Agriculture · Technical article

Low-pH liquid micronutrients with GLDA

Identity

R E L I A N T C H E M I C A L

Low-pH liquid chelate design with GLDA

Key Scientific Principles

GLDA is a chelating agent that can bind metals at multiple points (hexadentate), but it

is not as strong as EDTA. Because of this lower binding strength (lower stability constant

or “logK”), you typically need at least a 1:1 molar ratio of GLDA to metal, and in practice

often 10–50% excess GLDA to ensure stable chelation.

In theory, most metal-GLDA complexes form in a 1:1 ratio (one metal ion to one GLDA

molecule). However, real-world conditions, especially low pH, require extra GLDA to keep

the metal fully chelated.

At low pH (2.5–3.5), stability decreases significantly because GLDA becomes protonated,

meaning some of its binding sites are no longer available.

Hydrogen ions (H⁺) compete with metal ions for binding
Different metals also bind to GLDA with varying strength. The general order of stability

is:

Fe³⁺ (very strong)
Cu²⁺
Zn²⁺ ≈ Mn²⁺
Ca²⁺
Mg²⁺ (weakest)
This means weaker-binding metals (like Ca and Mg) require more excess GLDA, while

stronger-binding metals (like Fe³ ⁺) are more stable even at lower ratios.

Metal Common

Salt Used

Preferred

Salt (Low

pH)

Molar Ratio

(GLDA:Metal)

Weight

Ratio (GLDA

47% : Salt)

Stability @

pH 2.5–3.5 Notes

Zn²⁺ ZnSO₄·H₂O /

ZnO

ZnSO₄

(better

solubility)

1.2–1.5 : 1 ~2.5–3.2 : 1 Moderate

ZnO needs

acid pre-

Typical properties and specifications

dissolution

Mn²⁺ MnSO₄·H₂O MnSO₄ 1.2–1.4 : 1 ~2.3–3.0 : 1 Moderate Similar to

Fe³⁺ FeCl₃ /

Fe₂(SO₄)₃

FeCl₃

(more

soluble)

1.5–2.0 : 1 ~3.0–4.5 : 1

Good (best

among

GLDA)

Strong

complex

(logK ~15)

Fe²⁺ FeSO₄ FeSO₄

(fresh) 1.5–2.0 : 1 ~3.0–4.0 : 1 Poor–

moderate

Oxidizes →

Fe³⁺

Cu²⁺ CuSO₄·5H₂O CuSO₄ 1.2–1.5 : 1 ~2.5–3.2 : 1 Good Stronger

than Zn

Mg²⁺ MgSO₄·7H₂O MgSO₄ 1.3–1.6 : 1 ~2.5–3.5 : 1 Weak logK ~5.5

Ca²⁺ CaCl₂ /

Ca(NO₃)₂ CaCl₂ 1.5–2.0 : 1 ~3.0–4.5 : 1 Weak Easily

displaced

Summary table

R E L I A N T C H E M I C A L

Key Observations and Salt Selection

GLDA is borderline at low pH for most metals

At a low pH of 2.5–3.5, GLDA does not hold onto divalent metals very strongly. As a result,

some of the metal can come out of the complex (dissociate).

Metals affected: Zn, Mn, Mg, Ca
These complexes are only partially stable
You must use extra GLDA to compensate

Practical guideline:

Use at least 1.3–1.5 molar ratio of GLDA to metal

Iron (Fe) behaves very differently

Iron is a special case and needs separate consideration.

Fe³⁺ (ferric iron):

Forms very strong complexes with GLDA
Remains fairly stable even at low pH

Applications and benefits

Fe²⁺ (ferrous iron):

Unstable
Easily oxidizes to Fe³⁺
Can precipitate if not properly chelated

Recommendation:

Prefer Fe³⁺ + GLDA
Or intentionally oxidize Fe² ⁺ to Fe³⁺ before/while chelating

Calcium and Magnesium are the weakest

GLDA binds poorly with Ca and Mg, especially at low pH.

Stability strength:

Ca: moderate–weak
Mg: weak

At pH below 3, they are very prone to breaking apart

Expect:

Higher GLDA requirement
Lower overall stability compared to other metals

Salt Selection

Choosing the right metal salt makes a big difference in stability and solubility.

Sulfates (Best choice)

Stable, cost-effective, widely used
Examples: ZnSO₄, MnSO₄

Chlorides (Good option)

Improve solubility
Especially useful for Fe and Ca

Nitrates (Effective but expensive)

Very soluble
Used when clarity is critical

Oxides (Not recommended)

Examples: ZnO, CuO
Require acid to dissolve first
Can cause instability

Rule of thumb:

Always use fully soluble salts (sulfates or chlorides) for low pH liquids

R E L I A N T C H E M I C A L

Low-pH liquid chelate design with GLDA

Stabilization Strategies

1. Use excess GLDA (most important step)

Because GLDA is weaker than stronger chelants, extra ligand is required.

Zn / Mn / Cu → ~1.2× GLDA
Ca / Mg / Fe → ~1.5–2.0× GLDA

2. Follow the correct mixing sequence

Order of addition is critical to avoid precipitation . The following sequence is recommended:

Use levels, packaging, and storage

1. Dilute GLDA in water

2. Adjust pH to ~4–5

3. Add metal salts slowly

4. Allow chelation to complete

5. Then reduce pH to 2.5–3.5

This ensures proper complex formation before acid stress.

3. Use mixed chelation systems

GLDA alone may not be enough at low pH, especially in multi -metal systems. A better approach

is:

GLDA + DTPA → improves iron stability
GLDA + EDTA → improves Zn and Mn stability
GLDA + IDS or MGDA → balances performance + biodegradability

4. Control pH with buffering

Use:

Citric acid
Nitric acid (carefully controlled)

Maintain: Ideal pH range: 2.8–3.2

5. Prevent iron precipitation

For Fe²⁺ systems:

Add a mild reducing agent, or
Use Fe³⁺ with sufficient GLDA excess

6. Manage total salt load

High dissolved salts can destabilize chelates
Keep formulation balanced and not overly concentrated

When GLDA Alone is Not Enough

At low pH (2.5–3.5), consider stronger alternatives:

Fe → DTPA (best option)
Zn / Mn → EDTA or EDDS
Ca / Mg → GLDA works but remains weak
Multi-metal blends → GLDA + EDTA combination works best

Example: Practical Multi-Micronutrient Formula

For a Zn + Mn + Fe liquid at pH ~3:

GLDA 47%: 1.4–1.6 molar equivalents (total metals)

Metal salts:

ZnSO₄
MnSO₄
FeCl₃

Optional improvement: Replace 20–30% of GLDA with DTPA (for better Fe stability ). Final pH:

2.8–3.2