Glyoxylic acid is never sold as a single pure substance. It is sold as a main component plus a set of four acidic and aldehydic neighbours – glyoxal, oxalic acid, glycolic acid and residual nitric acid – and those four numbers, not the headline assay, decide whether the material works in your process. This article explains where each impurity comes from and how to read the pattern.
1. Three Acids That Look Interchangeable and Are Not
| Item | Glyoxylic acid | Glyoxal | Oxalic acid |
|---|---|---|---|
| CAS number | 298-12-4 (anhydrous), 563-96-2 (monohydrate) | 107-22-2 | 144-62-7 (anhydrous), 6153-56-6 (dihydrate) |
| Formula / MW | C2H2O3 / 74.04 | C2H2O2 / 58.04 | C2H2O4 / 90.03 |
| Functional groups | One aldehyde, one carboxylic acid | Two aldehydes | Two carboxylic acids |
| Behaviour in water | Strong acid, partially hydrated aldehyde | Neutral, extensively hydrated, slowly polymerises | Strong acid, strong calcium binder |
| Main industrial use | Vanillin, D-pHPG, HPAA, allantoin, plating | Textile and paper crosslinking resins, glyoxylic acid feedstock | Metal cleaning, rust removal, rare earth processing, oxalate salts |
| Number of oxygens gained from glyoxal | +1 | 0 | +2 |
All three are C2 molecules built from the same two carbons, and all three appear in the same reaction vessel. The last row of the table is the key: glyoxylic acid is glyoxal that has gained exactly one oxygen, and oxalic acid is glyoxal that has gained two. A commercial glyoxylic acid plant is therefore a plant that runs a partial oxidation and then has to prove, batch after batch, that it stopped in the middle.
2. The Impurity Triangle Is One Reaction, Three Ticks
The dominant manufacturing route is nitric acid oxidation of glyoxal, run with air or oxygen to re-oxidise the nitrogen oxides so that the net oxidant is oxygen:
2 (CHO)2 + O2 -> 2 OHC-COOH
Read the specification as a position on the reaction coordinate:
| Specification item | What it measures | If it is high | If it is low |
|---|---|---|---|
| Glyoxal, % max | Unreacted feedstock: oxidation did not go far enough | Reaction stopped short, or catalyst activity lost, or a stripping step was skipped | Deeper oxidation – check whether oxalic acid paid for it |
| Oxalic acid, % max | Over-oxidation: the second aldehyde was oxidised as well | Too much oxidant, too high a temperature, or too long a hold | Either good control or a generous glyoxal figure – they usually trade |
| Glycolic acid, max | Partial reduction of the aldehyde or a side reaction from the feedstock | Reducing conditions somewhere in the train, or a different feedstock | Normal |
| Nitric acid, % max | Unreacted oxidant left in the loop | Incomplete NOx recovery – a cost and a corrosion and a nitration risk | Good stripping, or a non-nitrate route |
| Colour, Hazen | Oxidation and metal-mediated degradation products | Iron or copper contact, warm storage, or a poorly stripped batch | Lined equipment and fresh material |
This is why the commercially published grades look inconsistent at first glance. Premium crystal allows 0.45% oxalic acid while Standard allows only 0.1% – because getting glyoxal down to 0.05% on a nitrate oxidation train means going deeper, and going deeper raises oxalic acid. The 50% solution grade family states the same thing openly: Premium I is glyoxal 0.05% with oxalic acid relaxed to 0.6%, Premium II holds both at 0.05% and 0.2% and charges for it in yield.
3. The Five Solution Grades as an Impurity Band
| Grade | Glyoxal, % max | Oxalic acid, % max | Nitric acid, % max | What it is for |
|---|---|---|---|---|
| Technical | 1.0 | 1.0 | 0.2 | Bulk synthesis where the product is isolated and purified anyway |
| Chelating | 0.5 | 0.5 | 0.2 | Water-treatment phosphonates, where oxalate must not enter the circuit |
| High purity | 0.25 | 0.25 | 0.2 | Balanced grade for most fine-chemical conversions |
| Premium I | 0.05 | 0.6 | not specified | Glyoxal-sensitive chemistry that can tolerate oxalate |
| Premium II | 0.05 | 0.2 | not specified | Both ends tight: flavour, pharmaceutical intermediates, colour-critical work |
Note what the grade names do not tell you: there is no monotonic ladder. Chelating grade is not simply “better” than high purity – it relaxes glyoxal to 0.5% in exchange for a tighter oxalic acid ceiling relative to technical, because in its application oxalate is the only impurity that matters. Grade selection is a matching exercise, not a ranking.
4. Downstream Sensitivity: Which Impurity Hurts Which Product
| Downstream product | Glyoxal | Oxalic acid | Nitrate | Colour bodies |
|---|---|---|---|---|
| Vanillin / ethyl vanillin (CAS 121-33-5 / 121-32-4) | High – becomes an aldehyde impurity in a flavour molecule | Moderate – purged in crystallisation but costs yield | Low after workup | High – food-grade requires white crystal |
| D-4-Hydroxyphenylglycine (CAS 22818-40-2) | High – impurity in a registered API side chain | Low | Low | Moderate |
| HPAA scale inhibitor (CAS 23783-26-8) | Moderate – reacts further in the phosphonation step | Very high – the inhibitor adds its own scale | Moderate – acid load in a water circuit | Moderate – colour in a liquid product |
| Allantoin (CAS 97-59-6) | High – odour and colour in cosmetic grade | Moderate | Low | High |
| Electroless copper plating | Moderate | High – oxalate complexes copper and shifts deposition kinetics | Low | Low |
The rule that emerges: glyoxal is a chemistry problem, oxalic acid is a system problem. Excess glyoxal turns into an impurity in the product you make; excess oxalic acid turns into calcium oxalate somewhere in your plant or your customer’s water. One is fixed by buying a tighter grade; the other is fixed by buying a different grade.
5. How the Numbers Are Measured
| Parameter | Common method | What to watch |
|---|---|---|
| Main assay | Titration of the acid or of the aldehyde function | Positive tolerance windows such as 99.5% + 0.2% show a titrimetric method; HPLC area percentage on a hydrate is not comparable |
| Glyoxal | HPLC with UV or RI detection, or derivatisation | Glyoxal hydrates and oligomerises in water, so sample age changes the result |
| Oxalic acid | Ion chromatography, or titration after precipitation | Report whether the figure is on the monohydrate or on the anhydrous acid |
| Nitric acid | Ion chromatography or colorimetric nitrate | Nitrate and nitric acid are not the same number at pH above 2 |
| Water | Karl Fischer | On a monohydrate the water of crystallisation makes the KF number large and uninformative; moisture is only meaningful on the solution grades |
| Colour | Hazen / Pt-Co, APHA | Iron and storage temperature dominate; a dark sample may be a packaging failure rather than a reactor failure |
6. Buying on Impurity Instead of Assay
- Ask for a full impurity table covering all four items plus colour, on the same basis as the assay.
- Ask which figure your process actually cannot tolerate, then set the acceptance limit on that one item and stop paying for the others.
- For water-treatment intermediates, ask for oxalic acid on every lot, not just on the first – the number is a reaction-control figure and it drifts.
- For pharmaceutical-adjacent work, ask for the glyoxal figure and the method used to get it.
- If a specification shows a crystal row labelled “ethylene glycol” and solution rows labelled “glyoxal”, ask which molecule the crystal figure refers to. On a nitrate-oxidation route the realistic carry-over is glyoxal; ethylene glycol is not an intermediate. A copied specification is the cheapest warning sign available.
7. FAQ
Can I remove glyoxal from glyoxylic acid myself?
Not economically. The two molecules are close in size and both water-soluble; separation relies on the plant’s stripping and crystallisation design. Grade selection at purchase is the cheap route.
Does a higher assay always mean fewer impurities?
No. Assay and impurity profile are separate axes, and the published grade tables show it: a high-assay crystal grade may carry a wider oxalic acid limit than a lower-assay grade.
Is residual nitric acid a safety issue or a quality issue?
Both. It is a quality issue for downstream nitration-sensitive chemistry and a materials issue for the plant, because it drives corrosion in unlined equipment.
Why does colour matter on a technical product?
Colour is the visible end of oxidation and metal contamination. It is a cheap early warning that the batch sat warm or touched iron, and in food-adjacent products it is a hard rejection criterion.
References and Notes
- Identity data verified against PubChem CID 760, CID 15620607, CID 7860 (glyoxal) and CID 971 (oxalic acid).
- Grade limits reproduce the specifications in current merchant circulation; batch COAs govern.
- Molecular weight and oxygen-balance statements are calculated from the formulae shown.
Disclaimer: technical information for industrial buyers only; not a specification and not a substitute for the supplier’s COA, SDS or your own incoming inspection.
Next Step
See the glyoxylic acid product page for the three crystal and five solution grades, or read the long-form article on grades, manufacture and industrial use. For water-treatment buyers, the oxalate budget in HPAA production works the oxalic acid limit through to kilograms of calcium per tonne. Related product pages: glyoxal, oxalic acid, glycolic acid, nitric acid.