A certificate of analysis (COA) is the quickest way to know what is really in a bag of caustic soda. Yet many buyers only look at the purity line. This guide explains every line on a typical COA for caustic soda flakes 99%, using a real batch certificate as the example, and what each result means for your process.
The header: is this certificate for your product?
Before reading any numbers, check the header. It should show the product name, a part number, the batch number, the manufacturing and expiry dates, and the chemical identifiers: molecular formula NaOH, molecular weight 40 g/mol, CAS number 1310-73-2 and EC number 215-185-5. The batch number on the COA must match the batch printed on your bags. A certificate without a batch number tells you nothing about the material you received.
The results, line by line
A well-written COA shows three columns: the test, the specification limit, and the result measured on that batch. Here is an example from a recent batch of caustic soda flakes:
| Test | Specification | Result |
|---|---|---|
| Appearance | White flakes | White flakes |
| Assay | Min. 97.0% | 97.75% |
| Purity | Min. 99% | 99.72% |
| Heavy metals (as Pb) | Max. 0.005% | Nil |
| Insoluble matter in water | Max. 0.05% | 0.05% |
| Carbonate | Max. 2.0% | 0.54% |
| Chloride (Cl) | Max. 0.05% | 0.024% |
| Sulphate (SO4) | Max. 0.05% | 0.024% |
| Arsenic (As) | Max. 0.0003% | 0.0003% |
| Iron (Fe) | Max. 0.005% | 0.00013% |
Appearance
Flakes should be white and free-flowing. Yellowish or grey flakes can mean contamination; hard lumps usually mean the product has absorbed moisture.
Assay and purity
Purity is the share of sodium hydroxide in the product. For industrial flakes, 99% is the standard. Assay is a titration result and can be reported on a different basis, which is why the two figures are not identical. What matters is that both meet their limits.
Carbonate
Sodium carbonate forms when caustic soda reacts with carbon dioxide in the air. A low carbonate result shows the flakes were made and stored properly. Carbonate also consumes acid in neutralisation and can leave deposits in some cleaning systems.
Chloride and sulphate
These come mostly from the salt brine used to make caustic soda. Chloride is the one to watch if solutions touch stainless steel, because it can cause pitting corrosion, especially when hot.
Iron, heavy metals and arsenic
Trace metals matter for water treatment, food processing and products where colour or purity is critical. Low iron also means less staining in cleaning applications.
Insoluble matter
Material that does not dissolve can block filters, nozzles and dosing pumps. Keep it low, and filter solutions if your dosing system is sensitive.
The conclusion and signature
The COA should end with a conclusion that the batch complies with the stated specification, and either a signature or a statement that it is a computer-generated document. Keep COAs with your receiving records; auditors often ask for them.
Where to see a real example
If you buy caustic soda in the UAE, you can download an example certificate of analysis for caustic soda flakes 99%, together with the full safety data sheet, from DUBI CHEM's caustic soda page. The page also shows the day's price per kg in AED and the rates for larger orders, with ready stock in Al Hayl, Fujairah and delivery across the UAE.