STPP in Ceramic Slurry: Dispersing Mechanism, Zeta Potential and Dosage Calculation

Published: May 2026 | Last Updated: May 2026 | Evidence Tier: B (Engineering Evidence — Industrial Chemistry Principles + Application Data)

⚡ Quick Answer

STPP (sodium tripolyphosphate, CAS 7758-29-4) disperses ceramic slurry by adsorbing its polyphosphate chains onto positively charged clay particle edges, reversing their surface charge to strongly negative (Zeta potential typically −25 to −40 mV), creating electrostatic repulsion that breaks particle clusters and dramatically lowers viscosity. Effective dosage for ceramic body slip is typically 0.3–0.8% by slurry dry weight, depending on clay content and mineralogy. Over-dosage beyond the optimal point causes charge reversal or ionic bridging, leading to re-flocculation and viscosity increase.

🔩 Quick Facts: Ceramic-Grade STPP

Parameter Typical Value Why It Matters Buyer Note
CAS No. 7758-29-4 Identifies sodium tripolyphosphate (STPP); distinguish from TSPP (CAS 7722-88-5) Confirm on COA before first order
Assay (as Na₅P₃O₁₀) ≥ 94.0% Lower assay = more inert filler, reduced dispersing efficiency per unit weight Request COA; ceramic grade should meet ≥94%
P₂O₅ Content ≥ 56.5% Direct indicator of active phosphate content; key for dosage calculation and batch consistency Verify against certificate; subject to shipped batch COA
Phase Composition Form I + Form II mixed, or Phase II dominant Phase I (high-temp form) hydrolyzes faster; Phase II preferred for stable slurry applications Specify application to supplier; confirm phase on TDS
Iron (Fe₂O₃) ≤ 0.05% Iron contamination causes discoloration in white or light ceramic bodies and glazes Critical for sanitaryware and floor tile; request COA value
Bulk Density 0.5–0.9 g/cm³ Affects dissolution rate in slurry; lower density dissolves faster but requires careful storage Confirm packing method (25 kg PP bag with inner PE liner recommended)
Typical Dosage (body slip) 0.3–0.8% of dry solids Starting reference; actual optimum must be verified by viscosity trial in your specific slurry See dosage section below; all values subject to COA

All specifications subject to shipped batch COA. Contact Goway for current batch data and ceramic-grade TDS.

📋 Request Ceramic-Grade COA / TDS: Tell us your application (body slip / glaze / sanitaryware), estimated slurry dry-solids content, and destination port. → Submit Inquiry

📚 Table of Contents

1. Why STPP Disperses Ceramic Slurry: The Electrochemical Mechanism

Ceramic body slurry is a suspension of clay minerals (primarily kaolinite, illite, ball clay), feldspar, quartz, and other non-clay raw materials in water. Without any dispersant, clay particles exhibit an asymmetric surface charge: the flat faces carry a permanent negative charge from isomorphous substitution, while the edges carry a pH-dependent positive charge from broken Si–O and Al–O bonds.

This charge asymmetry drives clay particles to form edge-to-face aggregates — often called a “house-of-cards” structure — that trap water within the network, producing a highly viscous, gel-like slurry that is difficult to pump, spray-dry, or press.

1.1 How STPP Changes the Particle Surface

STPP (Na₅P₃O₁₀) dissociates in water to release the triphosphate anion P₃O₁₀⁵⁻, a highly charged polyphosphate chain. This anion has a strong affinity for the positively charged clay particle edges. The mechanism proceeds in three steps:

  1. Adsorption: P₃O₁₀⁵⁻ anions adsorb onto the positive alumina-rich edge sites of clay particles through electrostatic attraction and ligand exchange.
  2. Charge reversal: The adsorbed high-charge phosphate layer converts the edge surface from positive to strongly negative, effectively eliminating the edge(+)/face(−) asymmetry.
  3. Electrostatic repulsion: Now uniformly negative on all surfaces, clay particles repel each other. The house-of-cards structure collapses, trapped water is released, and viscosity drops significantly — often by 40–70% at optimal dosage.
BEFORE STPP Addition                     AFTER STPP Addition

   [−FACE−]                              [−FACE−]
       |    ↗ edge(+)                     P₃O₁₀ chains adsorbed
  [−FACE−]  → forms house-of-cards    [−FACE−] ← repels ← [−FACE−]
       ↖ edge(+)                          ↑
                                     [edge now (−)]
                                     Electrostatic repulsion
                                     → slurry flows freely

Slurry state:  Highly viscous           Low viscosity, stable suspension
Edge charge:   Positive (attractive)     Negative (repulsive) after STPP
Zeta potential: ~0 to −10 mV            −25 to −40 mV (optimal range)

Figure 1: Schematic of STPP dispersing mechanism in ceramic slurry. The polyphosphate anion P₃O₁₀⁵⁻ adsorbs onto clay particle edges, reversing edge charge from positive to negative and creating electrostatic repulsion that breaks particle aggregates.

2. Zeta Potential Explained: The Dispersing Force Behind STPP

Zeta potential (ζ) is the electrokinetic potential at the shear plane of a colloidal particle in suspension. It is the most reliable single metric for predicting dispersion stability in ceramic slurry.

Table 2: Zeta Potential and Ceramic Slurry Stability
Zeta Potential (mV) Slurry State Practical Effect STPP Action
0 to −10 mV Flocculated / Unstable High viscosity; rapid sedimentation; pump blockage risk Pre-STPP addition; or over-dosage reversal zone
−10 to −20 mV Partially dispersed Moderate viscosity; some flow but inconsistent Under-dosage zone; increase STPP
−25 to −40 mV Well dispersed (Optimal) Low, stable viscosity; excellent spray-drying and pressing behavior Target zone for STPP dosage optimization
< −40 mV Over-dispersed / Risk zone Viscosity may plateau or begin to rise; risk of Na⁺ electrolyte effect at very high STPP Over-dosage territory; reduce STPP, verify with viscosity trial

2.1 Why Zeta Potential Matters More Than Viscosity Alone

Viscosity measurement tells you the current state of the slurry. Zeta potential tells you why the slurry behaves that way and whether it will stay stable. Two slurries can have similar viscosity readings but very different Zeta potentials — one approaching flocculation and one stable. If you are troubleshooting slurry instability without Zeta potential data, you are working with incomplete information.

For ceramic operations with access to a Zeta potentiometer (or laboratory testing service), measure Zeta potential at your target STPP dosage, at ±10% and ±20% of that dosage, and map the curve. The optimal STPP dosage for your specific clay body is the one that achieves the Zeta potential in the −25 to −40 mV range at the lowest addition rate.

3. Dosage Calculation: How to Find Your Optimal Addition Rate

There is no universal STPP dosage for ceramic slurry. The optimal rate depends on three primary variables: (1) clay mineral type and content, (2) slurry dry-solids concentration, and (3) water ionic strength. The following framework provides a starting point for laboratory trials.

3.1 Reference Range by Application

Table 3: STPP Starting Dosage by Ceramic Application (% of dry solids weight)
Application Typical Clay Content STPP Starting Dosage Process Notes
Wall / Floor Tile Body Slip 20–40% clay minerals 0.30–0.50% Most common application; start at 0.35% and adjust ±0.05% per viscosity trial
Sanitaryware Casting Slip 40–60% clay minerals 0.40–0.65% Higher clay content demands higher STPP; casting slip also requires longer milling time for STPP dissolution
Technical Ceramic / Alumina Slip Low clay, high alumina (>80%) 0.10–0.30% Alumina surface chemistry differs significantly from clay; STPP adsorption mechanism less effective; consider SHMP or other deflocculants
Porcelain / Fine China Body 25–45% clay, high feldspar 0.30–0.55% Feldspar releases K⁺/Na⁺ which compete with P₃O₁₀⁵⁻; may require slightly higher addition
Spray-Dried Granule Slurry Varies 0.35–0.60% Target slurry viscosity for spray drying typically 500–800 mPa·s (Ford Cup); adjust STPP to reach this range

Note: All dosages are expressed as % of total dry solids weight (not slurry total weight). All values are starting references for laboratory trials only. Actual optimum must be verified under your specific clay mineralogy, water quality, and milling conditions. (Source: Goway Application Data; all values subject to COA)

3.2 Step-by-Step Trial Protocol

  1. Fix your slurry parameters first. Establish a baseline: fixed dry-solids content (e.g., 65–68% wt), fixed water source (or deionised water for lab trial), fixed ball-milling time and intensity.
  2. Prepare STPP stock solution. Dissolve STPP in a small volume of warm water (40–50°C) to create a 10–20% concentration stock solution. Allow to cool before use. This ensures complete dissolution before addition to slurry.
  3. Run a dosage ladder. Prepare 6–8 samples at the following STPP levels (% dry solids): 0.10%, 0.20%, 0.30%, 0.40%, 0.50%, 0.60%, 0.70%, 0.80%. Add STPP solution at the start of milling, not after.
  4. Measure viscosity after equivalent milling. Use a Ford Cup No. 4 or a rotational viscometer at a standardised shear rate (e.g., 20 rpm). Record viscosity at T+30 min, T+60 min, and T+2 hr to assess time-stability.
  5. Plot the dosage–viscosity curve. Identify the minimum viscosity point. This is your preliminary optimum. Verify with at least two additional trials at ±0.05% around the optimum.
  6. If available, confirm with Zeta potential measurement. The optimum dosage should deliver Zeta potential in the −25 to −40 mV range. If Zeta potential is below −40 mV at minimum viscosity, you may have room to reduce dosage slightly.

3.3 Impact of Clay Content on Required Dosage

STPP dosage scales approximately linearly with clay mineral content in the body, because the polyphosphate chains adsorb specifically onto clay particle surfaces (not quartz or feldspar). A rough rule of thumb used in ceramic engineering:

Estimated STPP Dosage (% dry solids) ≈ Clay Content (%) × 0.012 to 0.018

Example:
  30% clay body → 30 × 0.015 = 0.45% STPP (starting estimate)
  50% clay body → 50 × 0.015 = 0.75% STPP (starting estimate)

⚠ This is an estimation formula only. Actual optimum requires laboratory viscosity trial.
   Kaolinite-rich bodies tend toward the lower coefficient (0.012).
   Ball clay-rich bodies (higher BET surface area) tend toward the higher coefficient (0.018).
   (Source: Goway Application Data, engineering estimation)

4. Dosage–Viscosity Relationship: Reading the Curve Correctly

The relationship between STPP addition rate and ceramic slurry viscosity follows a characteristic non-linear curve — often described as a “deflocculation curve” — that every ceramic engineer working with phosphate dispersants should understand.

Viscosity
(mPa·s)
   |
1800|  ●
1600|    ●
1400|      ●
1200|          ●
1000|              ●
 800|                  ●
 600|                      ●   ← Viscosity minimum (optimal zone)
 400|                        ●   ●
 200|                              ●   ←← Caution: viscosity begins to rise
   |                                      (over-dosage / re-flocculation zone)
   +----+----+----+----+----+----+----+----+--→  STPP Dosage (% dry solids)
      0.1  0.2  0.3  0.4  0.5  0.6  0.7  0.8

   Zone 1 (0–0.3%):  Under-dosage — insufficient charge reversal
   Zone 2 (0.3–0.6%): Deflocculation — viscosity drops rapidly
   Zone 3 (optimal):  Minimum viscosity — target zone (Zeta −25 to −40 mV)
   Zone 4 (>0.7%):   Over-dosage — viscosity may plateau or increase

Figure 2: Schematic STPP dosage–viscosity deflocculation curve for a typical
ceramic body slip (30–40% clay content, 65% dry solids, deionised water).
Exact inflection points vary with clay type, water quality, and slurry formulation.
All data points are illustrative; validate under your specific process conditions.
(Source: Goway Application Data)

4.1 The Three Critical Zones

Table 4: Three Zones of the STPP Deflocculation Curve
Zone STPP Level Slurry Behavior Action Required
Zone 1: Under-dosage Below optimal High viscosity; slurry may appear thick or gel-like; insufficient edge charge reversal; particles still aggregate Increase STPP in 0.05% increments; re-measure viscosity after 30 min mixing
Zone 3: Optimal At viscosity minimum Lowest stable viscosity; excellent flowability; consistent spray-drying behavior; Zeta potential −25 to −40 mV Lock this dosage as process parameter; verify batch-to-batch with Fordcup or viscometer QC check
Zone 4: Over-dosage Above optimal Viscosity stops falling or begins to rise; slurry may become thixotropic; over-dispersion leads to re-flocculation via ionic bridging Reduce STPP; do not continue increasing; see Section 5 for over-dosage diagnosis

5. Over-Dosage Warning: When More STPP Causes More Problems

Over-dosage is one of the most common STPP-related problems in ceramic production, particularly in plants that increase STPP “just to be safe” when slurry viscosity feels inconsistent. Understanding the mechanism prevents this mistake.

5.1 Two Mechanisms Behind Over-Dosage Re-flocculation

  1. Na⁺ ionic strength effect (electrolyte compression): At very high STPP dosages, the accompanying Na⁺ ions increase the ionic strength of the slurry water. High ionic strength compresses the electrical double layer around particles, reducing the effective repulsion range and allowing van der Waals attractive forces to dominate. Result: particles re-aggregate despite high phosphate concentration.
  2. Phosphate bridging: Excess polyphosphate chains can form bridges between two positively charged sites on different particle surfaces (or between Zeta-neutral zones that appear after full edge-site saturation), physically linking particles back together. This is more common in high-electrolyte or hard-water systems where Ca²⁺/Mg²⁺ compete for phosphate chains.

5.2 Over-Dosage Diagnostic Signs

Table 5: STPP Over-Dosage Symptoms and Diagnosis
Symptom What It Indicates Verify By
Viscosity plateaus or rises despite STPP increase Entered Zone 4; ionic strength effect beginning Plot dosage–viscosity curve; reduce dosage by 0.1% and re-measure
Slurry appears “slimy” or thixotropic after addition Phosphate bridging; gel network forming Dilute slurry slightly with water; measure viscosity at low shear (5 rpm)
White foam or excessive bubbling during milling May indicate excess STPP or interaction with Ca-bearing minerals Check water hardness (Ca²⁺/Mg²⁺); consider water softening or switch to SHMP
Slurry stable initially but re-gels after 2–4 hours Time-delayed flocculation; common in over-dosed systems or hard water Measure viscosity at T+0, T+1hr, T+2hr, T+4hr; map time curve 
OVER-DOSAGE Re-flocculation Mechanism (Schematic)

Case A: Na⁺ Electrolyte Compression          Case B: Phosphate Bridging

High [Na⁺] → compresses double layer         P₃O₁₀ chain bridges two particles
                                              ↕
[−]←thin EDL→ [−]←thin EDL→ [−]             [-P₃O₁₀-]
                                              ↕
van der Waals forces overcome repulsion       Physical link = re-flocculation
= re-aggregation

Both effects increase viscosity despite high STPP dosage.
Solution: reduce STPP; consider switching to SHMP for high-Ca slurries.
(Source: Goway Application Data; engineering principles)

6. Process Notes: Dissolution, Addition Sequence & Water Quality

The following process notes apply specifically to STPP addition in ceramic slurry preparation. They address variables that laboratory dosage data alone does not capture.

6.1 STPP Dissolution Before Addition

  • Pre-dissolve recommended: For consistent dosing, dissolve STPP powder in warm water (40–60°C) to create a 10–15% stock solution before adding to slurry. Direct addition of dry STPP powder to a running mill can result in uneven distribution and localised over-dosage zones.
  • Dissolution time: Ceramic-grade STPP typically fully dissolves in 5–10 minutes in warm water with agitation. Phase I STPP may take slightly longer at room temperature.
  • Stock solution stability: STPP solution hydrolyzes slowly to orthophosphate at room temperature. Use freshly prepared stock solution within 24–48 hours; do not store pre-mixed STPP solution for extended periods.

6.2 Addition Sequence in Ball Milling

  • Add STPP at the start of milling, not after. Adding dispersant to a partially milled slurry can produce inconsistent charge distribution across different particle sizes. Adding at the start of milling ensures STPP distributes evenly across the particle surface during size reduction.
  • Add water before STPP: Load clay minerals and milling water first, then add the STPP stock solution, then add non-clay materials. This sequence ensures the dispersant contacts clay surfaces before aggregate structures form.
  • Allow adequate mixing time: Zeta potential stabilises within 20–30 minutes of STPP addition under normal milling conditions. Take viscosity readings no earlier than 30 minutes after STPP addition.

6.3 Water Quality Impact

Table 6: Water Quality Effects on STPP Performance
Water Condition Effect on STPP Recommendation
Hard water (Ca²⁺/Mg²⁺ > 150 ppm) Ca²⁺ reacts with P₃O₁₀⁵⁻ to form insoluble calcium phosphate; significantly reduces effective STPP concentration; may require 30–50% higher dosage to achieve same dispersing effect Use water softener; or switch to SHMP (better Ca²⁺ sequestration); measure incoming water hardness monthly
High alkalinity (pH > 9.5) Accelerates STPP hydrolysis to pyrophosphate and orthophosphate, which have weaker dispersing action Adjust slurry pH to 8.5–9.5 for optimal STPP stability; measure slurry pH before and after STPP addition
High sulphate content (> 200 ppm SO₄²⁻) Sulphate competes with phosphate for edge-site adsorption; reduces dispersing efficiency Test with deionised water baseline; if performance gap is large, switch water source or increase STPP by up to 0.1%
Deionised / soft water STPP performs at maximum efficiency; dosage curve is most predictable Preferred baseline for laboratory dosage trials; use for quality benchmarking

7. Grade Boundary: Industrial Use Declaration

⚠ Industrial Grade — Not for Food Use The ceramic-grade STPP (sodium tripolyphosphate) described in this article is manufactured and supplied as an industrial processing additive for use in ceramic body slip, glaze preparation, and related ceramic manufacturing processes. It is not approved, labelled, or intended for use as a food additive or food contact material.
  • Intended use: Ceramic slurry deflocculation, glaze suspension, sanitaryware casting slip
  • Regulatory status: Industrial chemical; not subject to food additive regulations (E451, FDA 21 CFR 182.1810, FCC)
  • Documentation available: Industrial-grade COA, SDS (GHS format), TDS — see Section 8
For food-grade STPP applications and compliance documentation (EU E451, FDA GRAS, Halal/Kosher, FCC), please refer to our Food Grade STPP Compliance Checklist.

8. Documentation Package

Goway provides the following documentation for ceramic-grade STPP on a per-batch basis. All documents are available digitally upon inquiry.

Table 7: Available Documentation for Ceramic-Grade STPP
Document Contents Format Request Via
Certificate of Analysis (COA) Batch-specific test results: Assay (Na₅P₃O₁₀), P₂O₅, Fe₂O₃, water content, pH, bulk density, lot number, manufacture date PDF, per batch Provided with each shipment; can be requested pre-order for qualification
Safety Data Sheet (SDS) GHS-compliant 16-section SDS; hazard classification, first aid, handling, storage, disposal, transport PDF (GHS format) Available on request; specify destination country for localised version
Technical Data Sheet (TDS) Typical product specifications, application recommendations, dosage reference data, storage conditions PDF Available on request; specify “ceramic grade STPP TDS”
Packing List / Weight Certificate Per-shipment document; net/gross weight, container load, lot reference PDF, per shipment Issued with shipping documents

📋 Request Documentation: To receive ceramic-grade STPP COA, SDS, and TDS, submit your inquiry with: (1) application type, (2) estimated monthly quantity, (3) destination country/port. → Submit Inquiry Now

9. Frequently Asked Questions

9.1 Application & Function

Q1: Can STPP be used in glazes as well as body slip?

STPP can be used in glaze suspensions, but its effectiveness is more limited than in body slip due to the different surface chemistry of glaze raw materials (feldspars, frits, silica) compared to clay minerals. STPP works primarily through clay-edge adsorption; for glaze systems with low clay content or high calcium carbonate content, SHMP (sodium hexametaphosphate) is generally more effective due to its stronger calcium sequestration capability. For mixed body-glaze systems, a combination approach (primary: STPP for body, secondary: SHMP for glaze) may be considered. See our Ceramic Phosphate Dispersant Selection Guide for a full comparison.

Q2: How does STPP compare to sodium silicate (water glass) as a ceramic dispersant?

Both STPP and sodium silicate function as anionic dispersants for ceramic slurry, but through partially different mechanisms. STPP provides stronger, more stable charge reversal via polyphosphate adsorption and is more effective at lower dosages (0.3–0.5% vs. 0.5–1.5% for sodium silicate). Sodium silicate is lower cost and more available in many markets, but is more sensitive to pH and temperature, and can cause problems in slurries used at high temperatures or with acidic raw materials. Many ceramics operations use STPP + sodium silicate in combination — STPP for primary deflocculation and a small amount of sodium silicate for pH buffering and additional stability.

9.2 Specifications & Quality

Q3: What is the difference between Phase I and Phase II STPP, and which should I specify for ceramic use?

STPP exists in two crystalline forms: Phase I (α-form, high-temperature form) and Phase II (β-form, low-temperature form). Phase I has a higher hydration rate and dissolves faster in water but is also less stable during storage. Phase II is more stable, dissolves slightly slower, but provides consistent performance over time. For ceramic slurry applications requiring stable dosage performance across batches, Phase II dominant or a defined Phase I/II ratio is recommended. Specify your requirements to your supplier and ask for phase composition on the COA or TDS. Goway can provide phase composition data on request.

Q4: Does the iron content in STPP affect ceramic body colour?

Yes. Iron (measured as Fe₂O₃) in STPP can contribute to yellowish or brownish discolouration in white-firing ceramic bodies and glazes, particularly at dosages above 0.5% and in high-temperature firings (>1100°C). For standard tile bodies (non-white), this is typically not a concern at normal dosages. For sanitaryware, white wall tiles, or technical ceramics requiring high whiteness, specify low-iron ceramic-grade STPP with Fe₂O₃ ≤ 0.03% and request the iron value on the COA for each batch.

9.3 Packaging & Handling

Q5: What are the packaging options and storage requirements for ceramic-grade STPP?

Ceramic-grade STPP is supplied in 25 kg multi-wall paper bags with inner PE liner, or in 1,000 kg jumbo bags (super sacks) with liner. STPP is hygroscopic: it absorbs moisture from the air and can cake or partially dissolve, reducing dispersing efficiency. Store in a dry, ventilated warehouse away from moisture sources; ideal storage conditions are <30°C, <60% relative humidity, off the floor on pallets. Opened bags should be resealed immediately. Shelf life is typically 24 months from manufacture date under proper storage conditions.

9.4 Documentation & Supply

Q6: Can you provide a sample for laboratory dosage trial before we place a commercial order?

Yes. Goway provides laboratory samples of ceramic-grade STPP for qualified buyers conducting dosage trials. Sample requests should include your application (body slip, glaze, casting slip), approximate slurry formulation (if available), and your target viscosity range. Samples are typically shipped with a COA and TDS. To request a sample, use the inquiry form and specify “ceramic STPP sample request” in the message. → Submit Sample Request

9.5 Grade Boundary

Q7: Can industrial-grade STPP be used as a food additive in meat or seafood processing?

No. Industrial-grade ceramic STPP is not approved, labelled, or suitable for food use. Food-grade STPP (E451(i) in the EU; FDA 21 CFR 182.1810 GRAS in the USA) requires FCC-grade purity specifications, specific heavy metal limits, and food safety management certification (ISO 22000, HACCP). Industrial-grade STPP may contain higher levels of heavy metals and other impurities that are acceptable for ceramic processing but not for human food contact. For food-grade STPP requirements and documentation, see our Food Grade STPP Supplier Compliance Checklist.

9.6 Troubleshooting

Q8: I increased STPP dosage but viscosity went up, not down. What is happening?

This is the classic symptom of over-dosage re-flocculation — you have passed the viscosity minimum and entered Zone 4 of the deflocculation curve (see Section 4). The two most common causes are: (1) ionic strength effect from excess Na⁺ compressing the electrical double layer, and (2) phosphate bridging between particle surfaces. Do not continue increasing STPP. Reduce the dosage by 0.10–0.15% and re-measure. If hard water is involved, the problem may be amplified by Ca²⁺ consuming phosphate chains — test with softened water or consider switching to SHMP, which has better calcium sequestration properties.

Q9: Our slurry is well-deflocculated in the lab but re-gels in the production tank after 2–3 hours. Why?

Time-delayed re-flocculation after initial good deflocculation typically has three possible causes: (1) STPP hydrolysis — at elevated temperatures (>30°C in the storage tank) or high pH (>9.5), STPP slowly hydrolyzes to pyrophosphate and orthophosphate, which have much weaker dispersing action; add STPP closer to the point of use rather than at the start of a long hold time; (2) Electrolyte migration — soluble salts (Ca²⁺, Mg²⁺, K⁺) leaching from raw materials or water slowly displace phosphate from particle surfaces over time; check water hardness and raw material soluble salt content; (3) Biological activity — in summer or warm climates, microbial growth in standing slurry can destabilize dispersant performance; consider adding a biocide approved for industrial use.