Page 1Page 2Page 3Page 4Page 5Page 6Page 7Page 8Page 9Page 10Page 11Page 12Page 13Page 14Page 15Page 16Page 17Page 18Page 19Page 20Page 21Page 22Page 234.5.2.2-1-4.5.2.2 Phosphoric Acid ReactionWhile for the purpose of carbon isotopic studies, in principle, it does not much matter what acid isused to decompose a carbonate, this is not the case for oxygen isotope studies. Because only 2/3 ofthe element of interest is extracted, isotope fractionation occurs in the reaction, and, if water is2present, isotope exchange with it can further complicate the interpretation of the measured COisotopic composition. Hence, acids suitable for the purpose need to be anhydrous. Phosphoric acidis convenient in this regard, because the water content of high concentration acids can be further25reduced by the addition of appropriate amounts of P O , to produce nominally 100% phosphoric acid.The use of this acid was recommended by McCrea (1950) and his carbonate reaction protocol hasbeen duplicated widely since. The method was reviewed by Coplen et al. (1983). While McCrea34(1950) suggested that 100% phosphoric acid should be used and established that at this H PO2concentration isotope exchange with CO could not be detected, within the precision of themeasurements, not all researchers have followed his recommendation. For example, Mook (1968)suggested that the use of 95% phosphoric acid would yield more reproducible results, which in factis probably not the case, because at this concentration level isotope exchange between the acid and2the liberated CO can occur. The concentration of a particular phosphoric acid preparation can bedetermined readily by measuring its density (Table 4.5.2.2-1). 25The concentration of phosphoric acid can be increased to above 100% by further addition of P O ,34or by heating. If phosphoric acid is heated, two H PO molecules may combine under the removalof one water molecule, and pyrophosphoric acid is formed. 3442722HPO = HPO + HOFurther water can be removed by polymerizing three phosphoric acid molecules, forming tripoly-phosphoric acid3442753102HPO + HPO = HPO + HOThe reactions as written are incomplete, because the elimination of water molecules produces notonly pyrophosphoric and tripoly-phosphoric acid but also to longer linear molecules of various chainlengths and cyclic polymers. The polymerization of orthophosphoric acid leads to the formation of4polyphosphoric acid, a viscous liquid, whose acid chains consist of PO tetrahedra joined by ashared oxygen atom at the corner of the tetrahedra and connected by hydrogen bonds. Inconcentrated solutions these hydrogen bonds are responsible for the viscous nature of the acid, in4dilute solutions the phosphate ions are hydrogen bonded to the water rather than the PO ions.Huhti and Gartaganis(1956) found that even at nominal concentrations below 100% orthophosphoricacid, there were measurable amounts of pyrophosphoric acid and, hence, more water present thanPrint this SectionSaveZoom4.5.2.2-2-34the nominal concentration would indicate. For example, 99.8% wt. H PO , contained actually ~1.334moles per liter of water, compared to 0.2 moles expected on the basis of the nominal H POconcentration.An investigation into the composition of “strong” phosphoric acids by James (1959) demonstrated25that the degree of polymerization increases rapidly with increasing P O content of the acid. Polymerchains containing up to 14 phosphor atoms, as well as ring structures, were found. The relativeabundance of some of the phosphoric acid polymers is shown in Fig. 4.5.2.2-1.Figure 4.5.2.2-1. The abundance of phosphoric acid polymers with up to 6 P atoms25as a function of the P O content of the acid solution, after James (1959). The upper34 25x axis gives the nominal H PO concentration, the lower axis the corresponding P Ocontent. The numbers along the curves indicate the number of phosphor atoms in thepolymer. The abundance of polymers with more that 6 P atoms has not been shown,their concentrations decrease systematically with increasing chain length and are lowerthat the abundance of the 6 P atoms containing acid. For an enlarged image click onthe figure.4.5.2.2-3-The occurrence of water even in 100% phosphoric acid can be important for the preservation of the2isotope record of the CO liberated in the reaction with a carbonate. Keisch et al. (1958) examinedthe kinetics of the exchange of oxygen isotopes between phosphoric acid and water:2 34 2 34 H O* + H PO = H O + H PO *where the * indicates the exchanged atom. The authors find that the rate of exchange increasesrapidly with the phosphoric acid concentration; the half-time of exchange at 100 C decreases fromo34 34250 hours for ~45 wt.%H PO to 0.20 hours for acid concentrations close to 100wt% H PO .The rate expression that the authors find is consistent with the exchange occurring via three paths:34(a) direct oxygen exchange between water and H PO as indicated by the reaction above, (b) the42 7reversible formation and hydrolysis of H P O3442722HPO = HPO + HO53 10and (c) the reversible formation and hydrolysis of H P O .3442753102HPO + HPO = HPO + HO2The CO formed in the decomposition of the carbonate can exchange isotopes with this water, andvia these paths, also with the phosphate ion.In addition, one has to consider also that water is formed in the reaction of the carbonate with theacid, e.g., :322CaCO + 2H ==> CO + H O + Ca+ 2+and it is possible that isotope exchange with this water can occur.The importance of oxygen isotope exchange in the carbon dioxide-phosphoric acid systems for the2* O values of the CO formed was studied by Wachter and Hayes (1985). The oxygen isotope18exchange rate between solutions of concentrated phosphoric acid, ranging in concentration from 77.734 2to 103 wt.% H PO , and CO , enriched in O from 113.5 to 327.2 ‰, was measured as a function1834 34of temperature (25 C, 50 C, 75 C). While experiments for H PO concentrations below100% H POooodemonstrate that significant isotope exchange occurs and the observations made for them may shedlight on possible isotope exchange mechanisms, they are not relevant for the recommended use of100% phosphoric acid. For this acid strength, the estimated error introduced through the equilibration2of CO with the reagent acid at 25 C is estimated to be below 3*10 ‰ even after 1000 hours ofo-3equilibration. At 75 C, significant exchange was found to occur with anhydrous phosphoric