Iron and Water-Insoluble Limits in Sodium Metasilicate
Iron and water-insoluble limits by grade tier — Superior Fe ≤ 0.008–0.010%, Standard ≤ 0.015–0.020% — why each matters for white formulations, and how to verify them.
Published · Updated
Short answer
Iron is tiered: the Superior grade is controlled to Fe ≤ 0.010% on the anhydrous lines and ≤ 0.008% on pentahydrate granular, while the Standard grade carries ≤ 0.020% and ≤ 0.015% respectively. Water-insoluble matter follows the same split — Superior ≤ 0.06% on beaded granules against ≤ 0.20% Standard. Iron is the indicator to watch for white and light-coloured end products, where trace iron is the usual cause of discolouration over shelf life. Insolubles matter where the dissolved product passes through nozzles, filters or a treated surface, since undissolved residue accumulates rather than washing through. Both are measured per batch and reported on the COA.
Published impurity limits
Each figure on this table states its basis. Read the full rule on the technical index.
| Property | Value | Basis |
|---|---|---|
| Fe — granules (beaded) | Superior ≤ 0.010% · Standard ≤ 0.020% | Published specification |
| Fe — anhydrous powder | ≤ 0.010% (single grade) | Published specification |
| Fe — pentahydrate | Superior ≤ 0.008% granular / ≤ 0.010% powder · Standard ≤ 0.015% | Published specification |
| Water-insoluble matter — granules (beaded) | Superior ≤ 0.06% · Standard ≤ 0.20% | Published specification |
| Water-insoluble matter — pentahydrate | Confirmed by grade specificationStated against the grade at RFQ rather than published as a single figure. | On request |
| Batch-measured Fe and insolubles | Reported per shipment | Batch COA |
| Whiteness | Available on request for detergent applicationsReported as a measured value, separate from the iron limit. | On request |
Why iron is the indicator for white formulations
In a white powder detergent, a light-coloured cleaner or a bright paper application, trace iron is the impurity most likely to become visible. It does not usually show at the point of blending; it shows as gradual yellowing or as speckling over storage, which means it is discovered after the product has shipped rather than at incoming inspection. This is why detergent buyers specify an iron ceiling rather than relying on an appearance check.
An iron limit is a necessary condition, not a sufficient one. Colour in a finished product also depends on the other raw materials, on water quality, and on packaging and storage conditions. The limit tells you that the silicate is not the variable — it cannot tell you that the finished product will hold its colour.
What water-insoluble matter actually costs you
Water-insoluble matter is the fraction that does not dissolve when the product is made up into solution. In a dosing system it is the fraction that stays behind: accumulating in tanks, loading onto filters and strainers, and appearing as residue on a cleaned or treated surface. For spray systems and for metal pretreatment lines, insolubles usually create maintenance cost long before they create a quality complaint.
Where a process is sensitive, state the internal insolubles limit in the RFQ alongside the iron limit rather than accepting the published figure by default. A tighter limit is a specification question to settle before the first order.
Investigating an excursion
When an impurity excursion is suspected, the sequence that resolves it fastest is: identify the batch number from the bag or the packing list, request the COA for that specific batch, and compare the reported values against both the published limit and your own incoming test result. A disagreement between the COA and your own laboratory is a method question — sampling point, sample preparation and analytical method all move these numbers — and is worth settling before it is treated as a material question.
Where a batch is genuinely outside specification, batch traceability from the finished bag back to the production lot is what makes the investigation containable rather than open-ended.
How to verify this yourself
- Read Fe and water-insoluble matter from the batch-specific COA, not from a general product brochure.
- Record the batch number at goods-in so a later complaint can be traced to a specific production lot.
- Where your incoming result disagrees with the COA, compare analytical methods and sampling points before treating it as a material problem.
- Request a measured whiteness value separately if the end product is white — it is a different measurement from the iron limit.
What this page does not claim
- The iron limit does not constitute a commitment that a finished formulation will hold its colour; colour depends on the whole system.
- This page does not state a corrosion outcome for any substrate — that requires the buyer's own conditions and testing.
- Values other than those marked as published specification are confirmed per grade or per batch, not offered as standing limits.
FAQ
What is the iron content of sodium metasilicate?
Why does a detergent producer specify low iron?
What is water-insoluble matter and what limit applies?
My incoming test disagrees with your COA. What now?
Ready to RFQ
A quotation needs grade and monthly quantity (MT), plus packing, destination port and buyer type. The form will not submit without these. Approved TDS, SDS and stamped COA samples are on Downloads; a shipment COA is issued per order. No prices are shown on the form.
- Grade
- Monthly quantity (MT)
- Packing
- Destination port
- Documents needed