Summary. The SPC data sheet for caustic soda flakes (SPC-TDS-ALK-001, IS 252:2013, technical grade) states NaOH 97.0% minimum as supplied and 99.5% minimum on a dry basis. Both describe the same flakes: in general, the dry-basis figure leaves out the water, so it is higher. A purity quoted without a basis cannot be compared. Compare every offer on the same basis, then compare carbonate, chloride and iron limits.
Few chemical purchases cause as much avoidable confusion as caustic soda purity. One supplier lists one percentage, another a higher one, and both may be telling the truth about different things. This guide explains how to read the technical data sheet (TDS) and the batch Certificate of Analysis (CoA) for caustic soda flakes, using the figures in SPC data sheet SPC-TDS-ALK-001. Where something is general chemistry rather than an SPC figure, it is described as such.
The specification at a glance
The figures below are the limits in SPC-TDS-ALK-001. The source of the specification is recorded on the sheet as the manufacturers' published specifications and supplier certificates of analysis, to IS 252:2013. The grade is technical, and technical grade is not food grade or pharma grade.
| Parameter | Unit | Limit |
|---|---|---|
| Form and description | - | White flakes, free from foreign matter |
| Sodium hydroxide (as NaOH) | % mass | 97.0 min |
| NaOH on dry basis | % mass | 99.5 min |
| Sodium carbonate (as Na2CO3) | % mass | 0.40 max |
| Chlorides (as Cl) | % mass | 0.03 max |
| Sulphate (as SO4) | % mass | 0.030 max |
| Silicates (as SiO2) | % mass | 0.02 max |
| Iron (as Fe) | ppm | 20.0 max |
| Copper (as Cu) | ppm | 2.0 max |
| Manganese (as Mn) | ppm | 1.0 max |
| Chlorates (as NaClO3) | ppm | 10.0 max |
| Mercury (as Hg) | ppm | 0.02 max |
| Matter insoluble in water | % mass | 0.05 max |
As supplied versus dry basis
Caustic soda is hygroscopic: it takes up moisture from air (SDS section 7), so a sample of flakes can carry some water. Two numbers are used to describe its sodium hydroxide content.
- As supplied (97.0% min): the NaOH content of the flakes exactly as they arrive, water included. In general, this is the figure that decides how much NaOH you get per kilogram you buy.
- Dry basis (99.5% min): the NaOH content calculated as if the water were removed. Because it excludes the water, it reads higher than the as-supplied figure.
The data sheet does not list a moisture limit, so do not try to work one out from the two assay figures. When you request the batch CoA, ask for the NaOH assay with its basis and for moisture, and check them there.
Why this causes confusion
A listing that gives a high purity with no basis may be quoting a dry-basis or best-case figure. Set beside an as-supplied 97% it looks better, but the two cannot be compared. A purity quoted without a basis cannot be compared. Before you compare anything, ask the supplier one question: is this NaOH on an as-supplied or on a dry basis, and which test method?
What each impurity limit tells you
| Line | What it means (general chemistry) | Who should care |
|---|---|---|
| Sodium carbonate, 0.40% max | Formed when NaOH reacts with carbon dioxide in air; rises on poor storage | Anyone who needs full alkalinity or low carbonate in solution |
| Chlorides, 0.03% max | Residual salt from production; promotes corrosion of some metals | Users with stainless equipment, metal treatment or sensitive downstream chemistry |
| Sulphate, 0.030% max | Residual salt | Process streams sensitive to sulphate |
| Silicates, 0.02% max | Can form deposits | Water and boiler-related uses |
| Iron, copper, manganese | Trace metals that can catalyse unwanted reactions or colour products | Textile bleaching, colour-critical processes |
| Chlorates, 10.0 ppm max | Trace oxidising impurity | Users with reactive or oxidation-sensitive processes |
| Mercury, 0.02 ppm max | Trace heavy metal | Regulated or export-linked end uses |
| Water-insoluble matter, 0.05% max | Particles that remain undissolved | Anyone who filters or meters the solution |
The middle and right columns are general chemistry to help you decide which lines to watch; whether a given limit matters for your process is a question for your own technical team.
The TDS versus the Certificate of Analysis
The TDS states what the product is specified to meet. The CoA is the batch document, and SPC supplies a batch Certificate of Analysis with every delivery. A good habit is to put the TDS limits and the CoA readings side by side and check that every value reported sits inside its limit and states its basis. A CoA that gives an NaOH figure without saying whether it is as supplied or dry has not told you enough, so ask for the basis.
It is also wise to compare the document dates, the batch number on the CoA with the batch on the bags, and the standard cited. SPC is a Chennai trader and bulk distributor, not a manufacturer, so the data sheet names the producers it draws on: Grasim Industries Limited (Aditya Birla Group) and The Travancore-Cochin Chemicals Limited.
IS 252:2013 in a quote
SPC flakes are technical grade to IS 252:2013. When a quote cites the standard, confirm which edition of IS 252 it means, so that the numbers you are comparing come from the same one.
How to compare two caustic soda offers properly
Compare the NaOH figure on the same stated basis, for example 97.0% as supplied, or 99.5% on a dry basis.
- Write down the NaOH figure each offer states and the basis it uses.
- If one supplier gives only a dry-basis figure, ask for the as-supplied figure, or convert both to dry basis.
- Compare the pack and delivery location separately, so they do not hide inside the headline figure.
- Compare the impurity lines that matter to you: carbonate, chloride, iron.
Flakes versus lye
Do not compare a flakes TDS with a lye TDS. Caustic soda lye is a separate SPC product, a solution with 47% NaOH minimum to IS 252:2013 Grade-1, with its own data sheet (SPC-TDS-ALK-003) and tariff line. To compare flakes with lye, compare the NaOH content of each on the same basis, using the NaOH fraction of the solution for lye.
Checklist before you order
- Ask for the TDS, and for the batch CoA with the NaOH assay and its basis.
- Confirm the basis (as supplied or dry) printed next to every NaOH figure.
- Check the carbonate, chloride and iron lines against what your process tolerates.
- Check the form: white flakes, free from foreign matter, in 50 kg HDPE bags as documented.
- Check that the batch number on the CoA matches the batch on the bags at delivery.
- Check that the grade is technical, and say if you need something else, because technical grade is not food or pharma grade.
Common reading mistakes
- Comparing an as-supplied figure with a dry-basis figure.
- Reading the minimum on the TDS as the typical value on the batch; the CoA is the batch document.
- Ignoring carbonate when the flakes are old or were stored open.
- Comparing a technical-grade assay with a food or pharma claim.
- Reading purity from a safety data sheet. Use the SDS for hazards, handling and transport; the TDS gives the specification.
Where to find the documents
The TDS (SPC-TDS-ALK-001) and the Safety Data Sheet for caustic soda flakes are both downloadable PDFs on the caustic soda flakes page. Send your form, assay basis, quantity, destination and application and you will get a quote within 4 working hours, with the batch CoA supplied with delivery. For background on sourcing in the city, read our caustic soda supplier guide for Chennai.
Key takeaways
- 97.0% as supplied and 99.5% on a dry basis describe the same flakes.
- Always ask which basis a purity figure uses.
- Compare NaOH figures on the same stated basis.
- Read the CoA, not just the TDS, and check carbonate, chloride and iron.
- The liquid lye is a different product with a different data sheet.
This guide is general information based on the SPC data sheet; it is not legal, technical or purchasing advice.