2-ethylhexanoic acid, molybdenum salt

Dissociation constant

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

dissociation constant

Remarks:

dissociation of salts into individual ions

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with generally accepted scientific standards and described in sufficient detail

Qualifier:

no guideline followed

Principles of method if other than guideline:

zero-sink test that measures the dissociation of the metal carboxylate substance in two contrasting test media (low and neutral pH) based on the free metal ion concentration in the presence of a selective adsorbent that acts as a zero-sink for the metal cation

GLP compliance:

no

Dissociating properties:

yes

No.:

#1

Temp.:

20 °C

Remarks on result:

other: 98±<1% dissociation after 24h in simulated gastric fluid (pH 1.5)

No.:

#2

Temp.:

20 °C

Remarks on result:

other: > 100% dissociation after 1h in simulated interstitial fluid (pH 7.4)

Simulated gastric fluid 

Solution composition

The measured Mo concentration of the reference compound and MC in the gastric medium is given in Table 5. The percentage dissolved material of the MC at 1 g MC/L is low (5% on average). The reference compound is almost completely dissolved in the gastric fluid. In acidic conditions, the dissociation reactions of the MC are expected to be slower (Eqn. 6). The low Mo concentration in the MC might also be due to precipitation of Mo compounds, i.e. in the MC the Mo is expected to be present as Mo(IV) and MoO2 or unknown precipitates may form when Mo(IV) is oxidizing before it is Mo(VI). 

Table 5 Measured total Mo concentration (CMo) dissolved during 12h in the simulated gastric fluid and percent solubility (fsol) ± standard deviation. The nominal concentrations are the expected concentrations in solution based on the metal content of the compound and using the weighted mass of the compounds and volume of stock solution used to prepare the test solutions. 

	Measured	Nominal
Compound	CMo	CMo	fsol
	μM	μM	%
Reference salt
Na2MoO4.2H2O	27.6 ± 0.1	33.1	83 ± <1
	275 ± 4	331	83 ± 1
MC
(C8H15O2)4Mo	71.0 ± 1.2	1315	5 ± <1

Free metal ion concentration measurement 

For the reference salt (Na2MoO4.2H2O) the Mo concentration in solution decreased to < 2% of the initial concentration after 1h due to sorption of Mo on the adsorbent (Table 6). The measured Mo concentration in all diluted test solutions is above the quantification limit (LOQ) of the ICP-MS analysis. The Mo concentration in solution is the Mo (H2MoO4 or MoO3) in equilibrium with the adsorbent. Binding of the Mo to the adsorbent is almost instantaneous, the Mo concentration in solution CMo,e only slightly further decreased after 1h and thus the sorbed Mo concentration CMo,s on the adsorbent is almost constant after 1h. 

The initial Mo concentration of the MC after 12h dissolution in gastric medium, but before addition of the zero sink adsorbent is in the concentration range of the reference samples. The Mo concentration in solution of the MC decreased to about 45% of the initial concentration after 1h due to sorption of Mo on the adsorbent. The MC undergoes a dissociation reaction and exists in equilibrium with its dissociation products. The dissociated metal reacts with the adsorbent. The Mo concentration in solution of the MC is thus, in theory, the free Mo (present as MoO2, or oxidized H2MoO4, MoO3) in equilibrium with the zero-sink and undissociated MC. The Mo concentration in equilibrium with the adsorbent is calculated from the reference compounds and is after 1h only 0.38 ± 0.01 μM Mo. This is wel below to the total measured Mo concentration in solution of 32.3 ± 1.2 μM Mo. After 1h, the calculated dissociated fraction of metal carboxylate is thus only 55±2%. After 24h, the Mo concentration in solution is 1.30 ± 0.10 μM Mo, which approaches the expected Mo concentration based on equilibrium with the zero-sink. After 24h, the dissociated fraction thus increased to 98 ± <1%. Therefore, this test indicates that the MC dissociates fully after 24, but not instantaneously in the test media representing gastric fluid. The dissociation reaction is expected to be slower at low pH in agreement with the proposed reaction. 

Table 6 Measured Mo concentration in solution at the start (CMo,i) and after equilibration (CMo,e) and calculated concentration sorbed on the zero sink (CMo,s) ± standard deviation at different time points for the reference salts and the MC dissolved in the gastric medium. For the MC, the metal concentration in equilibrium with the zero-sink CMo is calculated from the reference compounds and used to calculate the dissociated fraction (fdiss). 

 

		Measured	Measured	Calculated	Calculated
Compound	time	CMo,i	CMo,e	CMo,s	CMo	fdiss
	h	μM	μM	μmol/g	μM	%
Reference salt	0	27.6 ± 0.1
Na2MoO4.2H2O	1		0.46 ± 0.22	2.72 ± 0.04
	6		0.05 ± <0.01	2.76 ± 0.01
	24		0.12 ± 0.01	2.75 ± 0.02
Reference salt	0	275 ± 4
Na2MoO4.2H2O	1		1.58 ± 0.04	27.3 ± 0.4
	6		0.47 ± 0.07	27.5 ± 0.4
	24		0.40 ± 0.07	27.5 ± 0.4
MC	0	71.0 ± 1.2
(C8H15O2)4Mo	1		32.3 ± 1.2	3.87 ± 0.23	0.38 ± 0.01	55±2
	6		12.3 ± 0.9	5.87 ± 0.20	0.10 ± <0.01	83±1
	24		1.30 ± 0.10	6.97 ± 0.12	0.14 ± <0.01	98 ± <1

Simulated interstitial fluid 

Solution composition 

The measured Mo concentration of the reference compound and MC in the interstitial fluid is given in Table 7. The percentage dissolved material of the MC at 1 g compound/L is low (11% on average), but higher than the solubility in pure water or than in the gastric fluid. The reference compound is completely dissolved in the interstitial fluid. 

Table 7 Measured total Mo concentration (CMo) dissolved during 12h in the simulated interstitial fluid and percent solubility (fsol) ± standard deviation. For the MC, the nominal concentrations are the expected concentrations in solution based on the metal content of the compound and using the weighted mass of the compounds and volume of stock solution used to prepare the test solutions. 

	Measured	Nominal
Compound	CMo	CMo	fsol
	μM	μM	%
Reference salt
Na2MoO4.2H2O	174 ±5	161	108 ± 3
	2178 ± 28	2011	108 ± 1
MC
(C8H15O2)4Mo	145 ± 6	1315	11 ± <1

Free metal ion concentration measurement 

For the reference salt (Na2MoO4.2H2O) the Mo concentration in solution decreased to < 65% of the initial concentration after 1h due to sorption of the Mo oxyanion MoO42- on the adsorbent (Table 8). The Mo concentration in solution in the reference salt is the MoO42- concentration in equilibrium with the adsorbent. Binding of Mo to the adsorbent is almost instantaneous since relatively small differences are observed between measurements at 1h, 6h and 24h. The relatively high Mo concentration in solution after equilibrium does indicate that the binding of Mo to the adsorbent is not very strong. 

The initial Mo concentration of the MC after 12h dissolution in simulated interstitial fluid, but before addition of the zero sink adsorbent is slightly below the concentration of the lowest reference sample. The Mo in solution of the MC is expected to be present as MoO2, or after oxidation as MoO42- oxyanion as in the reference compound (Eqn. 6 and 7). The Mo concentration in solution of the MC considerably decreased to about 21% of the initial concentration after 1h due to sorption of the Mo on the adsorbent. The Mo concentration in solution of the MC is thus, in theory, the Mo in equilibrium with the zero-sink and undissociated MC. The predicted Mo concentration in equilibrium with the adsorbent after 1h is 131 ± 13 μM Mo, this is higher than the measured Mo concentration of 31 ± 2 μM Mo, which indicates that the Mo concentration in equilibrium with the adsorbent is overestimated in the MC sample based on the reference samples. A possible explanation for this is that the pH in the MC samples has become lower than in the reference samples due to dissociation and possibly oxidation of the MC (see Eqn. 6 and 7), at lower pH a stronger sorption of Mo to the adsorbent is expected (e.g. due to increased positive charge on the Ti oxide adsorbent and less competition for sorption on the Ti oxide with hydroxyl ions at lower pH). We hypothesize that the high predicted dissociated fraction points to extensive dissociation in the interstitial fluid, but that accurate calculation of the dissociated fraction would require better correspondence in pH between the reference salt and the MC. Based on the dissociation reaction (Eqn.6), a faster dissociation is expected in pH neutral media. The calculated dissociated fraction remained relatively constant between 1h and 24h. Therefore, the results suggests that the dissociation of the MC in the test media representing interstitial fluid is fairly instantaneously and is complete (fdiss ≥ 149%). 

Table 8 Measured Mo concentration in solution at the start (CMo,i) and after equilibration (CMo,e) and concentration sorbed on the zero sink (CMo,s) ± standard deviation at different time points for the reference salt and the MC dissolved in the interstitial medium. For the MC, the Mo concentration in equilibrium with the zero-sink CMo is calculated from the reference compounds and used to calculate the dissociated fraction (fdiss). 

		Measured	Calculated	Calculated
Compound	time	CMo,i	CMo,e	CMo,s	CMo	fdiss
	h	μM	μM	μmol/g	μM	%
Reference salt	0	174 ±5
Na2MoO4.2H2O	1		70 ± <1	5.2 ± 0.2
	6		48 ± 2	6.3 ± 0.2
	24		37 ± 3	6.9 ± 0.1
Reference salt	0	2178 ± 28
Na2MoO4.2H2O	1		1383 ± 8	39.8 ± 1.0
	6		1309 ± 11	43.4 ± 0.8
	24		1254 ± 23	46.2 ± 0.3
MC	0	145 ± 6
(C8H15O2)4Mo	1		31 ± 2	5.72 ± 0.36	131 ± 13	169±8
	6		19 ± 3	6.33 ± 0.41	103 ± 13	158±9
	24		14 ± 2	6.59± 0.38	85 ± 11	149 ± 7

Conclusions:

The dissociation of 2-ethylhexanoic acid, molybdenum salt is complete in after 24h, i.e. fdiss = 98 ± <1 % in the gastric medium (pH 1.5) and fdiss 149 ± 7 % in interstitial medium (pH 7.4). The dissociation is instantaneous (< 1h) in the interstitial medium, but not instantaneous in the gastric medium. For the zero-sink test with the interstitial medium the results would benefit from better correspondence in pH between the reference samples and the MC, however, we expect that the conclusion would still be that the dissociation in this pH neutral test medium is instantaneous and complete.

Reference 2

Endpoint:

dissociation constant

Remarks:

dissociation of salts into individual ions

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

test procedure in accordance with generally accepted scientific standards and described in sufficient detail

Qualifier:

according to guideline

Guideline:

OECD Guideline 112 (Dissociation Constants in Water)

GLP compliance:

no

Dissociating properties:

yes

No.:

#1

Temp.:

25 °C

Remarks on result:

other: ≥70±3% dissociation after 24h in water

Solution composition 

The measured concentration of Mo and organic carbon (OC) in solution of the metal carboxylate and reference compounds prepared at 1 g/L are given in Table 2. Both reference compounds (Na2MoO4.2H2O and NaC8H16O2) are completely dissolved after 1h (fsol ≥ 100%) and the Mo or OC concentration in the reference samples remained constant in time (no precipitation). 

The Mo and OC concentration in solution of the metal carboxylate increased over time which means that the dissolution is not instantaneously (Table 2). The measured OC is high compared to the Mo concentration. The OC concentration after 24h (260 mg OC/L or 22 mM OC) is equivalent to a dissolved ligand concentration of 2.8 mM C8H15O2 and the total Mo in solution should be around 770 μM (based on the metal content of 15.77%, details of calculation not shown) which is by far not observed. The measured OC concentration of 22 mM OC is realistic given that the solubility of 2-ethylhexanoic acid is 1.4 g/L, which corresponds with 78 mM OC. The low Mo concentration suggest precipitation of Mo compounds, the Mo is expected to be present initially as Mo(IV) and MoO2 or unknown precipitates may form when Mo(IV) is oxidizing before it is Mo(VI). This is different for the Mo reference salt, because Mo is present initially in the oxidized Mo(VI) as soluble MoO42- (Table 2). The low Mo concentration in water after 24h is in agreement with previous work. The funder of this work provided a study performed by the Frauenhofer institute in which they refer to study EBR-093/7-80 where the solubility of this product in water was equivalent to a Mo concentration of 30 μM after 24h (consistent with Table 2), increasing to 139 μM after 16 days (Mo concentrations measured with ICP-OES and with UV-VIS that was calibrated with Na2MoO4). The Mo solubility in the acid gastric fluid and in the interstitial fluid is higher (70-140 μM Mo, see Tables 5 and 7) after only 12h compared to that after 24h in water (31 μM Mo). It is possible that Mo formed complexes with constituents present in the gastric and interstitial media, for instance Mo chlorides or oxychlorides, that increased the solubility of the metal carboxylate in these test media compared to pure water. 

Multi-element analysis with ICP-MS on the subsamples taken after 24h showed that the metal carboxylate solution contained 0.2 ± 0.04 mM Na, analysis of Na was not done in the subsamples taken after 1h and 6h. 

Table 2 Measured total concentration Mo (CMo,T) and organic carbon (COC,T) at 1 g/L and percent solubility (fsol) ± standard deviation at different samplings. The nominal concentrations are the expected concentrations in solution based on the metal content of the compound and using the exact weighted mass of the compounds in the test solution, the nominal organic carbon concentration is calculated assuming that the carboxylate content of the compound is 84,23% (i.e. 100%-reported metal content in the metal carboxylate). The percent solubility is calculated based on the metal and/or organic carbon concentrations. 

		Measured	Measured	Nominal	Nominal
Compound	time	CMo,T	COC,T	CMo,T	COC,T	fsol,Mo	fsol,OC
	h	μM	μM	μM	μM	%	%
Reference salt
Na2MoO4.2H2O	1	4124 ± 91		4136 ± 52		100 ± 1
	6	4250 ± 201				103 ± 4
	24	4347 ± 112				105 ± 1
Reference salt
NaC8H16O2	1		50237 ± 3737		48994 ± 2722		102 ± 2
	6		49336 ± 2176				101 ± 1
	24		49562 ± 2979				101 ± <1
MC (metal carboxylate)
(C8H15O2)4Mo	1	2.9 ± 0.3	3026 ± 399	1631 ± 4	46424 ± 125	< 1%	7 ± 3
	6	4.0 ± 0.6	7208 ± 962			< 1%	16 ± 8
	24	31.2 ± 2.4	22027 ± 1335			2 ± <1	47 ± 11

Electrical conductivity measurements 

The measured electrical conductivity for the metal carboxylate and reference compounds in function of time is given in Table 3. The dissociated fraction is calculated from the measured electrical conductivity. 

For the metal carboxylate, if only considering the dissociation and hydrolysis reaction, than the ions contributing to the conductivity are: RCOO- and H+, i.e. the electrical conductivity (σ) of the metal carboxylate is given by: 

σ = (F2RT)(D𝑅𝐶𝑂𝑂-zRCOO-2CRCOO-+D𝐻+zH+2CH+) (Eqn. 11) 

From the reaction stoichiometry follows that the deprotonated carboxylate concentration (CRCOO-) equals the proton concentration (CH+), thus the deprotonated carboxylate concentration can be calculated using the diffusion coefficients of the ions in solution and the measured σ based on Eqn.11. 

For the electrical conductivity σ (μS/cm) measured at 24h, at 25°C the constant F2/(RT) = 3.7554 106 sS/mol: 

CRCOO-=σ(DRCOO-+DH+) (RTF2)10-7 = 0.241 ± 0.002 mM (Eqn. 12) 

This means that the pH of the solution is expected to be 3.62 (CH+=0.241 mM), which is in agreement with the measured pH of 3.80. At pH 3.62, the largest fraction of the ligand is protonated, the ratio of protonated/deprotonated ligand is about 14 based on the pKa value of 2-ethylhexanoic acid (pKa = 4.76). The total predicted ligand concentration CL,diss derived from the conductivity measurement (protonated + deprotonated ligand) is thus equal to 3.582 ± 0.054 mM (Table 3), from which 0.241 ± 0.002 mM is anionic. The expected Mo concentration in solution is thus equal to 3.582/4 = 0.859 mM, the measured Mo concentration in solution (0.033 mM, Table 3) is much lower than calculated, which points to precipitation of Mo compounds. The total ligand concentration calculated from the measured conductivity CL,diss is higher than the total measured ligand concentration in solution (2.635 mM), which suggests that no additional undissociated metal carboxylate is present after 24h. The calculations for all time points are given in Table 3. Also for these time points good agreement between calculated and measured pH was found (after 1h, measured pH 4.35 and calculated pH 4.04; after 6h, measured pH 4.18 and calculated pH 3.84). 

If we consider subsequently oxidation of Mo, the ions in solution contributing to the conductivity are RCOO-, H+and MoO42-. Again based on the conductivity measurement after 24h, this would mean that the deprotonated ligand concentration RCOO- is equal to 0.162 ± 0.001 mM (Table 3) and the pH equal to 3.62. Based on the calculated pH and the pKa of 2-ethylhexanoic acid, the carboxylate ligand concentration (protonated + deprotonated) is equal to 2.342 ± 0.035 mM, this value is slightly below the measured total ligand concentration in solution of 2.755 ± 0.170 mM, which suggest that a small amount of undissociated metal carboxylate product is present after 24h, i.e. the dissociated fraction fdiss = 85 ± 4%. The calculations for the other time points are given in Table 3. Complete oxidation is however unlikely, because the concentration of Mo, which is present as the MoO42-ion in solution, would have been (2.342)/4 = 0.586 mM, which is by far not observed. Thus, in water presumably reaction 6 occurs and partly reaction 7. 

A final remark is that the test solution contained after 24h 0.20 ± 0.04 mM Na that will also contribute to the conductivity measurement. If for simplicity assumed that it is present as NaCl, than this contributes importantly on the measured conductivity, the conductivity of 0.20 mM NaCl is about 25 μS/cm. With correction for the contribution of NaCl, the dissociated fraction of metal carboxylate would be 16 ±3 % after 1h, 45 ± 4% after 6h and 70 ± 3 % after 24h (considering no oxidation, scenario of Eqn.6), which might be more realistic in terms of dissociation kinetics and extend compared to the reported values without NaCl correction in Table 3. However, more data is necessary to justify this correction (measurement of Cl concentrations and of Na concentration also in the samples taken after 1h and 6h). 

The dissociated fraction of reference compounds is expected to approach 100%, the deviation for both reference compounds (<100% for Na2MoO4.2H2O and >100% for NaC8H16O2) might be related to inaccuracies of the model. 

Table 3 Measured electrical conductivity (σ) and concentration of metal (CM,T) and carboxylate ligand (CL,T) of reference salts and metal carboxylate (in duplicate) at different points in time. For the Na2MoO4.2H2O reference salt, Ci is the dissociated metal concentration calculated from the conductivity data and fdiss the dissociated metal fraction (fdiss) ± standard deviation. For the metal carboxylate and NaC8H16O2 reference salt, the Ci is the deprotonated carboxylate ligand concentration calculated from the conductivity data. Because of the low pH, for the metal carboxylate also the protonated ligand concentration is taken into account to obtain fdiss, CL,diss is the total ligand concentration (protonated + deprotonated).The dissociated fraction of MC is calculated for two possible scenarios no oxidation (Eqn. 6) or complete oxidation (Eqn.8). 

 

Compound		Measured	Measured	Measured	Measured	Measured	Measured	Calculated	Calculated	Calculated
		Rep. 1	Rep. 1	Rep. 1	Rep. 2	Rep. 2	Rep. 2
	time	σ	CM,T	CL,T	σ	CM,T	CL,T	Ci	CL,diss	fdiss
	h	μS/cm	mM	mM	μS/cm	mM	mM	mM	mM	%
Reference salt
Na2MoO4.2H2O	1	980	4.06		1027	4.19		4.03 ± 0.13		98 ± 1
	6	993	4.11		1029	4.39		4.06 ± 0.10		95 ± 2
	24	960	4.27		992	4.43		3.92 ± 0.09		90 ± 1
Reference salt
NaC8H16O2	1	517		5.95	551		6.61	7.03 ± 0.32		112 ± 3
	6	519		5.97	553		6.36	7.05 ± 0.32		114 ± < 1
	24	503		5.93	535		6.46	6.83 ± 0.20		110 ± 2
MC
(C8H15O2)4Mo	1	33	0.003	0.343	35	0.003	0.375	0.091 ± 0.004	0.562 ± 0.004	155 ± 2
(Eqn. 6)	6	51	0.004	0.816	56	0.004	1.000	0.142 ± 0.009	1.312 ± 0.009	145 ± 3
	24	90	0.033	2.635	91	0.030	2.875	0.241 ± 0.002	3.582 ± 0.002	130 ± 6
MC
(C8H15O2)4Mo	1	33	0.003	0.343	35	0.003	0.375	0.061 ± 0.003	0.368 ± 0.028	103± 1
(Eqn. 8)	6	51	0.004	0.816	56	0.004	1.000	0.096 ± 0.006	0.859 ± 0.107	95 ± 2
	24	90	0.033	2.635	91	0.030	2.875	0.162 ± 0.001	2.342 ± 0.035	85 ± 4

Conclusions:

The results suggest that the dissociation of 2-ethylhexanoic acid, molybdenum salt is (almost) complete after 24h, i.e., the dissociated fraction is ≥70 ± 3 % in water.
The results of the conductivity test have to be interpreted with caution since assumption were made on the oxidation number of the Mo in the metal carboxylate (Mo4+), which has a consequence for the proposed chemical reactions that take place. To limit the uncertainty, different possible reactions were considered in the calculations. In addition, it is assumed that Na measured after 24h is present as NaCl and results are corrected for the contribution of NaCl to the measured conductivity. More data is necessary to fully justify this correction (measurement of Cl concentrations and of Na concentration also in the samples taken after 1h and 6h). Therefore, current results for the electrical conductivity test can be considered as a conservative (low) estimate of the dissociated fraction.

Description of key information

The results suggest that the dissociation of 2-ethylhexanoic acid, molybdenum salt is (almost) complete in all the test media after 24h, i.e., the dissociated fraction is ≥70 ± 3 % in water, 98 ± <1 % in the gastric medium (pH 1.5) and 149 ± 7 % in interstitial medium (pH 7.4), respectively. The dissociation is instantaneous (< 1h) in the interstitial medium, but not instantaneous in water or in the gastric medium. (Table 1). The results of the conductivity test have to be interpreted with caution since assumption were made on the oxidation number of the Mo in the metal carboxylate (Mo⁴⁺), which has a consequence for the proposed chemical reactions that take place. To limit the uncertainty, different possible reactions were considered in the calculations. In addition, it is assumed that Na measured after 24h is present as NaCl and results are corrected for the contribution of NaCl to the measured conductivity. More data is necessary to fully justify this correction (measurement of Cl concentrations and of Na concentration also in the samples taken after 1h and 6h). Therefore, current results for the electrical conductivity test can be considered as a conservative (low) estimate of the dissociated fraction.