Oxalate

Key takeaways

Oxalate belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Oxalate to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Oxalate from memory before moving on to harder problems.

Reference excerpt

Oxalate (systematic IUPAC name: ethanedioate) is an anion with the chemical formula C2O2−4. This dianion is colorless. It occurs naturally, including in some foods. It forms a variety of salts, for example sodium oxalate (Na2C2O4), and several esters such as dimethyl oxalate ((CH3)2C2O4). It is a conjugate base of oxalic acid. At neutral pH in aqueous solution, oxalic acid converts completely to oxalate.

Relationship to oxalic acid The dissociation of protons from oxalic acid proceeds in a determined order; as for other polyprotic acids, loss of a single proton results in the monovalent hydrogenoxalate anion HC2O−4. A salt with this anion is sometimes called an acid oxalate, monobasic oxalate, or hydrogen oxalate. The equilibrium constant (Ka) for loss of the first proton is 5.37×10−2 (pKa = 1.27). The loss of the second proton, which yields the oxalate ion, has an equilibrium constant of 5.25×10−5 (pKa = 4.28). These values imply, in solutions with neutral pH, no oxalic acid and only trace amounts of hydrogen oxalate exist. The literature is often unclear on the distinction between H2C2O4, HC2O−4, and C2O2−4, and the collection of species is referred to as oxalic acid.

Structure The oxalate anion exists in a nonplanar conformation where the O–C–C–O dihedrals approach 90° with approximate D2d symmetry. When chelated to cations, oxalate adopts the planar, D2h conformation. However, in the structure of caesium oxalate Cs2C2O4 the O–C–C–O dihedral angle is 81(1)°. Therefore, Cs2C2O4 is more closely approximated by a D2d symmetry structure because the two CO2 planes are staggered. Two structural forms of rubidium oxalate Rb2C2O4 have been identified by single-crystal X-ray diffraction: one contains a planar and the other a staggered oxalate.

The barrier to rotation about this bond is calculated to be roughly 2–6 kcal/mol for the free dianion, C2O2−4. Such results are consistent with the interpretation that the central C−C bond is regarded as a single bond with minimal π interactions between the two CO−2 units. This barrier to rotation about the C−C bond (which formally corresponds to the difference in energy between the planar and staggered forms) may be attributed to electrostatic interactions as unfavorable O−O repulsion is maximized in the planar form.

Occurrence in nature Oxalate occurs in many plants, where it is synthesized by the incomplete oxidation of saccharides. Calcium oxalate crystals known as raphides are also used by plants as a defense mechanism. Several plant foods such as the root and/or leaves of spinach, rhubarb, and buckwheat are high in oxalic acid and can contribute to the formation of kidney stones in some individuals. Other oxalate-rich plants include fat hen ("lamb's quarters"), sorrel, and several Oxalis species (also sometimes called sorrels). The root and/or leaves of rhubarb and buckwheat are high in oxalic acid. Other edible plants with significant concentrations of oxalate include, in decreasing order, star fruit (carambola), black pepper, parsley, poppy seed, amaranth, chard, beets, cocoa and chocolate, most nuts, most berries, fishtail palms, New Zealand spinach (Tetragonia tetragonioides), and beans. Leaves of the tea plant (Camellia sinensis) contain among the greatest measured concentrations of oxalic acid relative to other plants. However, the drink derived by infusion in hot water typically contains only low to moderate amounts of oxalic acid due to the small mass of leaves used for brewing. Some fungi of the genus Aspergillus also produce oxalic acid.

Physiological effects

Excess consumption of oxalate-rich foods has been linked to formation of salts such as calcium oxalate, a risk factor for kidney stones.

As a ligand for metal ions

Oxalate also forms coordination compounds where it is sometimes abbreviated as ox. It is commonly encountered as a bidentate ligand. When the oxalate chelates to a single metal center, it always adopts the planar conformation. As a bidentate ligand, it forms a 5-membered MC2O2 ring. An illustrative complex is potassium ferrioxalate, K3[Fe(C2O4)3]. The drug oxaliplatin exhibits improved water solubility relative to older platinum-based drugs, avoiding the dose-limiting side-effect of nephrotoxicity. Oxalic acid and oxalates can be oxidized by permanganate in an autocatalytic reaction. One of the main applications of oxalic acid is rust-removal, which arises because oxalate forms water-soluble derivatives with the ferric ion.

Excess An excess oxalate level in the blood is termed hyperoxalemia, and high levels of oxalate in the urine is termed hyperoxaluria.

Acquired Although unusual, consumption of oxalates (for example, the grazing of animals on oxalate-containing plants such as Bassia hyssopifolia, or human consumption of wood sorrel or, specifically in excessive quantities, black tea) may result in kidney disease or even death due to oxalate poisoning. The New England Journal of Medicine reported acute oxalate nephropathy "almost certainly due to excessive consumption of iced tea" in a 56-year-old man, who drank "sixteen 8-ounce glasses of iced tea daily" (roughly one gallon or 3.8 liters). The authors of the paper hypothesized that acute oxalate nephropathy is an underdiagnosed cause of kidney failure and suggested thorough examination of patient dietary history in cases of unexplained kidney failure without proteinuria (an excess of protein in the urine) and with large amounts of calcium oxalate in urine sediment. Oxalobacter formigenes in the gut flora may help alleviate this.

Congenital

Primary hyperoxaluria is a rare, inherited condition, resulting in increased excretion of oxalate, with oxalate stones being common.

References

Further reading Euler. "Ksp Table: Solubility product constants near 25 °C". chm.uri.edu. Retrieved 10 June 2021. Ibis, Fatma; Dhand, Priya; Suleymanli, Sanan; van der Heijden, Antoine E. D. M.; Kramer, Herman J. M.; Eral, Huseyin Burak (2020). "A combined experimental and modelling study on solubility of calcium oxalate monohydrate at physiologically relevant pH and temperatures". Crystals. 10 (10): 924. Bibcode:2020Cryst..10..924I. doi:10.3390/cryst10100924. ISSN 2073-4352. Ulmgren, Per; Rådeström, Rune (1999). "Solubility of calcium oxalate in the presence of magnesium ions, and solubility of magnesium oxalate in sodium chloride medium". Nordic Pulp & Paper Research Journal. 14 (4): 330–335. doi:10.3183/npprj-1999-14-04-p330-335. ISSN 2000-0669. S2CID 96834193.

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Illustrations

Worked examples

Example 1 — a first encounter with Oxalate

Start with the simplest possible case. Write down what Oxalate claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Oxalate before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Oxalate ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Oxalate

In research: Oxalate appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Oxalate in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Oxalate is common in secondary-school and first-year university syllabi. It links to neighbouring topics Anticoagulants, Carboxylate anions, Oxalates, so understanding it makes those chapters shorter.
In everyday life: Look for Oxalate outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Oxalate in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Oxalate means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Oxalate out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Oxalate in simple terms?

Oxalate (systematic IUPAC name: ethanedioate) is an anion with the chemical formula C2O2−4. This dianion is colorless.

Why does Oxalate matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Oxalate?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Oxalate.