EDTA MnNa2

Ethylenediaminetetraacetic acid manganese disodium salt, registered under CAS 15375-84-5 and represented by the molecular formula C10H12MnN2Na2O8, is a water-soluble chelated manganese raw material used in agricultural micronutrient fluids, fertigation concentrates, and animal feed premixes. The anhydrous formula mass is 389.13 g/mol. From formula stoichiometry, the theoretical manganese content is 14.12% w/w, the sodium content is 11.82% w/w, and the EDTA tetraanion content is 74.06% w/w. If the product is supplied as the trihydrate, the theoretical manganese content drops to 12.40% w/w because the molecular mass increases by 54.05 g/mol from three water molecules. This distinction between anhydrous and hydrous specifications is critical when comparing product models, because a label value of EDTA-Mn-13 refers to the guaranteed total Mn mass fraction on an as-received basis, not to the anhydrous-stoichiometric maximum.

The dry product is usually a pale pink to off-white free-flowing powder, although the hue can deepen when residual moisture rises because the Mn(II) centre becomes partially hydrated. A specification sheet should always be read against the certificate of analysis for total manganese, chelated manganese, free metal, sodium, moisture, pH of a 10 g/L solution, and water-insoluble residue. The total manganese value is determined after acid digestion by ICP-OES according to ISO 11885, while moisture is measured by Karl Fischer titration according to ISO 760. The pH reading is taken according to ISO 10523 using reagent water conforming to ASTM D1193 Type II or better. Water-insoluble matter is checked gravimetrically after filtration through a 0.45 μm membrane filter at 20 °C. Because the disodium salt contains labile water, storage under sealed conditions at or below 30 °C and 65% RH is required; if the packaging is left open above 60% RH, the powder can agglomerate within hours and analytical sampling becomes non-representative.

In bulk distribution, product model conventions frequently include a numerical suffix that declares the minimum manganese mass fraction. A product designated EDTA-Mn-13 therefore carries a specification of 13% Mn w/w on an as-is basis, while EDTA-Mn-12.5 may be used for a lot with higher hydration or a residual carrier. These suffixes are not harmonized under a single ISO standard, so procurement documentation must state whether the value applies to total Mn or chelated Mn and whether the sample is as-received or dried. The ratio of chelated Mn to total Mn is a lot-release criterion because uncomplexed manganese oxide or manganese carbonate impurities are not equally plant-available and may settle in concentrated tanks. In a properly chelated lot, the chelated Mn fraction should approach the total Mn value within the analytical repeatability of the supplier’s complexometric method.

What Limits Chelate Persistence in Iron-Contaminated Stock Solutions?

In multi-element liquid fertilizers and recirculating feedwater, ferric iron can destabilize the Mn-EDTA complex. Published stability-constant data for EDTA complexes list log K values at 25 °C and ionic strength 0.1 M of approximately 13.9 for Mn(II)-EDTA and 25.1 for Fe(III)-EDTA. Because this difference is roughly 11 orders of magnitude, Fe(III) acts as a displacing metal when it enters the concentrated stock solution, releasing Mn(II) from the ligand. The released manganese then becomes vulnerable to oxidative precipitation as MnO2 or hydroxide under neutral-to-alkaline conditions. This displacement is not an immediate catastrophic failure at low Fe concentrations; it is concentration-dependent and becomes detectable once the molar Fe(III) concentration approaches the EDTA-bound Mn concentration. Process lines carrying iron-contaminated groundwater should therefore use separate chelate stock tanks, or an excess of free EDTA must be maintained beyond the stoichiometric point. Free iron in solution can be monitored by colorimetric methods, while total Fe and Mn in the concentrate are cross-checked by ISO 11885 after 0.22 μm membrane filtration to distinguish dissolved species from precipitated metal oxides.

Conditional stability can be calculated from formation constants and protonation constants of EDTA. At pH 7.0 the fraction of total EDTA present as the fully deprotonated tetraanion is not unity; therefore conditional Mn-EDTA stability is lower than the thermodynamic log K. In hard water with 2.0 mmol/L Ca and 1.0 mmol/L Mg, calcium competition is not negligible even though Ca-EDTA log K is approximately 10.65. The rate of displacement increases when stock pH falls below 4.5 because protonated EDTA species have lower effective chelating strength. Above pH 7.5, calcium and magnesium compete more effectively for the ligand despite lower individual stability constants, lowering the conditional stability of Mn-EDTA. The operational window for concentrated Mn-EDTA stock solutions is therefore generally maintained between pH 5.5 and 7.0; outside this band, tank-mix compatibility testing is required before production scale-up.

Compared with manganese sulfate monohydrate, EDTA MnNa2 supplies Mn without the sulfate anion. That distinction matters in hard water, where sulfate can form gypsum with calcium and increase total dissolved solids in stock tanks. Manganese sulfate is highly water-soluble, but the free Mn(II) ion is susceptible to oxidation and precipitation when pH rises above approximately 6.0 or when aeration is high. EDTA MnNa2 keeps Mn in a complexed form through the same pH region, although it is not stable indefinitely at high pH. The difference in redox behavior is measurable by dissolved oxygen decay and optical density increase in aerated solutions; published data for this specific configuration is limited, but the effect is routinely observed in field fertigation lines where iron and manganese sulfate cause filter plugging before a chelated alternative.

Compared with manganous oxide, EDTA MnNa2 is water-soluble and can be metered through venturi injectors or diaphragm pumps, whereas MnO is not suitable for solution culture because its dissolution rate is negligible at neutral pH. Compared with DTPA-manganese, EDTA-MnNa2 has a less pH-tolerant ligand and a narrower alkaline stability window. DTPA is generally selected when source water pH exceeds 7.5 and bicarbonate exceeds 200 mg/L as CaCO3, because DTPA’s conditional stability for Mn remains higher under alkaline calcium-rich conditions. EDTA-MnNa2 is therefore appropriate for acidified or naturally neutral water and for foliar applications in which the spray solution is buffered between pH 5.5 and 6.5. Relative to iron-EDTA, manganese-EDTA has lower ligand affinity, so a combined stock solution with Fe-EDTA may be stable if the Fe fraction is chelated and the pH is controlled, but the reverse addition of free ferric salts is not recommended. The product should not be regarded as a substitute for Mn-DTPA in calcareous soils with high pH and high calcium cation saturation.

When Phosphate Loadings Exceed Chelate Exchange Capacity in Recirculating Nutrient Solutions

Concentrated phosphate fertilizers such as monoammonium phosphate and monopotassium phosphate can be combined with EDTA MnNa2 only after chelate-compatibility testing. Phosphate does not directly displace Mn from EDTA under neutral pH, but high ionic strength compresses the double layer and reduces the conditional stability constant, while local pH transients during mixing can protonate the ligand and release trace free Mn. Once free Mn(II) is present in a phosphate-rich solution, sparingly soluble manganese phosphates can form and produce fine precipitates that block 0.45 μm filters and drip emitters. Production-scale failures are most common when two concentrated stock solutions are injected into the same mixing chamber without sufficient dilution water or when the pH of the phosphate stock is below 4.0. Separate stock tanks for phosphorus and manganese chelate are therefore recommended in high-demand greenhouse operations; if single-tank blending is unavoidable, the combined solution should be maintained at pH 6.0–6.5 and used within the turnover interval specified by the supplier.

Oxidative sanitizers such as sodium hypochlorite attack the EDTA ligand itself. The amine and carboxylate coordination sites are oxidizable, and the resulting degradation products have weaker chelating ability. Manganese released after hypochlorite exposure can precipitate as oxide and stain pipe walls. In irrigation lines where chlorine injection is required for biofilm control, oxidation-reduction potential should be monitored upstream and downstream, and chelate injection should be separated from sanitizer injection by at least the full pipeline transit time necessary to dissipate free chlorine residuals.

Foliar, Fertigation, and Feed-Grade Application Windows

Foliar delivery requires the spray solution to remain clear and biologically available before drying, and the absence of sulfate reduces the tendency to leave crystalline residues under low-humidity drying conditions. The typical dilution for trace element sprays is determined by local extension data; a technical evaluation should always include leaf phytotoxicity screening because droplet size, leaf cuticle thickness, and ambient temperature interact with chelate concentration. The product can be mixed with nonionic surfactants and adjuvants only after jar testing, because some alkylphenol ethoxylate systems can cause complex dissociation under extreme pH or UV exposure.

For fertigation and hydroponics, the manganese dose is usually expressed as final-solution total Mn. A typical recirculating nutrient solution for leafy crops is maintained at 0.5–1.0 mg/L total Mn, with the chelated fraction expected to remain available. However, crop-specific demand varies widely and must be derived from published nutrient solution recipes, not from product specification. The amount of product to achieve a given Mn concentration is calculated from the guaranteed Mn mass fraction; for a 13% w/w product, a 1.0 mg/L Mn target requires approximately 7.7 g/m3 of formulated powder, assuming complete dissolution and no settling. This calculation is sensitive to hydration and should be corrected using the certificate of analysis.

In animal feed premixes, EDTA MnNa2 may be used where manganese chelates are permitted, but the regulatory status depends on regional feed additive registrations. Feed-grade material is subject to heavy-metal limits and may require documentation of dioxin and polychlorinated biphenyl levels. The sodium contribution from the disodium counterion should be included in total dietary sodium modeling.

On production lines, dissolution and powder handling are more demanding than for crystalline manganese sulfate. The chelated powder can form a sticky hydrated gel layer when wetted too quickly, reducing dissolution rate and leaving agglomerates. A high-shear dispersion eductor or recirculating venturi is used to wet the powder uniformly into warm water at 20–30 °C. Dust extraction is required because airborne manganese-containing dust is a workplace exposure risk; local exposure limits are jurisdiction-specific. The final solution should be filtered through a 100 μm screen before transfer to long-term storage tanks. Bulk packaging is typically heat-sealed 25 kg moisture-barrier sacks, and partial pallets should be resealed immediately after use. Shelf life claims vary by supplier; a common storage-stability window is 24 months in unopened packaging under controlled humidity, but lot-specific expiry dates on the certificate of analysis prevail.