Dissociation constants of calcite and CaHC03+ from 0 to 50°C
Dissociation constants of calcite and CaHC03+ from 0 to 50°C
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DOI：
10.1016/0016-7037(74)90112-4
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发表时间：
1974-02
影响因子：
5
通讯作者：
R. Jacobson;D. Langmuir
中科院分区：
文献类型：
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作者：
R. Jacobson;D. Langmuir
电导测量表明，在0℃、25℃和60℃时，CaHCO3+离子对在±0.05单位内的解离常数pK(CaHCO3+)的负对数分别为0.7、1.0和1.35。重新评估已公布和未公布的数据得到25°C下的PK(CaCO30)≈3.2。使用这些PK根据已公布的方解石在纯水中的溶解度计算方解石的离解常数(Kc)，得到的pKc值随着离子强度的增加而显著增加。然而，如果忽略离子对，计算的pKc值与离子强度几乎是恒定的。所有通过调整离子活度系数和/或K(CaCO30)值来消除pKc趋势的合理尝试都失败了，因此两难境地仍然存在。从最可靠的方解石溶解度数据计算的Kc值与基于本研究中测量的5°C、15°C、35°C和50°C的溶解度数据计算的Kc值非常一致。研究结果可以用方程logKc=13.870-(3059/T)−0.04035T准确地表示，并分别对应于0、25和50°C下的−8.35、−8.42和−8.635。对于25°C下的方解石分解反应，其对数表达式为：ΔHRO=−2420±300卡/摩尔，ΔCP=−110±2卡/度摩尔，ΔSRO=−46.6±1.0卡/度摩尔，当假设CaCO30和CaHCO3+时，K随温度的变化由logKc=13.543−(3000/T)−0.0401T描述，在0，25和50°C时得到−8.39，−8.47和-8.70。这给出了25°C下ΔHRO=−2585±300卡/度，ΔCP=−109±2卡/度摩尔，ΔSr0=−47.4±1.0卡/度摩尔。
From conductance measurements, the negative logarithm of the dissociation constant of the CaHCO3+ion pair, pK(CaHCO3+), is 0.7, 1.0 and 1.35 within ±0.05 units at 0, 25 and 60°C, respectively. A revaluation of published and unpublished data yields pK(CaCO30) ≈ 3.2 at 25°C. Use of these pK's to compute the dissociation constant of calcite (Kc) from published calcite solubility measurements in pure water gives pKcvalues which increase markedly with ionic strength. However, if the ion pairs are ignored, computed pKcvalues are nearly constant with ionic strength. All reasonable attempts to eliminate the trend in pKcby adjusting ion activity coefficients, and/or values ofK(CaCO30) failed, so the dilemma remains.Kcvalues computed from the most reliable published calcite solubility data are in good agreement with such values based on solubility data measured in this study at 5, 15, 35 and 50°C. Study results ignoring ion pairs are accurately represented by the equation logKc= 13.870 — (3059/T) −0.04035T, and correspond to −8.35, −8.42, and −8.635 at 0, 25 and 50°C, respectively. The logarithmic expression leads toΔHro= −2420 ± 300 cal/mol,ΔCp= −110 ± 2 cal/deg mol, andΔSro= −46.6 ± 1.0 cal/deg mol for the calcite dissociation reaction at 25°C. The dependence ofKcon temperature when CaCO30and CaHCO3+are assumed, is described by logKc= 13.543 − (3000/T) − 0.0401Twhich yields −8.39, −8.47, and -8.70 at 0, 25 and 50°C. This givesΔHro= −2585 ± 300 cal/mol,ΔCp= −109 ± 2 cal/deg mol, andΔSr0= −47.4 ± 1.0 cal/deg mol at 25°C.