Oxalic acid project

With the more widespread use of subnitrogen cryogenic temperatures, use of smaller samples made possible by brighter sources, and rapid developments in detector technology and computational methods, the conclusions of the oxalic acid project are now of mainly historical importance. However, the project remains an example of the value of collaborative efforts in establishing the validity of a scientific method. 

Oxalic acid dihydrate, studied by several laboratories as part of the HJCr oxalic acid project, contains a short hydrogen-bond of 2.481 AO O distance, linking the oxalic acid and water molecules. All experiments are in agreement that the lone-pair peak of the water-molecule oxygen atom is polarized into the short hydrogen bond. The deformation density in the plane perpendicular to the water-molecule plane, bisecting the H—O—H angle, for one of the experiments is shown in Fig. 12.6. 

The process is one of electrolytic reduction and the apparatus is similar to that shown in Fig. 77, p. 144. It consists of a small porous cell (8 cm. x 2 cm. diam.) surrounded by a narrow beaher (ii cm. X 6 cm. diam.). The oxalic acid, mixed w lth too c.f. 10 per cent sulphuric acid (titrated against standard baryl.a solution] forms the cathode liquid and is placed in Iht beakei. The porous cell is filled with the same strength of siilphuiic acid and foims the anode liquid. The electrodes ara made from 01 dinary clean sheet lead. The anode consists of i thiu strip projecting about two inches from the cell and tliu... 

To examine the reliability of X-ray charge densities at a time of rapid development of new methods, the Commission on Charge, Spin and Momentum Densities of the lUCr organized a project under which a single substance, a-oxalic acid dihydrate, was studied in a number of laboratories using X-ray, neutron, and theoretical methods. The report by Coppens on the study, published in 1984, established unequivocally the qualitative reproducibility of chemically significant features in deformation density maps, which had not been generally accepted. 

A very careful study of oxalic acid di-hydrate by Garrett [11], working with Levy and Peterson.- again illustrates the two general points which we have emphasized earlier. In this crystal, of which a projection is shown in Fig. 5, there are two different hinds of hydrogen... 

Figure 11.5. The face-centered crystal structure of orthorhombic oxalic acid. On the left is the cell projected along the a0 axis. Only one molecule at a corner and three molecules in the centers of the faces are shown for clarity. On the right, a packing drawing shows all molecules in the cell with the same view.

Fig. 14.10. (a) The planar molecule of oxalic acid in the structure of oxalic acid dihydrate, (COOH)3.aH20. ( >) Plan of the unit cell of the monoclinic structure, projected on a plane perpendicular to the y axis. The heights of the atoms are indicated in units of b/ioo and hydrogen bonds between oxalic acid and water molecules are represented by broken lines. 

Most chemical and reaction path models currently account only for the aqueous carboxylic acids and their cation complexes, amino acids, some liquid hydrocarbons, alcohols, and certain other compounds entered into the data base for project-specific purposes. Adsorption of trace metals onto solid humic substances, for example, requires the user to create a fictitious solid and use an empirical adsorption coefficient. Scattered reports of carboxylic acid solids such as calcium oxalate (Marlowe 1970 Naumov et al. 1971 Galimov et al. 1975 Graustein et al. 1977 Campbell and Roberts 1986) emphasize the necessity to perform sensitivity analyses on the formation of such solids and indicate another area of uncertainty in the interpretation of chemical and reaction path model results. 

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