Glyoxylic acid is the smallest molecule in commercial supply that carries both an aldehyde and a carboxylic acid on the same carbon. That single structural fact explains almost everything about how it is made, sold and used: it cannot be isolated as a stable anhydrous liquid, it is traded as a monohydrate crystal or as a 40-50% aqueous solution, and nearly every industrial reaction it enters is an alpha-hydroxy acid formation followed by oxidation, decarboxylation or condensation.
This article covers the two CAS numbers that matter (298-12-4 for the anhydrous acid, 563-96-2 for the monohydrate), the three crystalline and five solution grades that are actually quoted in the market, the manufacturing routes and their impurity fingerprints, and the stoichiometry that determines how much acid a downstream plant needs per tonne of vanillin, D-4-hydroxyphenylglycine, HPAA or allantoin.
1. Identity: Two CAS Numbers, One Product Family
| Item | Anhydrous glyoxylic acid | Glyoxylic acid monohydrate |
|---|---|---|
| CAS number | 298-12-4 | 563-96-2 |
| Molecular formula | C2H2O3 | C2H4O4 (C2H2O3 . H2O) |
| Molecular weight | 74.04 | 92.06 |
| PubChem CID | 760 | 15620607 |
| InChIKey | HHLFWLYXYJOTON-UHFFFAOYSA-N | MOOYVEVEDVVKGD-UHFFFAOYSA-N |
| SMILES | O=CC(=O)O | O=CC(=O)O.O |
| Anhydrous acid equivalent | 100% | 80.43% (74.04 / 92.06) |
The monohydrate is not “glyoxylic acid with a bit of moisture”. It is a defined crystal in which the aldehyde carbon is present as a geminal diol, which is exactly why the solid exists at all: the free aldehyde acid polymerises and is not handled as a neat liquid in commerce. Buyers who compare a crystal quotation with a solution quotation without converting to anhydrous equivalent are comparing two different products.
Conversion is simple and worth memorising: kg anhydrous acid = kg monohydrate crystal x 0.8043. One tonne of 98% monohydrate crystal therefore contains 789 kg of glyoxylic acid, and one tonne of 50% solution contains 500 kg.
2. Physical and Chemical Properties
| Property | Value / behaviour | Practical consequence |
|---|---|---|
| Appearance (monohydrate) | White crystalline powder | Hygroscopic; cakes in humid air |
| Solubility | Freely soluble in water; slightly soluble in ethanol, diethyl ether and benzene | Aqueous reactions are the default; alcoholic systems need the acid dried or the salt pre-formed |
| Acidity | Carboxyl pKa around 3.3 (alpha-keto acid, stronger than acetic acid at 4.76) | Reacts with amines and carbonates directly; salt formation can pre-empt the aldehyde chemistry you wanted |
| Functional groups | Aliphatic aldehyde plus carboxylic acid on C2 | Two competing reactivities in one molecule – control of pH and order of addition decides the product |
| Aqueous speciation | Partially hydrated at the aldehyde carbon | Reactions with nucleophiles can be slower than the aldehyde label suggests |
| Thermal behaviour | Free acid tends to polymerise; the monohydrate is the stable commercial solid | Do not attempt to dry the monohydrate to “pure” acid for storage |
| Corrosivity | Causes serious eye damage and skin corrosion | Acid-resistant PPE, lined equipment, eyewash at the point of use |
3. Manufacture: Nitric Acid Oxidation of Glyoxal, and Its Alternatives
The dominant commercial route is oxidation of glyoxal (CAS 107-22-2) with nitric acid, usually run so that nitrogen oxides are re-oxidised by air and recycled. The net reaction is deceptively simple:
2 (CHO)2 + O2 -> 2 OHC-COOH
In practice the nitric acid is both oxidant and oxygen carrier, and the process is a controlled partial oxidation. Stopping at the mono-acid is the whole engineering problem: push too little and glyoxal survives into the product; push too far and the second aldehyde is oxidised as well, giving oxalic acid. Every commercial specification for glyoxylic acid is therefore an expression of where that balance was struck, and the residual nitric acid figure (0.2% max in solution grades) is the unreacted remainder of the NOx loop.
| Route | Feedstock | Character | Status in the market |
|---|---|---|---|
| Nitric acid oxidation | Glyoxal | High yield per pass, oxalic acid and residual nitric acid as the signature impurities | Dominant, especially in China |
| Ozonolysis / oxidative cleavage | Maleic anhydride or maleic acid | Clean oxidant, no nitrate residue, higher capital cost | Niche and specialty capacity |
| Catalytic oxidation | Ethylene glycol or glyoxal with air | Avoids nitric acid; catalyst selectivity is the constraint | Subject of continuous process improvement |
| Electrochemical reduction | Oxalic acid | Electricity as the reductant; couples to renewable power | Development and small scale |
| Biological oxidation | Glycolic acid or ethylene glycol | Very high selectivity in water at ambient temperature | Research and early pilot |
The route determines the impurity you will live with. A nitrate-oxidation producer will always show glyoxal, oxalic acid, glycolic acid and nitrate on the COA; an ozonolysis producer will not show nitrate at all. When two suppliers quote the same assay with very different impurity tables, they are almost certainly on different routes.
4. The Grade Structure and What Each Specification Controls
4.1 Crystalline monohydrate (CAS 563-96-2)
| Parameter | Technical | Standard | Premium |
|---|---|---|---|
| Monohydrate assay (C2H4O4), % min | 93.0 | 98.0 | 99.5 (+0.2) |
| Ethylene glycol / glyoxal carry-over, % max | 0.3 | 0.1 | 0.05 |
| Oxalic acid, % max | 0.1 | 0.1 | 0.45 |
| Glycolic acid, max | 3 g/L | 3 g/L | 3 g/L |
| Residue on ignition, % max | 0.05 | 0.05 | 0.05 |
4.2 50% solution (five grades in common circulation)
| Parameter | Technical | Chelating | High purity | Premium I | Premium II |
|---|---|---|---|---|---|
| Assay as C2H2O3, % | 50 +/- 0.5 | 50 +/- 0.5 | 50 +/- 0.5 | 50 +/- 0.5 | 50 +/- 0.5 |
| Glyoxal, % max | 1.0 | 0.5 | 0.25 | 0.05 | 0.05 |
| Oxalic acid, % max | 1.0 | 0.5 | 0.25 | 0.6 | 0.2 |
| Nitric acid, % max | 0.2 | 0.2 | 0.2 | not specified | not specified |
| Colour, Hazen | pass | pass | pass | 100 max | 100 max |
Three features of these tables are worth more attention than the headline assay.
- The upper window on Premium crystal. “99.5% + 0.2” describes a titrimetric assay with a small positive tolerance, not a purity above 100%. If a supplier reports 99.8% by HPLC area normalisation instead, the number is not comparable – area percentage does not see water, and water is the largest single component of a monohydrate.
- Oxalic acid moves in the opposite direction on the top grade. Premium crystal tolerates 0.45% oxalic acid while Standard tolerates 0.1%. The 50% table makes the same trade-off explicit: Premium I relaxes oxalic acid to 0.6% to achieve glyoxal at 0.05%, while Premium II holds both. This is the oxidation-depth trade-off described in section 3, expressed as a specification. Buy the grade whose loose parameter your process can tolerate.
- Mixed units hide a basis. Glycolic acid is limited in g/L while everything else is a percentage, which only makes sense if the analytical sample has a defined concentration. Do not compare that row across suppliers without asking how the sample was prepared.
5. Where the Volume Goes
Glyoxylic acid consumption is concentrated in a small number of outlets that share one mechanistic step: electrophilic attack on an aromatic ring or on a nitrogen, giving an alpha-hydroxy acid that is then transformed. The table gives the stoichiometric requirement per tonne of product at 100% yield; apply your own yield factor.
| Downstream product | Partner | kg anhydrous acid per t | Equivalent t of 50% solution | Why this grade |
|---|---|---|---|---|
| Vanillin (CAS 121-33-5) | Guaiacol | 487 | 0.97 | Premium grades – colour and food-grade purity |
| Ethyl vanillin (CAS 121-32-4) | Guethol | 445 | 0.89 | Premium grades |
| D-4-Hydroxyphenylglycine (CAS 22818-40-2) | Phenol | 443 | 0.89 | Low glyoxal – the impurity becomes an amino acid impurity |
| HPAA (CAS 23783-26-8) | Phosphorous acid | 474 | 0.95 | Chelating grade – oxalate must not enter a water circuit |
| Allantoin (CAS 97-59-6) | Urea | 468 | 0.94 | Low glyoxal, low colour |
The arithmetic for vanillin is worth doing once by hand, because it is the number that appears in most procurement discussions: vanillin is C8H8O3 at 152.15, glyoxylic acid is 74.04, and one mole of each is required, so a tonne of vanillin consumes 152.15 / 74.04 = 487 kg of anhydrous acid, or just under one tonne of 50% solution. A plant making 5,000 t/a of vanillin therefore needs roughly 2,435 t/a of anhydrous glyoxylic acid – which is why vanillin capacity and glyoxylic acid capacity are built near each other.
6. Application Detail
6.1 Vanillin and ethyl vanillin
Guaiacol (2-methoxyphenol) is condensed with glyoxylic acid under acidic or alkaline conditions to give 4-hydroxy-3-methoxymandelic acid, which is then oxidatively decarboxylated to vanillin. The mass account is: guaiacol 124.14 + glyoxylic acid 74.04 = mandelic acid 198.18, which loses 46.03 (CO2 plus two hydrogens removed by the oxidant) to give vanillin 152.15. Ethyl vanillin follows the same route from guethol (2-ethoxyphenol) and lands at 166.18.
Only a small fraction of world vanillin comes from vanilla pods – well under 1%. The rest is synthetic, and within synthetic supply there are three competing sources: guaiacol plus glyoxylic acid (the dominant petroleum route), oxidative depolymerisation of lignin (the Norwegian wood-based route, the only large-scale non-petroleum process), and ferulic acid from rice bran (a small natural-labelled Japanese route). The glyoxylic acid route is the one that scales, and it is the one whose economics move with glyoxylic acid pricing.
6.2 D-4-Hydroxyphenylglycine
Phenol is condensed with glyoxylic acid to 4-hydroxymandelic acid, which is converted to the D-amino acid by enzymatic or chemical resolution. D-pHPG (C8H9NO3, 167.17) is the side chain of amoxicillin, ampicillin and cefadroxil; one tonne of amoxicillin (365.40) needs about 457 kg of D-pHPG, and one tonne of D-pHPG needs 443 kg of anhydrous glyoxylic acid. Glyoxal carry-over matters here more than oxalic acid, because any aldehyde impurity ends up as an impurity in a registered active pharmaceutical ingredient rather than in a technical product.
6.3 HPAA and phosphonate scale inhibitors
Addition of phosphorous acid across the aldehyde gives 2-hydroxyphosphonoacetic acid (C2H5O6P, 156.03), a phosphonocarboxylic acid used as a calcium carbonate and calcium sulphate scale inhibitor in cooling water, boiler water and oilfield water flooding. One tonne of HPAA needs 474 kg of anhydrous glyoxylic acid and 526 kg of phosphorous acid. This is the application where the oxalic acid limit becomes a performance parameter rather than a purity decoration – see our dedicated article on the oxalate budget for HPAA production.
6.4 Allantoin
Condensation with urea gives allantoin (C4H6N4O3, 158.12): 2 x 60.06 urea plus 74.04 glyoxylic acid, less two water molecules. Consumption is 468 kg of acid per tonne. Cosmetic and pharmaceutical grades live or die on colour and odour, so residual glyoxal and heavy metals matter more than the headline assay.
6.5 Electroless copper plating
Glyoxylic acid is used as a non-formaldehyde reducing agent in alkaline electroless copper baths for printed circuit board metallisation. This is a small but technically demanding outlet: the plating bath is sensitive to oxalate build-up, because oxalate complexes copper and changes deposition kinetics, so chelating grade material is normally specified.
7. Supply Landscape and What to Compare
| Supplier type | Typical strength | What to watch |
|---|---|---|
| Integrated Chinese nitrate-oxidation producers | Volume, price, both crystal and solution in one plant | Batch-to-batch oxalic acid variation; NOx permitting and therefore future supply stability |
| Specialty / pharmaceutical-adjacent producers | Low glyoxal, documentation, small lots | Price per tonne of anhydrous acid, not per tonne of product |
| European and Japanese traders | Documentation, REACH compliance, reliability | Origin is usually the same Chinese capacity – ask for the original manufacturer on the COA |
| Downstream-integrated vanillin producers | Captive acid, no merchant dependency | Not a source for third parties, but they set the marginal demand that prices your purchase |
The comparison that matters is cost per kilogram of anhydrous acid, not cost per kilogram of product. At equal price per tonne, 50% solution is 1.58 times the cost per unit of acid compared with 98% monohydrate crystal, before you count the freight you pay on water. Do that calculation before you are impressed by a solution price.
8. Buying Checklist
- State the form and the basis. Crystal or solution, monohydrate assay or anhydrous assay, and which grade. “98% glyoxylic acid” without a basis is an incomplete purchase order.
- Ask for the impurity table, not just the assay. Glyoxal, oxalic acid, glycolic acid, nitrate and colour. Then map each one to your own reaction’s sensitivity.
- Confirm the isomer of the specification. If a supplier cannot say whether the crystal “ethylene glycol” row means ethylene glycol or glyoxal, the specification was copied rather than measured.
- Check the assay method. Titration and HPLC area percentage are not interchangeable on a hydrate.
- Agree packaging against your humidity. Bags with an inner PE liner, 25 kg net, or 250 kg drums and 1250 kg IBCs for solution. Bulk tanker only with dedicated lined equipment.
- Plan the discharge. Crystal dust is corrosive to eyes; solution damages concrete floors and unlined steel. Design the unloading point, not just the purchase order.
9. Safety, Storage and Compliance
Glyoxylic acid solutions are corrosive: they cause serious eye damage and skin corrosion, and the solid is equally aggressive once it dissolves in sweat or tears. Use chemical splash goggles plus a face shield, acid-resistant gloves and apron, and provide an eyewash within ten seconds of the handling point. Store away from strong bases, strong oxidisers, metal powders and amines. Keep the solid sealed and dry – a caked bag has already absorbed water and lost assay. Keep solutions out of direct sunlight, below about 30 degC, and out of contact with iron and copper, both of which catalyse oxidation and push colour and oxalic acid upwards over time.
Glyoxylic acid is an industrial intermediate, not a food or cosmetic ingredient as such; its downstream products are the regulated articles. Transport classification and packing group differ between the crystalline monohydrate and the solutions, and they differ by concentration, so take the UN number and packing instruction from the SDS issued for the exact grade rather than from a generic data sheet.
10. FAQ
Is glyoxylic acid the same as glyoxal?
No. Glyoxal is the dialdehyde (CAS 107-22-2, C2H2O2, 58.04); glyoxylic acid is the mono-acid (C2H2O3, 74.04). Glyoxal is the feedstock that is partially oxidised to make glyoxylic acid, and residual glyoxal is a standard specification item in the finished acid. See the comparison article for how the two impurity families behave downstream.
Why is there no anhydrous glyoxylic acid liquid?
Because the free aldehyde acid is not stable as a neat liquid – it polymerises. Commerce solved this two ways: crystallise it as the monohydrate (CAS 563-96-2), or keep it in water at 40% or 50%. Both are the same molecule; only the water content differs.
Which grade should a scale inhibitor plant buy?
Chelating grade at 50%. The oxalic acid limit is the reason: an inhibitor carrying oxalate into a hard-water circuit works against itself.
Can I use technical grade for pharmaceutical intermediates?
Rarely. Technical grade allows 1.0% glyoxal, and in a route such as D-pHPG that aldehyde impurity becomes an impurity in an API. Most pharmaceutical-side users specify 0.05% glyoxal or lower.
What is the shelf life?
Crystal keeps for months in sealed, dry storage; assay and colour are the parameters to re-test. Solutions are stable but slowly accumulate colour and oxalic acid, particularly in warm storage or in contact with iron, so a 12-month retest interval is typical rather than a fixed expiry.
References and Notes
- Identity data verified against PubChem CID 760 (anhydrous) and CID 15620607 (monohydrate).
- All consumption figures are stoichiometric at 100% yield, calculated from the molecular weights in this article; add your own yield factor.
- Specification tables reproduce the grades quoted in current merchant supply and are grade-specific – request the COA for the batch you are buying.
Disclaimer: this article is technical information for industrial buyers. It is not a specification, and it does not replace the supplier’s COA, SDS or your own process validation. Regulatory status of glyoxylic acid and of its downstream products varies by jurisdiction and must be confirmed for the destination market.
Next Step
PolyblueChem supplies glyoxylic acid as crystalline monohydrate (93%, 98%, 99.5%) and as 50% and 40% solution, in the grades described above. For a grade-specific COA, a sample and a quotation, see the glyoxylic acid product page or send your specification with the physical form, grade and assay basis stated.
Related reading: glyoxylic acid vs glyoxal vs oxalic acid and the oxalate budget in HPAA production. Product pages for the main downstream partners: vanillin, D-p-hydroxyphenylglycine, HPAA, glyoxal, oxalic acid, glycolic acid, nitric acid and phenol.