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The absorption of polybromide ions on an anion exchange resin
J. Inorg. Nucl. Chem., 1964, Vol. 26, pp. 2235 to 2239. Pergamon Press Ltd. Printed in Northern Ireland
THE
ABSORPTION OF POLYBROMIDE IONS ON AN ANION EXCHANGE RESIN H. IRVING and P. D. WILSON
Department of Inorganic and Structural Chemistry, University of Leeds (Received 13 April 1964) Abstract---The absorption of polybromide ions on an anion exchange resin in the bromide ion form from solutions of bromine in potassium bromide has been studied. The equilibria at 25°C and /z = 0.500 M (KBr) can be reproduced by exchange coefficients K1 = 1"9 × 108 and K2 = 9.5 × l0 b for the species Br3- and Brs-; the hepta-bromide ion BrT- does not participate significantly up to 0.2 M total concentration of bromine used.
IT has been reported °) that an anion exchange resin in the bromide form can be made to absorb up to three molecules of bromine per equivalent of resin, corresponding to the formation of Br C ions. The absorption ofpolybromide ions on such a resin has been utilised for some time past in the purification of hydrobromic acid from its oxidation products. ~2) The present paper presents a quantitative study of these phenomena. EXPERIMENTAL De-acidite FF anion exchange resin (14--52 mesh), in the chloride form as supplied, Was packed into a colunm and washed with 2 M sodium hydroxide solution until the effluent was free from chloride ions and then with water until it was free from hydroxide ions. A known volume of Mhydrobromic acid, completely free from polybromides~2~and roughly equivalent to twice the quantity of resin taken, was passed slowly through the column, and the resin was then washed with distilled water until the washings were neutral. The combined acid effluent and washings were then titrated against standard alkali to obtain the number of equivalents of abSOrbed bromide. The resin was then removed quantitatively from the column, dried at 80°C, and weighed. Its equivalent weight, F_~, was thus found to be 3.463 mequiv/g. The bulk supply of resin thus prepared was stored in an airtight container until required, to prevent changes in water content. Solutions were prepared from pure elemental bromine and a concentrated solution of AnalaR potassium bromide, together with sufficient hydrobromic acid to prevent hydrolysis of the bromine. All solutions had an ionic strength of 0.500 M and a pH of -~2.5. The total initial concentration of bromine '~.[Br2]° in each solution was determined by adding excess potassium iodide and titrating with standard thiosulphate. Fifteen ml portions were then equilibrated with ,-.0.5 g of the resin by shaking in stoppered tubes for 24-48 hr at 25°C. The supernatant liquid was then removed and the total equilibrium concentration of bromine in the aqueous phase, ~;[Br2], determined as before. The total quantity of bromine absorbed by the resin was determined by difference, and, from a knowledge of the weight of resin used, the concentration of total absorbed bromine per gram of dry resin, Z = ~[Br2]R, was calculated. Here and subsequently the subscript R will be used to distinguish the resin from the aqueous phase. lx~ j. AV~TON and D. A. EVEREST,Chem. and lndustr. 1238 (1957). cs~ H. IRVaNOand P. D. WILSON, Chem. and Industr. 653 (1964). 2235
2236
H.M.N.H.
IRVINO and P. D. WILSON
RESULTS AND DISCUSSION The results of typical experiments are given in the first three columns of Table 1 : the fourth column gives the calculated values o f Z and the ninth column gives values o f the distribution coefficient
K~=
total concn, of absorbed bromine per g r a m o f resin total concn, o f bromine in the aqueous phase
= ~ n[Br~+l]R/([Br2] + ~ n[Br~+l]) n=l TABLE I . - - T H E
(1)
n=l ABSORPTION OF POLYBROMIDE IONS ONTO DEAC1DITE-FF IN THE BROMIDE ION FORM
z= Wt. resin(g) Y.[Br2]° 104Z[Br2] 103Y,[Br~]R 105[Br~] [Br-] 104[Br3-] 0'5140 0"5063 0"5019 0-5070 0-5004 0'5057 0"5094 0"5125 0"5003 0.5100 0'5033 0.5056 0-5102 0.5016
0.2035 0.1895 0.1708 0'1502 0-1163 0-1043 0.0793 0.0784 0.0777 0.0750 0-0739 0.0684 0.0671 0.0663
180 152 81"4 49-6 16"0 10.8 4.97 4"92 5"78 3-74 4"22 3.74 3"70 3"80
5"4134 5"1639 4"8613 4'2970 3'4383 3"0617 2"3205 2"2802 2'3123 2.1949 2'1899 2-0182 1.9602 1.9713
198 166 88'5 53"6 17"3 11.6 5'34 5'29 6"21 4.02 4"54 4"02 3"98 4"09
0.484 0-487 0.493 0"496 0.498 0.499 0.500 0-500 0.500 0'500 0.500 0.500 0.500 0.500
159 134 72"4 44-1 14-3 9"64 4.44 4"39 5.16 3.34 3'77 3.34 3-30 3.30
KD KD 10a[Br5-] (exp.) (calc.) 4760 3360 967 357 37'2 16"9 3"58 3"50 4"82 2'02 2"57 2.02 1'97 2'09
0.301 0"340 0'597 0"866 2'149 2"835 4"669 4"635 4"001 5"869 5'189 5"396 5-302 5-188
0.297 0"345 0"576 0"858 2.066 2.772 4.612 4"637 4"211 5-432 5'433 5.432 5.425 5.352
Overall stability constants for the species Br 3- and Brs-, defined by fll = [Bra-]/ [Br--][Br2] = 16.6 and /~12 = [Brs-]/[Br-][Br2] 2 = 25.12, have been reported ta~ for solutions o f ionic strength 0.5 M (sodium perchlorate) at 25°C. N o value is available for the h e p t a b r o m i d e ion, Br7-, which has apparently been reported only in equilibrium with very high concentrations of bromine, tl~ In view of the k n o w n trend in the stability constants fll and fll~, the low equilibrium concentration o f b r o m i n e ([Br2] 2 × 10--a), at which the degree o f information of the p o l y b r o m i d e system is only = 0.03, and the relatively low loading of the resin, it can be assumed that the species Br 7- can be neglected. Since the functional groups o f the resin are strongly basic and the b r o m i d e solutions are o f low p H , hydroxyl ions and h y p o b r o m i t e ions need not be considered, and only the species [Br-], [Br3-] and [Brs-] will enter into the equilibria. The interphase adsorption equilibria (Br3-) + (Br-)rt ~- (Bra-)rt A- (Br-)
(2a)
(Brs-) -f- (Br-)rt ,~-(Brs-)rt q- (Br-)
(2b)
lal H. J. V. TYRRELLand D. B. SCAIr~, Quoted in Chem. Soc. Special Publ. No. 7, Stability Constants of Metal Ion Complexes, Part II, 115 (1958). ~4~B. G. F. CARLrSONand H. IRWNG,J. Chem. Soc. 4390 (1954).
The absorption of polybromide ions
2237
are governed by the exchange constants Kx -- [Br3-]rt[Br-]/[Bra-][Br-]rt
(3a)
Ks = [Brs-]R[Br-]/[Brs-][Br-]rt
(3b)
Equation (1) may now be written
KD=
[Br3-IR ÷ 2[Brs-]n [Br2] + [Br3-] + 2[Brs-] [Br-]R(glfll -r- 2K2~[Br~I) 1 + fll[Br-] -k 2fl2[Br-][Br2]
(4)
In the limit, as [Br2] --+ 0, KD ~ [Br-]R Klfll/(1 -t- fll[Br-]) whence
//1 _~ KD(1 + fll[Br-l)/fl~[Br-]R KDtBr-I/[Br-IR
(5) (5a)
provided fll[Br-] >> 1. At the lowest concentrations of bromine used here, KD ~ 5 when Z[Br2]a ,-~ 2 × 10-3. If we assume that all this bromine is present as tribromide ions (and thus that [Brs-]n = 0, then since [Br-IR + [Br3-IR + [Brs-] = ER = 3"463 × 10-3
(6)
it follows that [Br-]R =_~ 1.5 x 10-3. Since [Br-] = 0.500 M throughout we arrive at an estimate/(1 = 5 × 0.5/1.5 × 10-a = 1.7 x 10a from Equation (5a). A better value is obtained from Equation (5) by inserting the known value of fll = 16.6: this gives//1 = 1.9 × 10s, which should be compared with the final value calculated below. A full analysis of the system can be made as follows. (a) In the aqueous phase, from the total concentration of combined bromine E[Br2] = [Br2] ÷ [Br3- ] + 2[Br5-]
(7)
and the total concentration of bromide ion taken 0.500 M = X[Br-] ---- [Br-] q- [Bra- ] + [Brs- ] we obtain
X[Br2] -- [Br~] = 0.500
fll[Br2] + 2fl~[ar2] 2 1 + fll[Br2] ~- fll~[Br2]z
(8) (9)
From experimental values of Z[Br2] in Table 1, column 3, and the known values of fll and fl12,we calculate first the corresponding value of the free bromine concentration, [Br2], and thence the values of [Br-], [Br3-] and [Br5- ] : these are tabulated in columns 5, 6, 7, and 8 of Table 1. (b) In the resin we have Z = [Bra-]R q- 2[Br5-]~ =
whence
K~[Br3-][Br-]R 2K2[Br5-][Br-]R + [Br-] [Br-] 1 [Br-]R
~
=
(KIX-~
2K~Y)/Z
(10)
(11)
2238
H.M.N.H.
_1
IRVING and P. D. WILSON
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XIE-Z)/(2E-Z)Y FIG. 2
FIos. 1 and Z--Linear plots for the calculation of/(71 and Kv where
X = [Br3-]/[Br-] ---=fll[Br2]
(12a)
and
Y = [Brs-]/[Br-] = fl12[Br2]2
(12b)
Similarly from Equation (6) 1
[Br-]R
- - (1 +
K1X + K SY)/E
(13)
F r o m (11) and (13), by eliminating the term [Br-]R and rearranging, Z X(E -- Z) Z Y(2E -- Z)
--KI+ =K2+
K z Y(2E -- Z)
(14a)
X(E -- Z) KaX(E -- Z) Y(2E -- Z)
(14b)
The absorption of polybromide ions
2239
The appropriate linear plots are shown in Figs. I and 2. By a least squares treatment there results Kl=(l'9~z0"09)
× 10z
and
K S ~ ( 9 . 5 ~ k 0 " 6 ) x l0 s .
Values of KR were now calculated from Equations (4) and (13) explicitly through KD ----
E(KI#I + 2Kz/51z[Br=]) (1 + Kafl[Brz] + K2fll~[Br2]2)(1 + fla[Br-] + 2#x2[Br-][Br2l)
109.22 + 1.655 × 105[Brg.] = (9.300 + 2.933 × i05[Br2] + 2.233 × 108[Br2]2 + 6-014 x 10a[Br=]z and are given in Table 1, column 10. The differences between the experimental and calculated values of K~ are small, and show no systematic trend; this confirms the
,00 ...
I
I
\
%
I
l
/Br~
50--
25
"~'x,
-
.... 0'05
0"10 0"15 TotoI initiol bromine, ~[Brzl °~
-
4•0.20
FIO. 3.--The percentage of polybromides absorbed on Deacidite-FF as a function of the total initial concentration of bromine. assumption that the species Br 7- need not be taken into consideration under the present conditions. However, even with the present low concentrations of bromine the bromine ion on the resin is substantially replaced by the species Brn- and Brb-, as shown in Fig. 3. The maximum concentration of tribromide ion is reached when the initial total bromine is only 0.12 M, and thereafter it is replaced by pentabromide, while the concentration of bromide ion becomes very small indeed. The constancy of the stoicheiometric exchange constants K1 and Ka requires that activity coefficients should remain constant in each phase over their range of validity. The ionic strength was certainly maintained constant (effectively 0.500 M (KBr)), but the usual device for minimising changes in the resin phase, i.e. the use of radiotracers and minimum loading, was in conflict with the objectives of the present study. It would appear, however, that the very substantial replacements of Br- by other uninegative ions, viz. Br 3- and Brb-, despite their greater deformability, does not affect the average environment sufficiently to alter the values of K 1 and K 2.
THE
ABSORPTION OF POLYBROMIDE IONS ON AN ANION EXCHANGE RESIN H. IRVING and P. D. WILSON
Department of Inorganic and Structural Chemistry, University of Leeds (Received 13 April 1964) Abstract---The absorption of polybromide ions on an anion exchange resin in the bromide ion form from solutions of bromine in potassium bromide has been studied. The equilibria at 25°C and /z = 0.500 M (KBr) can be reproduced by exchange coefficients K1 = 1"9 × 108 and K2 = 9.5 × l0 b for the species Br3- and Brs-; the hepta-bromide ion BrT- does not participate significantly up to 0.2 M total concentration of bromine used.
IT has been reported °) that an anion exchange resin in the bromide form can be made to absorb up to three molecules of bromine per equivalent of resin, corresponding to the formation of Br C ions. The absorption ofpolybromide ions on such a resin has been utilised for some time past in the purification of hydrobromic acid from its oxidation products. ~2) The present paper presents a quantitative study of these phenomena. EXPERIMENTAL De-acidite FF anion exchange resin (14--52 mesh), in the chloride form as supplied, Was packed into a colunm and washed with 2 M sodium hydroxide solution until the effluent was free from chloride ions and then with water until it was free from hydroxide ions. A known volume of Mhydrobromic acid, completely free from polybromides~2~and roughly equivalent to twice the quantity of resin taken, was passed slowly through the column, and the resin was then washed with distilled water until the washings were neutral. The combined acid effluent and washings were then titrated against standard alkali to obtain the number of equivalents of abSOrbed bromide. The resin was then removed quantitatively from the column, dried at 80°C, and weighed. Its equivalent weight, F_~, was thus found to be 3.463 mequiv/g. The bulk supply of resin thus prepared was stored in an airtight container until required, to prevent changes in water content. Solutions were prepared from pure elemental bromine and a concentrated solution of AnalaR potassium bromide, together with sufficient hydrobromic acid to prevent hydrolysis of the bromine. All solutions had an ionic strength of 0.500 M and a pH of -~2.5. The total initial concentration of bromine '~.[Br2]° in each solution was determined by adding excess potassium iodide and titrating with standard thiosulphate. Fifteen ml portions were then equilibrated with ,-.0.5 g of the resin by shaking in stoppered tubes for 24-48 hr at 25°C. The supernatant liquid was then removed and the total equilibrium concentration of bromine in the aqueous phase, ~;[Br2], determined as before. The total quantity of bromine absorbed by the resin was determined by difference, and, from a knowledge of the weight of resin used, the concentration of total absorbed bromine per gram of dry resin, Z = ~[Br2]R, was calculated. Here and subsequently the subscript R will be used to distinguish the resin from the aqueous phase. lx~ j. AV~TON and D. A. EVEREST,Chem. and lndustr. 1238 (1957). cs~ H. IRVaNOand P. D. WILSON, Chem. and Industr. 653 (1964). 2235
2236
H.M.N.H.
IRVINO and P. D. WILSON
RESULTS AND DISCUSSION The results of typical experiments are given in the first three columns of Table 1 : the fourth column gives the calculated values o f Z and the ninth column gives values o f the distribution coefficient
K~=
total concn, of absorbed bromine per g r a m o f resin total concn, o f bromine in the aqueous phase
= ~ n[Br~+l]R/([Br2] + ~ n[Br~+l]) n=l TABLE I . - - T H E
(1)
n=l ABSORPTION OF POLYBROMIDE IONS ONTO DEAC1DITE-FF IN THE BROMIDE ION FORM
z= Wt. resin(g) Y.[Br2]° 104Z[Br2] 103Y,[Br~]R 105[Br~] [Br-] 104[Br3-] 0'5140 0"5063 0"5019 0-5070 0-5004 0'5057 0"5094 0"5125 0"5003 0.5100 0'5033 0.5056 0-5102 0.5016
0.2035 0.1895 0.1708 0'1502 0-1163 0-1043 0.0793 0.0784 0.0777 0.0750 0-0739 0.0684 0.0671 0.0663
180 152 81"4 49-6 16"0 10.8 4.97 4"92 5"78 3-74 4"22 3.74 3"70 3"80
5"4134 5"1639 4"8613 4'2970 3'4383 3"0617 2"3205 2"2802 2'3123 2.1949 2'1899 2-0182 1.9602 1.9713
198 166 88'5 53"6 17"3 11.6 5'34 5'29 6"21 4.02 4"54 4"02 3"98 4"09
0.484 0-487 0.493 0"496 0.498 0.499 0.500 0-500 0.500 0'500 0.500 0.500 0.500 0.500
159 134 72"4 44-1 14-3 9"64 4.44 4"39 5.16 3.34 3'77 3.34 3-30 3.30
KD KD 10a[Br5-] (exp.) (calc.) 4760 3360 967 357 37'2 16"9 3"58 3"50 4"82 2'02 2"57 2.02 1'97 2'09
0.301 0"340 0'597 0"866 2'149 2"835 4"669 4"635 4"001 5"869 5'189 5"396 5-302 5-188
0.297 0"345 0"576 0"858 2.066 2.772 4.612 4"637 4"211 5-432 5'433 5.432 5.425 5.352
Overall stability constants for the species Br 3- and Brs-, defined by fll = [Bra-]/ [Br--][Br2] = 16.6 and /~12 = [Brs-]/[Br-][Br2] 2 = 25.12, have been reported ta~ for solutions o f ionic strength 0.5 M (sodium perchlorate) at 25°C. N o value is available for the h e p t a b r o m i d e ion, Br7-, which has apparently been reported only in equilibrium with very high concentrations of bromine, tl~ In view of the k n o w n trend in the stability constants fll and fll~, the low equilibrium concentration o f b r o m i n e ([Br2] 2 × 10--a), at which the degree o f information of the p o l y b r o m i d e system is only = 0.03, and the relatively low loading of the resin, it can be assumed that the species Br 7- can be neglected. Since the functional groups o f the resin are strongly basic and the b r o m i d e solutions are o f low p H , hydroxyl ions and h y p o b r o m i t e ions need not be considered, and only the species [Br-], [Br3-] and [Brs-] will enter into the equilibria. The interphase adsorption equilibria (Br3-) + (Br-)rt ~- (Bra-)rt A- (Br-)
(2a)
(Brs-) -f- (Br-)rt ,~-(Brs-)rt q- (Br-)
(2b)
lal H. J. V. TYRRELLand D. B. SCAIr~, Quoted in Chem. Soc. Special Publ. No. 7, Stability Constants of Metal Ion Complexes, Part II, 115 (1958). ~4~B. G. F. CARLrSONand H. IRWNG,J. Chem. Soc. 4390 (1954).
The absorption of polybromide ions
2237
are governed by the exchange constants Kx -- [Br3-]rt[Br-]/[Bra-][Br-]rt
(3a)
Ks = [Brs-]R[Br-]/[Brs-][Br-]rt
(3b)
Equation (1) may now be written
KD=
[Br3-IR ÷ 2[Brs-]n [Br2] + [Br3-] + 2[Brs-] [Br-]R(glfll -r- 2K2~[Br~I) 1 + fll[Br-] -k 2fl2[Br-][Br2]
(4)
In the limit, as [Br2] --+ 0, KD ~ [Br-]R Klfll/(1 -t- fll[Br-]) whence
//1 _~ KD(1 + fll[Br-l)/fl~[Br-]R KDtBr-I/[Br-IR
(5) (5a)
provided fll[Br-] >> 1. At the lowest concentrations of bromine used here, KD ~ 5 when Z[Br2]a ,-~ 2 × 10-3. If we assume that all this bromine is present as tribromide ions (and thus that [Brs-]n = 0, then since [Br-IR + [Br3-IR + [Brs-] = ER = 3"463 × 10-3
(6)
it follows that [Br-]R =_~ 1.5 x 10-3. Since [Br-] = 0.500 M throughout we arrive at an estimate/(1 = 5 × 0.5/1.5 × 10-a = 1.7 x 10a from Equation (5a). A better value is obtained from Equation (5) by inserting the known value of fll = 16.6: this gives//1 = 1.9 × 10s, which should be compared with the final value calculated below. A full analysis of the system can be made as follows. (a) In the aqueous phase, from the total concentration of combined bromine E[Br2] = [Br2] ÷ [Br3- ] + 2[Br5-]
(7)
and the total concentration of bromide ion taken 0.500 M = X[Br-] ---- [Br-] q- [Bra- ] + [Brs- ] we obtain
X[Br2] -- [Br~] = 0.500
fll[Br2] + 2fl~[ar2] 2 1 + fll[Br2] ~- fll~[Br2]z
(8) (9)
From experimental values of Z[Br2] in Table 1, column 3, and the known values of fll and fl12,we calculate first the corresponding value of the free bromine concentration, [Br2], and thence the values of [Br-], [Br3-] and [Br5- ] : these are tabulated in columns 5, 6, 7, and 8 of Table 1. (b) In the resin we have Z = [Bra-]R q- 2[Br5-]~ =
whence
K~[Br3-][Br-]R 2K2[Br5-][Br-]R + [Br-] [Br-] 1 [Br-]R
~
=
(KIX-~
2K~Y)/Z
(10)
(11)
2238
H.M.N.H.
_1
IRVING and P. D. WILSON
I I I I Iool
I I_~
3
hi
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I
I
-25
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-20
I
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I
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15
I04Y(2E-7)/X(E-Z) Flo. 1 12
t
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8
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0
I
I
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2
3
4
5
IO 3
XIE-Z)/(2E-Z)Y FIG. 2
FIos. 1 and Z--Linear plots for the calculation of/(71 and Kv where
X = [Br3-]/[Br-] ---=fll[Br2]
(12a)
and
Y = [Brs-]/[Br-] = fl12[Br2]2
(12b)
Similarly from Equation (6) 1
[Br-]R
- - (1 +
K1X + K SY)/E
(13)
F r o m (11) and (13), by eliminating the term [Br-]R and rearranging, Z X(E -- Z) Z Y(2E -- Z)
--KI+ =K2+
K z Y(2E -- Z)
(14a)
X(E -- Z) KaX(E -- Z) Y(2E -- Z)
(14b)
The absorption of polybromide ions
2239
The appropriate linear plots are shown in Figs. I and 2. By a least squares treatment there results Kl=(l'9~z0"09)
× 10z
and
K S ~ ( 9 . 5 ~ k 0 " 6 ) x l0 s .
Values of KR were now calculated from Equations (4) and (13) explicitly through KD ----
E(KI#I + 2Kz/51z[Br=]) (1 + Kafl[Brz] + K2fll~[Br2]2)(1 + fla[Br-] + 2#x2[Br-][Br2l)
109.22 + 1.655 × 105[Brg.] = (9.300 + 2.933 × i05[Br2] + 2.233 × 108[Br2]2 + 6-014 x 10a[Br=]z and are given in Table 1, column 10. The differences between the experimental and calculated values of K~ are small, and show no systematic trend; this confirms the
,00 ...
I
I
\
%
I
l
/Br~
50--
25
"~'x,
-
.... 0'05
0"10 0"15 TotoI initiol bromine, ~[Brzl °~
-
4•0.20
FIO. 3.--The percentage of polybromides absorbed on Deacidite-FF as a function of the total initial concentration of bromine. assumption that the species Br 7- need not be taken into consideration under the present conditions. However, even with the present low concentrations of bromine the bromine ion on the resin is substantially replaced by the species Brn- and Brb-, as shown in Fig. 3. The maximum concentration of tribromide ion is reached when the initial total bromine is only 0.12 M, and thereafter it is replaced by pentabromide, while the concentration of bromide ion becomes very small indeed. The constancy of the stoicheiometric exchange constants K1 and Ka requires that activity coefficients should remain constant in each phase over their range of validity. The ionic strength was certainly maintained constant (effectively 0.500 M (KBr)), but the usual device for minimising changes in the resin phase, i.e. the use of radiotracers and minimum loading, was in conflict with the objectives of the present study. It would appear, however, that the very substantial replacements of Br- by other uninegative ions, viz. Br 3- and Brb-, despite their greater deformability, does not affect the average environment sufficiently to alter the values of K 1 and K 2.