Dissociation constants and activity coefficients of methionine in KCl aqueous solutions

doi:10.11949/0438-1157.20210291

Abstract

Abstract:

The dissociation constants of methionine in KCl aqueous solutions with the molality of 0.25—4.0 mol/kg at 25℃, 30℃ and 35℃ were determined by changing the pH value, and the protonation enthalpies of methionine species were calculated. Methionine activity coefficients of 0.1—1 mol/kg KCl in 0.025—0.200 mol/kg DL-methionine solution were measured by electromotive force method at 25℃, and the activity coefficients of methionine in KCl aqueous solutions were calculated. The results showed that the dissociation constants of methionine in KCl aqueous solution decreased with the increase of temperature and increased with the increase of KCl concentration. The activity coefficients of methionine in KCl aqueous solutions at 25°C decreased first and then increased with the increase of methionine concentration. Debye-Huckel model and Pitzer model were used to fit the experimental datas of dissociation constants and activity coefficients, and the relevant parameters were obtained. The results show that the reliability of the Pitzer model is better.

Key words: dissociation constant, activity coefficient, DL-methionine, KCl, equilibrium, enthalpy

摘要：

采用改变pH法在25、30和35℃下测定了质量浓度为0.25~4.0 mol/kg的KCl水溶液中蛋氨酸的解离常数，计算得到了蛋氨酸的质子化焓；采用电动势法在25℃测量了0.025~0.200 mol/kg蛋氨酸溶液中浓度为0.1~1 mol/kg的KCl的活度系数，计算得到了KCl溶液中蛋氨酸的活度系数。结果表明，蛋氨酸在KCl水溶液中的解离常数随温度上升而减小、随KCl浓度增加而增大；25℃下，蛋氨酸在KCl水溶液中的活度系数随蛋氨酸浓度增大先减小后增大。运用德拜-休克尔模型和Pitzer模型对解离常数和活度系数的实验数据进行拟合，得到了相关参数，结果表明Pitzer模型的可靠性较好。

关键词: 解离常数, 活度系数, DL-蛋氨酸, 氯化钾, 平衡, 焓

CLC Number:

TQ 021.8

Zhirong CHEN, Yun TONG, Shenfeng YUAN, Hong YIN. Dissociation constants and activity coefficients of methionine in KCl aqueous solutions[J]. CIESC Journal, 2021, 72(9): 4469-4478.

陈志荣, 童云, 袁慎峰, 尹红. 蛋氨酸在KCl溶液中的解离常数与活度系数[J]. 化工学报, 2021, 72(9): 4469-4478.

Figures/Tables 14

Fig.1 Activity coefficient measuring system

Table 1 pKi*（±S.D.） for methionine in KCl solutions at different temperatures

T/℃	m_KCl/(mol/kg)	pK₁^*	pK₂^*	c_KCl/(mol/L)	pK₁^*	pK₂^*
25	0.25	2.163±0.004	9.098±0.003	0.25	2.167	9.102
	0.5	2.186±0.006	9.108±0.006	0.49	2.193	9.115
	1.0	2.240±0.003	9.138±0.004	0.97	2.254	9.152
	1.5	2.299±0.007	9.225±0.001	1.43	2.319	9.244
	2.0	2.361±0.009	9.300±0.006	1.89	2.387	9.326
	3.0	2.485±0.009	9.463±0.007	2.75	2.523	9.501
	4.0	2.616±0.010	9.640±0.007	3.55	2.667	9.691
30	0.25	2.158±0.002	8.969±0.001	0.25	2.162	8.973
	0.5	2.181±0.004	8.979±0.003	0.49	2.188	8.986
	1.0	2.230±0.002	9.028±0.004	0.97	2.243	9.041
	1.5	2.288±0.006	9.093±0.004	1.43	2.307	9.112
	2.0	2.344±0.005	9.167±0.008	1.89	2.370	9.193
	3.0	2.456±0.010	9.328±0.009	2.75	2.494	9.366
	4.0	2.587±0.011	9.500±0.012	3.55	2.640	9.551
35	0.25	2.155±0.003	8.846±0.004	0.25	2.159	8.850
	0.5	2.174±0.005	8.855±0.006	0.49	2.181	8.862
	1.0	2.223±0.002	8.903±0.005	0.97	2.236	8.916
	1.5	2.276±0.004	8.967±0.006	1.43	2.295	8.986
	2.0	2.330±0.006	9.038±0.005	1.89	2.356	9.064
	3.0	2.445±0.008	9.199±0.008	2.75	2.483	9.237
	4.0	2.564±0.010	9.369±0.011	3.55	2.615	9.420

Fig.2 pKi*(molar scale) for methionine in KCl solutions as a function of ionic strength

Table 2 Enthalpy of dissolution at different ionic strength

I/(mol/kg)	-ΔH₁^*/(kJ/mol)	-ΔH₂^*/(kJ/mol)
0.25	1.4	44.3
0.5	2.1	44.5
1.0	3.1	45.0
1.5	4.1	45.4
2.0	5.4	46.1
3.0	7.0	46.4
4.0	9.1	47.7

Fig.3 Comparison of pKi* (molar scale) for methionine in KCl, NaCl and KNO3 solutions(T=25℃)

Table 3 Values of electrochemical cell（1）

m_KCl/(mol/kg)	E₊/mV	E_-/mV	ΔE⁽¹⁾/mV	$γ_{\pm}^{(1)}$	ln(m_s $γ_{\pm}^{(1)}$ )
0.1	151.3	48.2	103.1	0.76777	-2.5669
0.3	175.5	21.0	154.5	0.68627	-1.5805
0.5	187.3	8.6	178.7	0.64902	-1.1254
0.7	195.4	0.6	194.8	0.62599	-0.8251
1.0	203.6	-8.2	211.8	0.60389	-0.5044

Table 3 Values of electrochemical cell（1）

m_KCl/(mol/kg)	E₊/mV	E_-/mV	ΔE⁽¹⁾/mV	$γ_{\pm}^{(1)}$	ln(m_s $γ_{\pm}^{(1)}$ )
0.1	151.3	48.2	103.1	0.76777	-2.5669
0.3	175.5	21.0	154.5	0.68627	-1.5805
0.5	187.3	8.6	178.7	0.64902	-1.1254
0.7	195.4	0.6	194.8	0.62599	-0.8251
1.0	203.6	-8.2	211.8	0.60389	-0.5044

Table 4 Ratio of mean ionic activity coefficient of KCl in methionine aqueous solution to that in water at different molalities of KCl and methionine

m_A/(mol/kg)	m_KCl/(mol/kg)
m_A/(mol/kg)	0.1	0.3	0.5	0.7	1.0
0.025	1.0018	1.0022	1.0025	1.0029	1.0035
0.050	1.0026	1.0035	1.0041	1.005	1.0062
0.100	1.0026	1.0042	1.0058	1.0074	1.0099
0.150	1.0018	1.0045	1.0067	1.0093	1.0129
0.200	1.0019	1.0051	1.0082	1.0117	1.0165

Table 5 Coefficients of Eq.（29）

参数	参数值
a₁	1.5854×10^-1
a₂	1.5835×10^-1
a₃	-1.6440
a₄	6.6315×10^-4
a₅	4.3176
a₆	1.3558×10^-2

Table 6 Ratio of mean ionic activity coefficients of methionine in the presence of KCl aqueous solution to that in water at different molalities of KCl and methionine

m_A/(mol/kg)	m_KCl/(mol/kg)
m_A/(mol/kg)	0.1	0.3	0.5	0.7	1.0
0.025	1.0093	1.0330	1.0641	1.1031	1.1784
0.050	1.0035	1.0153	1.0338	1.0595	1.1125
0.100	0.9967	0.9950	0.9998	1.0111	1.0408
0.150	0.9965	0.9944	0.9987	1.0097	1.0389
0.200	1.0027	1.0132	1.0305	1.0550	1.1064

Fig.4 Effect of methionine molality on γA(2)/γA(1)

Fig.5 Effect of KCl molality on γA(2)/γA(1)

Table 7 Parameters of DH model

i	A_i	$- l g K_{i}^{0}$	S.E.
1	0.2095	2.437	0.019
2	0.2380	9.339	0.0074

Table 7 Parameters of DH model

i	A_i	$- l g K_{i}^{0}$	S.E.
1	0.2095	2.437	0.019
2	0.2380	9.339	0.0074

Table 8 Pitzer coefficients for methionine species in KCl media at different temperatures

T/K	Species	$l n K_{i}^{}$	$β_{i}^{0}$	$β_{i}^{1}$	$C_{i}$	S.E.
298.15	H₂B⁺	-4.939	0.1009	9.316	0.03460	0.0019
303.15	H₂B⁺	-4.912	0.1340	9.199	0.03002	0.0077
308.15	H₂B⁺	-4.931	0.1040	9.348	0.0348	0.00169
298.15	B^-	-22.10	0.1530	-5.810	0.05501	0.0110
303.15	B^-	-21.82	0.1594	-5.807	0.05449	0.0125
308.15	B^-	-21.55	0.1593	-5.771	0.05533	0.0128

Table 8 Pitzer coefficients for methionine species in KCl media at different temperatures

T/K	Species	$l n K_{i}^{}$	$β_{i}^{0}$	$β_{i}^{1}$	$C_{i}$	S.E.
298.15	H₂B⁺	-4.939	0.1009	9.316	0.03460	0.0019
303.15	H₂B⁺	-4.912	0.1340	9.199	0.03002	0.0077
308.15	H₂B⁺	-4.931	0.1040	9.348	0.0348	0.00169
298.15	B^-	-22.10	0.1530	-5.810	0.05501	0.0110
303.15	B^-	-21.82	0.1594	-5.807	0.05449	0.0125
308.15	B^-	-21.55	0.1593	-5.771	0.05533	0.0128

Fig.6 Deviations between the measured and calculated pKi*

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