Buffers, formic acid,

A little later, Russell et al.19 tried to obtain methanol from carbon dioxide by electrolysis. Reduction of carbon dioxide to formate ion took place in a neutral electrolyte at a mercury electrode. On the other hand, formic acid was reduced to methanol either in a perchloric acid solution at a lead electrode or in a buffered formic acid solution at a tin electrode. The largest faradaic efficiency for methanol formation from formic acid was ca. 12%, with poor reproducibility, after passing 1900 C in the perchloric acid solution at Pb in a very narrow potential region (-0.9 to -1.0 V versus SCE). In the buffered formic acid solution (0.25 M HCOOH + 0.1 M... 

In contrast to aqueous buffer, formic acid and glacial acetic acid caused the triazolo[4,3-c]pyrimidines to rearrange completely to their [1,5-c] counterparts. This merely seems to be a catalytic effect operative on intermediate (324). 

A column of Amberlite IR-120, 0.9 X 28 cm, held constant at 37°C and formic acid-pyridine buffers are employed. With a buffer formic acid-pyridine 0.2 N, pH 2.50-2.60, kynurenic, xanthurenic, and o-amino-hippuric acids are eluted successively from the column. By increasing molarity and pH, respectively, to 0.3 N and 4.20 there emerge kynurenine, 3-hydroxyanthranilic acid, and 3-hydroxykynurenine, which are collected automatically in fractions of 2 ml. Figure 3 gives an example of chromatographic separation. 

Buffer Formic acid-acetic acid-water (25 75 900) Component Solvent ... 

We have already mentioned some of the commonly used mobile phase additives for MS-compatible pH control. The most important attribute is the volatility of all buffer components. The most frequently used mobile phase additives are formic acid and acetic acid, together vhth the true buffers formic acid/ammonium formate and acetic acid/ammonium acetate. In the alkaline pH range, the preferred additive is ammonia, and the buffer of choice is ammonium hydrogencarbonate. One can also use the ammonium ion with volatile counterions such as formate or acetate to establish a true buffer at the pfQ of the ammonium ion (pKj = 9.24). One finds sometimes in the literature the reported use of ammonium formate or acetate at neutral pH. It needs to be pointed out that this has little to do with pH control, since these salt solutions have no buffer capacity at pH 7 ... 

On the other hand, metals such as Ta, Nb, Ti, Zr, Al, etc. (the valve metals ) do not exhibit transpassive behaviour, and in appropriate electrolyte solutions film growth at high fields rather than corrosion and/or oxygen evolution is the predominant reaction thus aluminium can be anodised to 500 V or more in an ammonium borate buffer titanium can be anodised to about 400 V in formic acid and tantalum can be anodised to high voltages in most acids, including hydrochloric acid. 

Iodine was determined by an iodometric titration adapted from White and Secor.(3) Instead of the normal Carius combustion, iodide was separated from the samples either by slurrying in 6M NaOH, or by stirring the sample with liquid sodium-potassium (NaK) alloy, followed by dissolving excess NaK in ethanol. Precipitated plutonium hydroxides were filtered. Iodine was determined in the filtrate by bromine oxidation to iodate in an acetate buffer solution, destruction of the excess bromine with formic acid, acidifying with SO, addition of excess KI solution, and titrating the liberated iodine with standard sodium thiosulfate. The precision of the iodine determination is estimated to be about 5% of the measured value, principally due to incomplete extraction of iodine from the sample. 

C18-0050. A buffer solution made from formic acid (HCO2 H) and sodium formate (NaHC02) has a pH in the range of 3-5. Write balanced equations that show how this buffer system neutralizes H3O+ and OH. ... 

C18-0095. The pH of a formic acid/formate buffer solution is 4.04. Calculate the acid/conjugate base ratio for this solution. Draw a molecular picture that shows a small region of the buffer solution. (You may omit spectator ions and water molecules.) Use the following symbols ... 

C18-0119. In a biochemistry laboratory, you are asked to prepare a buffer solution to be used as a solvent for isolation of an enzyme. On the shelf are the following solutions, all 1.00 M formic acid (Za = 1.8 X 10 ), acetic acid = 1.8 x 10 ), sodium formate (NaHC02), and sodium acetate (NaCH3 CO2). Describe how you would prepare 1.0 L of a pH = 4.80 buffer solution... 

While earlier papers cited buffer systems or aqueous o-phosphoric acid to achieve satisfactory peak resolution, most recent investigations involved acetic acid or formic acid systems. " Representative examples are 0.2% and 1% HCOOH for betacyanins and betaxanthins, respectively, the latter requiring a lower pH for chromatographic resolution. Methanol or acetonitrile are most commonly used as modifiers, either undiluted or diluted with purified water at ratios of 60 40 or 80 20 (v/v), respectively. - Typical HPLC fingerprints for yellow and red beet juice are shown in Figure 6.4.1. 

Buffers are necessary to adjust and maintain the pH. Buffering agents can be salts of a weak acid and a weak base. Examples are ammonium, potassium, sodium carbonates (caustic soda), bicarbonates, and hydrogen phosphates [1345]. Weak acids such as formic acid, fumaric acid, and sulfamic acid also are recommended. Common aqueous buffer ingredients are shown in Table 17-8. 

Udapa et al.16 showed that C02 was reduced to formic acid at a mercury electrode in a 0.05 M phosphate buffer (pH 6.8) solution. A current efficiency of 81.5% was obtained at a current density of 20 mA/cm2 and a cell voltage of 3.5 V. On the other hand, Bewick and Greener17 reported that malate and glycolate were produced at Hg and Pb electrodes, respectively, using aqueous quartenary... 

Halmann reported in 1978 the first example of the reduction of carbon dioxide at a p-GaP electrode in an aqueous solution (0.05 M phosphate buffer, pH 6.8).95 At -1.0 V versus SCE, the initial photocurrent under C02 was 6 mA/ cm2, decreasing to 1 mA/cm2 after 24 h, while the dark current was 0.1 mA/cm2. In contrast to the electrochemical reduction of C02 on metal electrodes, formic acid, which is a main product at metal electrodes, was further reduced to formaldehyde and methanol at an illuminated p-GaP. Analysis of the solution after photoassisted electrolysis for 18 and 90 h showed that the products were 1.2 x 10-2 and 5 x 10 2 M formic acid, 3.2 x 10 4 and 2.8 x 10-4 M formaldehyde, and 1.1 x 10-4 and 8.1xlO 4M methanol, respectively. The maximum optical conversion efficiency calculated from Eq. (23) for production of formaldehyde and methanol (assuming 100% current efficiency) was 5.6 and 3.6%, respectively, where the bias voltage against a carbon anode was -0.8 to -0.9 V and 365-nm monochromatic light was used. In a later publication,4 these values were given as ca. 1% or less, where actual current efficiencies were taken into account [Eq. (24)]. 

Hiratsuka et al102 used water-soluble tetrasulfonated Co and Ni phthalocyanines (M-TSP) as homogeneous catalysts for C02 reduction to formic acid at an amalgamated platinum electrode. The current-potential and capacitance-potential curves showed that the reduction potential of C02 was reduced by ca. 0.2 to 0.4 V at 1 mA/cm2 in Clark-Lubs buffer solutions in the presence of catalysts compared to catalyst-free solutions. The authors suggested that a two-step mechanism for C02 reduction in which a C02-M-TSP complex was formed at ca. —0.8 V versus SCE, the first reduction wave of M-TSP, and then the reduction of C02-M-TSP took place at ca. -1.2 V versus SCE, the second reduction wave. Recently, metal phthalocyanines deposited on carbon electrodes have been used127 for electroreduction of C02 in aqueous solutions. The catalytic activity of the catalysts depended on the central metal ions and the relative order Co2+ > Ni2+ Fe2+ = Cu2+ > Cr3+, Sn2+ was obtained. On electrolysis at a potential between -1.2 and -1.4V (versus SCE), formic acid was the product with a current efficiency of ca. 60% in solutions of pH greater than 5, while at lower pH... 

In the determination of formic acid in more complicated reaction-mixtures (for example, in the presence of buffers,22 69a in solutions containing non-volatile acids,49- 67 and in solutions containing ammonia234), it was necessary to distil the formic acid from the reaction solution (after destruction of the excess periodate with ethylene glycol or arsenite) before it could be titrated. 

For formic acid, pATa = - log(1.8 x 10 4) = 3.74. The Henderson-Hasselbalch equation provides the pH of the original buffer solution ... 

The most controversial issue is the number and exact stoichiometries of the iron(III)-sulfito complexes formed under different experimental conditions. Earlier, van Eldik and co-workers reported the formation of a series of [Fe(SO ) ]3-2" (n = to 3) complexes and the [Fe(S03)(0H)] complex (89,91,92). The stability constants of these species were determined by evaluating time resolved rapid-scan spectra obtained from the sub-second to several minutes time domain. The cis-trans isomerization of the complexes was also considered, under feasible circumstances. In contrast, Betterton interpreted his results assuming the formation and linkage isomerization of a single complex, [Fe(SC>3)]+ (93). In agreement with the latter results, Conklin and Hoffmann also found evidence only for the formation of a mono-complex (94). However, their results were criticized on the basis that the experiments were made in 1.0 M formic acid/formate buffer where iron(III) existed mainly as formato complex(es). Although these reactions could interfere with the formation of the sulfito complex, they were not considered in the evaluation of the results (95). Finally, van Eldik and co-workers re-examined the complex-formation reactions and presented additional data in support of... 

Triethylamine may act to buffer the pH, which changes as formic acid is consumed during the reaction. An excess of formic acid over substrate is often used. Though not essential (as will be discussed later), it is sometimes preferable to charge TEAF during the reaction in order to ensure a high yield of product. 

Ion pair formation appears to become relevant only for stronger complexing agents. The partitioning of the protonated form of DMA increases significantly in the presence of formic acid/formate buffer at pH 3, and is most likely due to complex formation between the anilinium ion and formate [120]. 

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