296
R.R.IRAXI .WD
benzene found in the present study to that of the color reaction in water studied by various workers. Acknowledgment.-The author wishes to express
C. F. C~LLIS
T'ol. G 3
his hearty thanks to Prof. K. Yamamoto and Prof. K. Shibata for their kind guidance and valuable suggestions throughout this investigation.
METAL CO31PLEXISG BY PHOSPHORUS COMPOUSDS. 111. THE COJIPLESISG OF CALCIUM BY IMIDODI- AND DII,141DOTRIPHOSPHATF,4TE BY R. R. IRANI AND C. F. CALLIS N o m a n t o Chemical Company, Research Department, Inorganic Chewticals Division, St. Louis 66, 71fo Receaued July 27, 1960
The stabilitiea of the complexes of imidodiphosphate, diimidotriphosphate and their monohvdrogen forms with calcium have been evaluated from nephelometric titrations in the presence of oxalate, as a function of pH, temperatiire and ionic strength. The free-energy changes a t 25', taking unit mole fraction standard states, for the formation of the complexes CaP20s(NH)-2, CaHP20s(NH)-I, CaP308(NH)z-3and CaHP308(NH)t_*from the corresponding ions wrre found to be -5.6, -2.2, -7.4 and -3.9 kcal./mole, respectively. For the complexing of calcium by P&(NH) - 4 and PK)8(?1TH)?-6, the enth:ilpies \rere evaluated to be - 5.i and - 9.1 kcal./mole, respectively, with the corresponding entropy changes being about zero
Introduction In a previous paper' of this series, the thermodynamics of association of linear polyphosphates with calcium was reported. In order t o investigate the effect 011 complexing of the groups connecting the phosphorus at oms in the linear chain, the complexing of calcium by imidophosphates is reported in this paper. Experimental Chemicals.-'The hexahydrate sodium salts of imidodiand diimidotriphosphate were used as the source for the imidophosphate anions. These sodium salts were kindly supplied by Dr . M. L. Nielsen of Monsanto's Research and Engineering Division. Infrared spectra2 as well as nuclear magnetic resonance and wet chemical analyses3 established that the imidodi - and diimidotriphosphate were better than 98 and 96% pure, respectively. The imidophosphates were converted to the tetramethylammonium salts using ion exchange, as previously described. However, due to their relatively low hydrolytic stability, the stock solutions were maintained a t room temperature and a pH of 13, and were used within a few days, during which time no significant hydrolysis is expected .4 The tetrametliylammonium bromide and hydroxide were Eastman Kodalt reagent grade. All other chemicals were C .P . grade. Procedure.-The nephelometric procedure, previously described,' was iitilized as a measure of the competition for thy? calcium ions between the oxalate and imidophosphate anions. However, the temperature of the solutions was controlled to better than *0.1' using a highly sensitive heat-sensing element? The concentrations of calcium and oxalate were taken into account in calculating the total ionic strength which was made up to the desired value using tetramethylammonium bromide. No PZOj was detected in the precipitates formed upon addition of a slight excess of C a + + to a solution containing oxalate and imidophosphate a t the end-point. X-Ray as well as wet-chemical analyses also established the preripitates to bP CaC204.H20.
TABLE I Ca+"-H
SUMMARY OF DATA FOR THE
f-C?Oi"-IMIDOPHOS-
PHATE SYSTEM"
Imidophosphate (CI) X 10-8,
Temp.,
"C.
PH
25
12.0
9.5 37
12.0
50
12.0
1M
Oxalate Cc. (y) of 8.86 X 10-2 M Ca (CQ) X s o h . to nephelometric end10-2, point at ionic strengths of BI 0.1 0.3 1.0
Imidodiphosphate 2.33 0.745 2.42 .. 1.65 1.49 1.73 I . 62 1.26 2.23 1.44 0.95 0.90 2.33 0.3i2 0.71 .. 1.21 ,745 0.35 .. 0 67 2.33 .745 _. 1.08 .. 1.49 1.85 .. 0.88 2.23 1.30 .. 2.33 0.745 .. .. 1.21 1.49 1.76 .. 0.74 2.23 1.53 .. ..
Diimidotriphosphate 1.71 0.745 4.24 .. 3.i7 1.49 4.05 3.47 3.67 2.23 3.88 2.80 3.12 9.0 1,il 0.372 3.46 .. .. .. 0.745 2.68 1.55 .. .. 1.40 1.00 12.0 1.71 37 0.745 .. .. 3.24 1.49 3.40 .. 2.84 3.26 2.23 .. .. 50 12.0 1.71 0.745 .. .. 3.22 1.49 3.55 3.02 2.23 3.32 .. The total volume of the solution mas 250 ml. in a11 caws.
25
12.0
Results and Discussion Table I gives the cc. of 8.86 X M Ca(Pu'O& methylammonium oxalate and imidophosphate t o that must be added to solutions containing tetra- reach the point of incipient precipitation of CaC204. Using previously described procedures, it was (1) R. R. Irani and C. F. Callis, J. Phgs. Chem., 64, 1398 (1960). found that the experimental data are best fit by (2) J. V. Pustinrrer, W. T. Cave and hl. L. Nielsen, Spectrochzm. assigning only one complexing site per calcium, as Acta, 11, 909 (1969). (3) hI. L. Nielsen. private communication. shown in Table 11. Obviously, a negative number (4) 0 T. Quinibz., A . Naratii and 1;. H. I ohman, J . A m . Chem. SOC., of phosphorus atoms per site is meaningless. 83, 1099 (1060). Therefore, the general equation for the association ( 5 ) For details, cf. R. R. Irani and C. F. Callis, J . Phgs. Chem., 64, reaction is as follows, omitting charges on the ions 1741 (1960).
Feb., 1961 C%.(I%O).
COMPLEXING OF CALCIUM BY INIDODIAKD DIIMIDOTRIPHOSPHATE
+ i'(e).(H~O)b= ct&s(e).(HzO),+
29i
given in Table 111; the errors shown as i are the statistical 95y0 confidence limits. The data in Table I at 25" and lower pH values where a, 6 and c are hydration numbers, and 9 is the number of phosphorus atoms per calcium-complex- were then utilized to evaluate the dissociation coning site, S . Xs shown in Table 11, the value of 0 stants of the calcium monohydrogen imidowas found to be two and three for imidodiphosphate phosphate species using equation 2 and the already and diimidotriphosphate, respectively, immaterial evaluated dissociation constants of the calcium of whether the imidophosphate had a hydrogen imidophosphates and the previously determined hound to it or not. Therefore, the simple mass acid dissociation constants6 at the same temneraaction expression for combination of ligand with ture and ionic strength. The results are also &en in Table 111. metal ion applies here. Attempts to evaluate the stability of the possible TABLE I1 complex between calcium and the first homolog the imidophosphate family, namely, the anion of phosSITESOCCUPIED B Y ONE CALCIUM phoramidic acid were unsuccessful with the proceS o . of Calcd. no. of phosphorus sites per atoms per site to) dure employed in this work, due to unfavorable Complexing anion calciiini Range Average competition with the oxalate ion. Within the Iniidodiphosphate 1 1 . 8t o 2 . 3 2.0 sensitivity of the procedure, the presence of up to 2 -4 t 0 0 . 3 .. 0.5 M phosphorimidate did not change the point 3 -10 to -3 of precipitation of calcium oxalate, so that the posniimidotriphosphate 1 2.5to3.6 3.3 sible calcium phosphorimidate complex must have 2 l.0t03.0 2.1 a dissociation constant greater than 3 -0.5 t 0 2 . 6 .. The thermodynamic dissociation constants at 25, At t'he point of incipient precipitation of CaC204, 37 and 50' are shown in Table IV and were eraluated by extrapolating the apparent dissociation conit can be shown' that stants to infinite dilution.',' Since the extrapolation covers a wide concentration range, larger errors consistent with previous extrapolations (1) are assigned to these constants. Table IV is also a com1 pilation of the changes at 25' in free energy, AFO, (2) PC~HS(O) [1 P H s ( e ) / [ H + ] enthalpy, AH', and entropy, ASo, accompanying where C1 and Cz are the molar concentrations of the association reaction typified by equation 1. imidophosphate and oxalate, respectively, A the These thermodynamic quantities were computed total volume of solution in ml., and y is the ml. of a from the constants in Table I11 as previously dex molar Ca++ solution that must be added to reach scribed.' However, they were corrected to the abthe point of incipient precipitation of CaC204. solute scale of a hypothetical unit mole fraction Kspis the solubility product of CaCz04at the appro- standard state by adding 2.4 kcal. and subtracting priate ionic strength and temperature, and /3caS(o) 7.9 e.u. from the AFO and ASo obtained for the and PHS(~) are the apparent dissociation constants hypothetical one molal standard state.8 of the calcium and hydrogen imidophosphates, In contrast with the complexing of calcium by respectively pyrophosphate and tripolyphosphate, the enthalpies for the association reactions of calcium with imidophosphates are significantly more exothermic and the entropies of association are much lower. These differences reflect having an -KH- rather than an -0- linking the phosphorus atoms. I n the [Ca++] = Ksp/[C,l ( 5) case of the calcium pyrophosphate and calcium At a pH of 12, only the non-hydrogen phosphate tripolyphosphate complexes, the large entropy species exist,6namely changes accompanying association contributed significantly toward the free-energy term and mere attributed to release of waters of hydration. For the complexing of calcium by iniidodiphosI I I I I phate and diimidotriphosphate, one of two possi-0 0-0 00bilities may be valid. The first possibility is to so that equation 2 reduces t o assume that neither maters of hydration were released nor any significant structural changes were involved, and assume that the stability of the calcium imidophosphates is solely due to some type of because PCaHS(0) is much larger than /3cas(o) and direct bonding. However, calculation of Bjerrum ~ ) smaller than 1. The mdiig,10 the ratio [ H + ] / / ~ H Sis(much for the distance of closest approach between data at a pH of 12 in Table I were interpreted with calcium and imidodiphosphate or diimidotriphosequation 6 to obtain the apparent dissociation con(7) A . E. Martell and >I. C a l r m , "Chemistry of the Metal Chelate stants of the calcium imidodiphosphate and calcium Compounds," Prentice-Hall, Inc , New York, N. Y., 1952, p. 133. ( 8 ) A . W. Adarnson. J. A m . Chem Soc., 76, 1578 (1954). diimidotriphosphate at several combinations of (9) N. Bjerrum, Kgl. Danske Selskab, 7, No. 9 (1936). temperatures and ionic strengths. The results are (10) R. M. Vuoss and C . i. Kraus, J. A m . Chrm. S O C ,66, 1019 (a
+ b - c ) H ~( 1 )
.
+
(6) R. R. Irani and C.
I
1
F. Callis, ibid., 68, in press (1961).
(1933)
298 TABLE I11 THEAPPARENTDISSOCIATION C O N S T ~ KOF T SC a ~ c rair IWDOPHOSPH ~TC\ Calcium cornplev of
Temp.,
Iinidodiphosphatt-1
25
5 59 f 0 5 3i f 5 14 f t 3 3 3 f G 74 It F 21 i 0 01 i. 1 44 f
3i
11onohydrogen imidodiphoqphatc I,liiniidotripho.phatf,
11onohrdrogen diimidotriphosphate C H 4NGES I N
log of dissociation constant a t ionic stlengths of-03 10
---Negative 01
OC.
50 25 25 37 50 25
07 02 04 10 12 05 08 10
TABLE IT' THERMODYNAMIC QUANTITIES FOR C 4 L C I I W
Calcium complex of
Negative log of Temp,, thermodynamic molal "C. dissociation constant
5 20 f 0 05
4 59 It 0 4 14 i 3 8G f .i15 f 5 GG =t 5 1 L i 4 86 It 4 16 It
G 1 1 i . 0 OF
09 09 04 10 12 10
10 IO
COMPLEXISG BY IiVIDOPHOSPHATES [ A F a ] , " ?b
kcal.
AH", kcal.
[ A S o ] , a >b B.U.
25 607f01 - 5 6 f O 1 -57+00 0 f3 37 5 8 9 i I - 6 l f 1 50 5 7 6 f 1 - 6 1 5 1 25 3 4 f 2 -222t 2 AIonohydrogen imidodiphosphate Iliimidoi riphosphate 25 7 1 o i 2 - i 4 f 2 -9 l f 2 -6 i7 37 6 9 0 5 2 - 7 4 f 2 50 660& 2 -74i 2 Monohyirogen diimidotriphosphate 25 4 6 f L -39f 2 * Taking the standard state as a hypothetical unity mole fraction. b The influence of temperature was taken into acroiint in applying the corrections to obtain the unit mole fraction standard state.
Imidodiphosphate
phate yields values of 3.1 and 3.5 .&., respectively, so that bonding by this criterion is ionic rather than covalent. The second possibility is to assume that waters of hydration were released, together with the forma-
tion of a more ordered structure, with both entropy change contributions cancelling one another. Acknowledgment.-The authors thank Mr. TT' W. Morgenthnler for making qome of the measurements.
OXIDATION OF ,4MMONIL4 IS FLAMES BY C. P. FENIMORE ASD G. W.JONES General Electric Research Laboratory, Schenectady, A'. I'. Recezved J u l y $8 1960
The rate of the reaction of nitric oxide with ammonia has been measured in lox pressure flat flames a t 1'700 to 1900'K. by probe sampling, and simultaneous estimates of [HI obtained. The data support the vim- that the equilibrium K[?;H?]-
+
- I,
[H2]= [ SH3][HI is satisfied for small ratios of [NO]/[H,], and that nitric oxide decays vau reaction I, KO YH2 . , with k l / K = 6 X 1010 1. mole-' sec.-l. Reaction 1 is important in other, suprrficially different, deeventually K2 compositions of ammonia also. Much nitric oxide is generated in the reaction zone of H2-NH1-02 flames, probablv by attack of 0 atoms on ammonia, and nitrogen is formed chiefly by reaction 1. Furthermore, residual ammonia siirvives into the post-flame gas of rich enough ammonia flames and decomposes there, in the absence of appreciable oxygen or nitric oxide, much more slowlp than in the reaction zone; and this too map he due to a slow formation of transient nitric oxide follomd b j rapid 1.
+-
Introduction Adams, Parker and Wolfhard' showed that nitric oxide decays faster in ammonia than in hydrogen flames, and since nitric oxide rapidly scavenges NH?radicals according to kl
NO
- NH2 +
+eventually N2 +
(1)
even a t room temperature123 they suggested that the same process occurs in flames. I n this paper we show that the decomposition of ammonia in (1) G. SGC, 14, (2) C. (3) A (19.59)
K Adam?, W. G. Parker a n d H. G. Wolfhard, Dmc. Paraday 97 (1953) H . Barnford, Ibzd., 38, 568 (1939). Sereuicz and W. Albert Noyes, Jr., J . Phys. Chem., 63, 843
many flames involves reaction 1. If nitric oxide is not supplied as a reactant, it is generated by oxidation of part of the ammonia and then consumed via reaction 1. Adams and co-workers considered that R reNO + N2 OH, might be imaction, S H portant as well. But under the conditions we used, it was unnecessary to assume an important role for S H radicals. Experimental
+
+
A water-cooled, porous, flat flame burner,d mounted in a bell jar with movable quartz probe and quartz coated thermocouple, was fed with reactants metered through (4) W. E. Kaskan, "6th Symposium on Combustion,'' Reinhold Publ. Corp., New York, N. Y.,1957, p. 134.
