants, defined as KO =
(c'c~), are
convenient for dt comparing rates fur various solutions and are listed in Table 11. They can also be compared to the values of K I given in Figure d (C 'Co) 2, since K1 = KO if the initial rates -arec the same. ~
dt
Figure 4. Effect of temperature on degradation rate in concentrated solution
for breaking a P-OP bond and is in agreement with the mechanism postulated above. Effects of Fe(III), Ca(II), C1-, SO1-*, and EDTA on Degradation of Tripolyphosphate. The compositions of the test solutions for these studies at 70' C . are presented in Table 11. Since no buffering agent was present. significant p H changes, shown in Figure 5, were noted during the reaction. The degraded products, hoLvever, have some buffering capacity, and the rate of change of p H decreased as the reaction proceeded. Eventuall), the p H approached 7.0. 'The degradation of tripolyphosphate (Figure 5) under these conditions was pseudo zcro order. Apparently, the decrease in rate \vith decreaiing tripolyphosphate concentration which is expected for first-order kinetics was offset by a n increase in
The degradation rate of the pure, unbuffered tripolyphosphate solutions increased \vith concentration essentially in the manner shoivn in Figure 2 for the buffered solutions. T h e change in the ratio of pyro- to orthophosphate in the products also follo\ved the pattern shown for buffered solutions (Figure 3). Both sodium chloride and sodium sulfate increased the degradation rate. There is a good correlation between the rate constant and the sodium ion concentration for a given tripolyphosphate concentration, as shown in Figure 6. Since the types and concentrations of anions vary considerably, this implies that these anions have little. if any, effect on the degradation rate. T h e presence of Fe(II1) increased the degradation rate, while the presence of Ca(I1) slowed the rate slightly. In both cases, when EDTA was present to sequester the multivalent cation ( in effect replacing it Mith sodium ion), the degradation rate was faster. In dilute solutions, small amounts of multi-
Table II.
Composition of Unbuffered Tripolyphosphate lest Solutions Tofol Soh. .Va6P~Olo, Concentrotion of KO 705, 5, A,o. yG Other AdditiLes Sodium M i n . -1 5.0 0.0 1 56 7.15 10.0 0.0 3 12 9.56 n n 14.0 4 37 11.13 10.0 300 p.p.m. Fe as FeC13, 3 14 10,jl 0 OljSC EDT.ln 10.0 300 p.p.m. Fe as FeC13 3 12 10.36 250 p.p.m. Ca as CaC12, 10.0 3 14 9.67 0 015c; EDTA" 10.0 3 12 9.39 5 0 3 18 10.23 5.0 3 53 11.50 a Disodium (etii~[enedinitriio)t?troacetate dihydrate.
x
wt.
VOL. 5
NO. 2
JUNE 1 9 6 6
99
DH
Figure
5.
Degradation of tripolyphosphate in unbuffered solutions at
70" C.
Conclusions
5 % Na5P3OlO
20% Na,SO, 10% Na2Si0,
Studies on the degradation rate of tripolyphosphate in relatively concentrated solution indicate that it is dangerous to extrapolate from results obtained for dilute solutions without further checks. T h e degradation rate increases exponentially with total solids or sodium ion concentration. Although the degraded products contain disproportionately more pyrophosphate with increasing concentration, the over-all reaction kinetics in buffered solutions appear to remain first order. This anomaly may be explained by the dependence of the degradation products upon the manner in which a slowly formed activated tripolyphosphate complex reacts. Multivalent cations have relatively less effect on the degradation rate in concentrated solution than in dilute solution. literature Cited
B
5-
I
I
I
I
I
I
2
4
6
8
10
12
Wt. % Sodium in Solution Figure 6. Effect of sodium ion concentration on STP degradation rate 70' C., 5% initial
NagP3010 concentration
valent cations increased the degradation rate more than much larger amounts of univalent ions (73), possibly indicating the incorporation of tripolyphosphate in complexes or ion groups, in which it was more vulnerable to degradation. The present results show that no such great differences exist in concentrated solution.
100
l&EC P R O D U C T RESEARCH A N D D E V E L O P M E N T
(1) Bell, R. N., Ind. Eng. Chem. 39, 136 (1947). (2) Benckiser, J. A., Ger. Patent 1,103,312 (Oct. 12, 1959). (3) Br@nsted,J. N., Z. Physik. Chem. 102,169 (1922). (4) Buyers, -4.G., J . Phys. Chem. 66, 939 (1962). (5) Crutchfield, M. M., Irani, R. R., J . A m . Chem. SOL.87, 2815 (1965). ( 6 ) Kolloff, R. H., ASTM Bull. 237, 74 (TP-94-TP-100) (April 1959). (7) Mabis, A. J., Quimby, 0. T., Anal. Chem. 25, 1814 (1953). (8) Pfrengle, O., Fette-Sezfen-Anstrichmlttel 58, 1029 (1956). (9) Shen, C. Y.,Dyroff. D. R., Anal. Chem. 34, 1369 (1962). (10) Shen, C. Y.,Metcalf, J. S.,IND.ENG.CHEM.PROD.RES. DEVELOP. 4, 107 (1965). (11) Shen, C. Y.,Metcalf, J. S., O'Grady, E. V., Ind. Eng. Chem. 51.717 11959). (12) 'Van h a z e r , J. R., Griffith, E. J., McCullough, J. H., J . Am. Chem. SOC. 77, 287 (1955). (13) Wieker, W., Thilo, E., 2. Anorg. Allgem. Chem. 306, 48 (1960). RECEIVED for review October 21, 1965 ACCEPTED March 7. 1966
