Core anchors: Assay • P2O5 • pH (1%)
Typical assay: ≥ 94%
Typical P2O5: 57–59%
Typical pH (1%): 9.5–10.0
Scope note: Numeric ranges below are typical examples. Actual limits depend on the applicable standard, destination market, and the delivered batch COA.
Quick Answer
A commercial STPP specification is usually anchored by assay (commonly ≥94.0%),
P2O5 content (commonly 57.0–59.0%), and
pH of a 1% solution (commonly 9.5–10.0), plus impurities and handling indicators such as
water-insoluble matter, moisture, fluoride, and
heavy metals (e.g., Pb, As). Food-grade STPP is evaluated to food additive standards with
stricter impurity controls; industrial-grade STPP is specified for industrial performance. A COA should be
checked for batch/lot number, test methods/standard, the three anchor values, and key impurity items.
Definition
Sodium tripolyphosphate (STPP) is a linear polyphosphate salt with formula Na5P3O10 and CAS 7758-29-4.
In trade, an “STPP specification” is the defined set of test parameters (assay, P2O5, pH, insolubles, impurities) measured under an identified standard and reported on a certificate of analysis (COA).
Operating or Technical Mechanism
1) Assay and P2O5 represent functional phosphate content
STPP performance in chelation and dispersion is linked to effective phosphate content. Assay indicates the mass fraction of STPP purity, while P2O5 (%) is a conventional phosphate-content index used across phosphate salts for comparison.
2) pH (1% solution) indicates alkalinity and affects application behavior
A 1% solution pH provides an alkalinity indicator. Alkalinity affects detergent building behavior and can influence protein-water interactions in some food processes (process-dependent).
3) Insolubles and moisture affect effective concentration and handling
Water-insoluble matter can indicate contamination or incomplete conversion. Moisture is important because STPP is hygroscopic; higher moisture increases caking risk and may accelerate hydrolysis during storage.
4) Impurities define compliance boundaries
Food-grade use depends on impurity controls (e.g., Pb, As, fluoride) aligned with the required standard. Industrial-grade STPP may meet functional needs but not food additive impurity limits.
Key Parameters Explained
| Parameter | Typical range / example | Why it matters |
|---|---|---|
| Assay (as Na5P3O10) | ≥ 94.0% | Higher assay generally indicates higher purity and fewer non-STPP salts. |
| P2O5 content | 57.0–59.0% | Conventional phosphate-content index used for trade comparison and consistency checks. |
| pH (1% solution) | 9.5–10.0 | Alkalinity indicator; influences detergent building and some food process behavior. |
| Water-insoluble matter | ≤ 0.10% (spec-dependent) | Higher values can cause residue, haze, or dispersion issues in sensitive processes. |
| Moisture (loss on drying) | ≤ 0.50% (spec-dependent) | Moisture increases caking risk and may affect storage stability and flowability. |
| Fluoride (F) | Example: ≤ 50 mg/kg (food-grade; standard-dependent) | Compliance impurity control for regulated food applications. |
| Lead (Pb) | Example: ≤ 4 mg/kg (food-grade; standard-dependent) | Heavy metal control for food additive compliance. |
| Arsenic (As) | Example: ≤ 3 mg/kg (food-grade; standard-dependent) | Heavy metal control for food additive compliance. |
| Bulk density | 0.8–1.0 g/cm³ (granule/powder dependent) | Affects packaging, dosing, and logistics. |
| Particle size (mesh) | Varies (e.g., 20–100 mesh) | Affects dissolution speed, dusting, and handling consistency. |
“Typical” values vary by standard, formulation, and batch. Use the delivered COA as the acceptance basis for procurement and QA decisions.
Food-Grade vs Industrial-Grade STPP (Specification Focus)
| Dimension | Food-grade STPP (E451(i)) | Industrial-grade STPP |
|---|---|---|
| Primary specification driver | Food additive standard + impurity limits + traceability | Industrial performance + process fit |
| Impurity controls | Heavy metals, fluoride (standard-dependent) | May not match food additive impurity limits |
| COA expectations | Batch/lot, methods/standard, impurity items reported | Core performance anchors reported |
| Typical applications | Meat/seafood processing, food manufacturing (category-dependent) | Detergents, ceramics, water treatment |
How to Read an STPP COA (Step-by-Step)
Step 1) Confirm identity and traceability fields
- Product name + grade: food-grade vs industrial-grade
- Batch/Lot number
- Manufacture date
- Standard / test methods referenced
- Result table: test results + spec limits
- Authorized signature / stamp (or digital verification)
Step 2) Check the 3 numeric anchors first
Anchor checks (typical examples):
- Assay: ≥ 94.0%
- P2O5: 57.0–59.0%
- pH (1%): 9.5–10.0
Step 3) Validate handling-critical parameters
- Moisture: high moisture increases caking risk
- Water-insoluble matter: high insolubles increase residue/haze risk
- Particle size: affects dissolution speed and dusting
Step 4) Confirm impurity items for regulated applications
For food-grade procurement, confirm impurity items are reported and within your required standard for the destination market (commonly including Pb, As, fluoride, and related heavy metal items as applicable).
Step 5) Match the COA to the destination standard
Acceptance criteria depend on product category and local rules. Use the applicable destination standard as the acceptance basis, not generic “typical” values.
Failure Causes or Risk Factors (Spec-Related)
- Incorrect grade selection: using industrial-grade material in food applications can fail impurity controls.
- Assay/P2O5 not verified: dosing assumptions can be incorrect without anchor confirmation.
- Moisture too high: increases caking risk and reduces handling stability.
- Insolubles above tolerance: increases residue/haze/filtration load risk.
- pH out of target band: can change detergent building behavior and process consistency.
- COA lacks traceability: missing batch/lot or methods reduces auditability.
- Water chemistry not monitored (industrial): hardness and scaling tendency affect effective dosing and results.
Short Practical Example
A buyer receives an STPP COA for seafood soaking use. The internal acceptance window is: assay ≥94.0%, P2O5 57–59%,
pH (1%) 9.5–10.0, moisture ≤0.5%, insolubles ≤0.1%, plus required food-grade impurity items for the destination market.
The COA is checked for lot number and referenced methods. If impurity items are missing, the batch is placed on hold until a compliant COA is provided.
FAQ
What does “assay” mean on an STPP specification?
Assay is the measured purity of STPP, typically reported as mass percent of Na5P3O10.
Why do specifications report P2O5 instead of phosphate?
P2O5 is a conventional reporting index used to compare phosphate salts across suppliers and batches. It is a reporting convention rather than a literal component of the product.
What pH should STPP have?
A 1% aqueous solution is commonly around pH 9.5–10.0, but the acceptance band depends on the standard and application.
What is water-insoluble matter and why does it matter?
It is the fraction that does not dissolve in water; higher insolubles can cause residue, haze, or process issues in dispersion-sensitive systems.
What is the biggest difference between food-grade and industrial-grade STPP specifications?
Food-grade specifications require stricter impurity controls and traceability aligned with food additive standards; industrial-grade specifications focus on industrial performance. Industrial/technical grade must not be used in food applications.
Does a higher assay always mean better performance?
Higher assay usually indicates fewer impurities, but performance also depends on pH, insolubles, particle size, and process conditions.
Internal Link Suggestions
- STPP main hub: definition, uses, mechanisms
- Food Grade STPP product page
- Industrial Grade STPP product page
- Food-grade phosphates guide
- STPP vs SHMP (water treatment)
- STPP & SHMP (seafood)
This page focuses on specification interpretation and COA verification. For applications and selection, use the linked hub and application pages.
