Eluents benzoic acid

The concentrations of benzoic acid, aspartame, caffeine, and saccharin in a variety of beverages are determined in this experiment. A Gig column and a mobile phase of 80% v/v acetic acid (pH = 4.2) and 20% v/v methanol are used to effect the separation. A UV detector set to 254 nm is used to measure the eluent s absorbance. The ability to adjust retention times by changing the mobile phase s pH is also explored. 

The influence of NH., and CO, on the chromatographic behaviour of benzoic acid and its derivatives (o-, m-, p-hydroxybenzoic, nitrobenzoic, aminobenzoic, chlorobenzoic acids) was studied. The work was carried out by means of upgoing TLC on Sorbfil plates. Isopropanol- and ethyl acetate-containing water-organic eluents were used as mobile phases in the absence or presence of gaseous modifiers in the MP. The novel modification of TLC has been found to separate benzoic acids with different values of their dissociation constants more effectively than water-organic mobile phases. 

Tezuka s group (Tezuka and Ando, 1985 Tezuka et al., 1986) was able to isolate and characterize the benzenediazo ether of 1-naphthol (6.10). They stirred a solid mixture of the molecular complex 6.9 formed between an a-azohydroperoxide acid and benzene with an excess of 1-naphthol at room temperature in the dark for several hours. The separation of this solid by thin layer chromatography (silica gel, with a benzene-ethyl acetate mixture [9 1] as eluent) afforded the diazo ether 6.10 as a yellow oil in 17 % yield, together with 4- and 2-phenylazo-l-naphthol (6.11 and 6.12, 4% and 42%, respectively), 4-phenylbenzaldehyde (32%), benzoic acid (23%), and traces of other compounds (Scheme 6-6). Higher yields of the diazo ether (up... 

Retention volumes of monosubstituted benzenes, benzoic acid, phenols, and anilines have been measured in RPLC [76]. Buffered acetonitrile/water and tetrahydrofuran/water eluents were used with an octadecylsilica adsorbent. From the net retention volumes, a substituent interaction effect was calculated and described with the linear free energy relationship developed by Taft. The data was interpreted in terms of hydrogen bonding between the solutes and the eluent. 

Benzoic acid, SA, and EsHBA can be also well separated using an isocratic elution. After separation on a Sep-Pak C)g cartridge, the sample was injected on a Rhodex RSpak DS-613 column containing a rigid polystyrene-divinylbenzene resin. An eluent, 0.05 M potassium dihydrogen phosphate-acetonitrile (60 40), was directed to the UV detector, which operated at 230 nm (2,55). 

Diphenylchlorophosphate (0.3 mmol) was added dropwise to solution of benzoic acid (0.3 mmol) and triethylamine (0.3 mmol) dissolved in 1 ml dry CH2C12, then stirred 60 minutes at ambient temperature. This mixture was treated with the Step 5 product (0.3 mmol) and triethylamine (0.6 mmol) dissolved in 0.5 ml in dry CH2C12 and stirred overnight at ambient temperature. The solution was diluted with 2 ml CHC13, then washed with 2 ml 5M NaOH, and concentrated. The residue was dissolved in 5 ml methyl alcohol and purified by HPLC on a C18 silica gel column using acetonitrile/water gradients containing 0.05% trifluoroacetic acid as eluent and the product isolated. 

Scenario 4. Buffered eluents must be used when analyzing ionizable species, lonizable species are prone to solvation by the mobile-phase components and the solvation equilbira may lead to poor peak shapes. In Figures 8-12A and 8-12B, two acidic compounds, benzoic acid (pKa 4.2) and sorbic acid (pKa 4.8), are analyzed at pH 3.5 (a pH lower than the analyte pKa) and at pH 7.0 (a pH greater than the analyte pKa). Acceptable peak shapes are obtained at both... 

The concept of employing free acids as eluents dates back to the investigations of Fritz et al. [67]. In 1981, they already demonstrated that free benzoic acid may be employed as eluent instead of the sodium or potassium salts that were utilized until then. This is understandable, taking into account that benzoic acid is partially dissociated in aqueous solution. The degree of dissociation may be calculated from the dissociation constant Ka according to the Ostwald dilution law [54] ... 

With benzoic acid as the eluent, generally higher sensitivities are obtained for the analyte anions in comparison to sodium or potassium benzoate, respectively. This is illustrated in the following reaction scheme ... 

Eluents based on sodium benzoate or benzoic acid, respectively, exhibit about the same elution power as bicarbonate and are thus used for the analysis of monovalent anions. When looking for less strongly adsorbing acids, Fritz, DuVal, and Barron [68] found other compounds that may also be employed as eluents. The properties of these compounds, that are important in this connection, are listed in Table 3-17. 

If this equilibrium is disturbed by a sample injection, a new equilibrium is established via relaxation i.e., the kind of relaxation process depends on the pH value of the sample injected. If the sample pH is lower than the pH value of the mobile phase, benzoate ions in the mobile phase are protonated due to the sample injection. Thus, the concentration of molecular benzoic acid in the mobile phase increases as does the concentration of the amount adsorbed to the stationary phase. The amount not adsorbed travels through the column and appears as a chromatographic signal the system peak. A qualitatively similar chromatogram is obtained when a sample containing the solute ions and the corresponding eluent component is injected into the system. However, only the position of the system peak is comparable, not its area and direction. 

Fig. 3-68. Equilibrium processes between the stationary phase of an anion exchanger and benzoic acid as an example of a weak organic acid that acts as an eluent.

Fig. 3-86. Comparison between the retention behavior of inorganic anions and several organic carboxylic acids, respectively. - Separator column IonPac AS4 eluent 0.0028 mol/L NaHC03 + 0.0022 mol/L Na2C03 flow rate 1.6 mL/min detection suppressed conductivity injection 50 pL solute concentrations a) 1.5 ppm fluoride (1), 2 ppm chloride (2), 5 ppm orthophosphate (3) and bromide (4), 10 ppm nitrate (5), and 12.5 ppm sulfate (6), b) 5 ppm formic acid (7), 40 ppm benzoic acid (8), 20 ppm succinic acid (9), 10 ppm malonic acid (10), 20 ppm maleic acid (11), tartaric acid (12), and oxalic acid (13).

According to Tanaka and Fritz [9], comparably low background conductivities were obtained with benzoic acid as the eluent, because benzoic acid is only partly dissociated. However, the acid strength of a dilute benzoic acid solution is sufficient to elute aliphatic carboxylic acids with a good peak shape. 

Mobile and stationary phases need to harmonize with each other a certain eluent will not necessarily interact well with a given ion exchanger. Typical eluents for separations with electronic suppression are diluted phthalic or benzoic acid, perhaps with a low amount of acetone or methanol (to influence the selectivity) for anions, and nitric, oxalic, tartaric, citric or dipicolinic acid (the latter one for complex formation)... 

To a solution of imine (65 mg, 0.25 mmol), 2Sa (15 mg, 0.025 mmol), and benzoic acid (0.15 mg, 0.0125 mmol) in dichloromethane (1.0 ml) was added MVK (42 ml, 0.5 mmol) at room temperature. Then reaction mixture was stirred at room temperature for 10 h. After the reaction completed, the solvent was removed under reduced pressure and the residue was purified by a flash column chromatography (SiO2, eluent EtOAc/petroleum ether = 1/5) to afford 4 methyl N (2 methylene 3 oxo 1 phenylbutyl) benzenesulfonamide as a color less solid. 

For non-suppressed ion chromatography to be successful, the ion exchanger used in the separation column must have a low exchange capacity and a very dilute eluent must be used. In the separation of anions, the resin must have an exchange capacity between about 0.005 mequiv/g and 0.10 mequiv/g. Typical eluents are 1.0 x 10 M solutions of sodium or potassium salts of benzoic acid, hydroxybenzoic acid, or phtha-lic acid. These eluents are sufficiently dilute that the background conductivity is quite... 

Figure 3.3. Separation of five anions on three different resins of similar capacity. The resins were packed in a 500 x 2.0 mm i.d. column and a solution of 0.0001 M benzoic acid was used as the eluent at a flow rate of 0.93 mL/min (From Ref. [8], with permission].

Lithium, sodium, potassium, or other salts of benzoic acid, phthalic acid, sulfoben-zoic acid, citric acid, and others are useful eluents for anions. These are rather large organic anions that are less mobile than most inorganic anions and therefore have lower equivalent conductances. For example. Table 4.1 shows that the benzoate anion has a limiting equivalent eonductance of 32 S cm equiv, while ehloride, nitrate, sulfate, and other typieal sample anions have higher equivalent conductances (approximately 70 S cm equiv ). If a sodium benzoate eluent is used, the equivalent conductance is the sum of sodium ion (50) and benzoate (32), or 85 S cm equiv". The equivalent conductance of an anion is the sum of equivalent eonductances of the sodium ion (50) and the anion (70), or 120 S cm equiv. On an equivalent basis, this amounts to almost a 50 % increase in conductance. 

The detection sensitivity in NSIC can be improved markedly if the acidic form of benzoic acid is used, rather than the alkali metal salt, in the eluent. Gjerde and Fritz... 

X 10 M solution of benzoic acid is 20% ionized. Thus this eluent has a hydrogen ion and a benzoate ion concentration each of 2.50 x 10 M. Comparison of this benzoic acid solution with an eluent containing 2.50 x 10 potassium benzoate shows that the two eluents give very similar retention times for inorganic anions (see Table 6.7). 

Table 6.7. Comparison of anion retention times with benzoic acid and potassium benzoate eluents that have the same benzoate ion concentrations. (From Ref. 113]).

Thus the concentration of anion A in an elution peak reduces the benzoate concentration by an equivalent amount. However, the eluent equilibrium is dynamic, and it is always 20 % ionized. This means that 80 % of the hydrogen and benzoate ions in the exchange reaction come from molecular benzoic acid and are converted into highly ionized H A. This effect enhances the detection sensitivity considerably because the counterion of the A has an unusually high equivalent conductance. A discussion of the phenomenon is given in Section 6.3.3.6. 

Several other organic acids have been investigated as eluents in ion chromatography [14]. One objective was to find an eluent acid that is not adsorbed by the resin matrix and that attains equilibrium faster than benzoic acid. 

Calculation of the expected conductance change with a benzoic acid eluent is accomplished with Eq. 6.10. An 8.40 x 10 M benzoic acid eluent is calculated to be... 

Tanaka and Fritz [4] found that the conductivity detection of aliphatic carboxylic acids is much improved if a dilute solution of benzoic acid is used as the eluent. The background conductance is much lower than with aqueous solutions of strong mineral acids. A fairly low background conductance is obtained because benzoic acid is only partly ionized and a very dilute (5.0 mM) solution of benzoic acid is used. Nevertheless, this eluent is sufficiently acidic to give a good separation of aliphatic carboxylic acids with well-shaped peaks. 

In a dried vial 1 (2 equiv., 1 mmol), benzoic acid (0.025 equiv) and aldehyde (1 equiv., 0.5 mmol) were mixed and stirred at 20 °C. The reaction was quenched with water (2 mL) and the organic layer extracted in Et20 (3x5 mL). The crude product was desilylated by Kruger and Carreira s procedure and the resulting mixture purified by silica gel chromatography (eluent CHC13-Et20 9/1). 

Fig. 4.4.14. Separation of sorbic acid (1) and benzoic acid (2) on a /iBondapak C,g column (0.4x 30 cm) using three various eluents as stated. Reprinted from Ref. 26 with permission.

Figure 6.22 The relationship between predicted (fcpred) and measured (femes) capacity ratios for benzoic acid derivatives in eluents of various pH.

Some new phenacyl labels [104] for carboxylic acids have been synthesized recently, and l-(4-hydroxy-phenyl)-2-bromoethanone (4-HBE) was found to be a useful UV derivatizing agent. 4-HBE reacted with aliphatic and aromatic adds. A 4 2 1 molar ratio of 4-HBE, triethylamine and organic add was used. The reaction was carried out at 80 °C for 1 h with 76% conversion. HPLC separation was carried out on an Ultrasphere RP-18 column with methanol/water (60 40 v/v) as eluent. Detection was at 289 nm, which avoided most of the interference found at lower wavelengths. The detection limit was 1 ng for the derivative of benzoic acid. 

Table 7 Dependence of eluent pH on LC separation factors of benzoic acid versus benzoic acic ...

Synthesis of 4,4 -bis 6-[acryloyloxy)hexoxy]benzoate l,r-biphenyl (BABB-6). (3). TOs reaction was carried out under dry conditions as shown in Scheme 1. The 4,4 -bis 6-[acryloyloxy)hexoxy]benzoate)-l,r-biphenyl, 3, was prepared from the reaction of 4-(acryloyloxyhexyloxy)benzoic acid cMoride and 4,4 -biphenoL The crude was purified by flash chromatography using a mixture of methylene chloride/ethyl acetate (98 2) as the eluent The iR NMR is shown in Figure la. 

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