MultiTech Chelator A

Solubility

The solubility of MGDA as a function of pH is shown in figure 2. Like most aminocarboxylic chelates, the solubility is greatest for the fully ionized form that is present at high pH, quite similar to that of NTA. The solubility of MGDA is surpassed by the extraordinary high solubility of GLDA (Dissolvine® GL) across the entire pH range.

Table 2 lists the solubility of several chelates in various media. Here too the solubility of MGDA is similar to NTA, which may enable Dissolvine® M-40 to be used as a direct replacement for NTA in many formulations. Unlike NTA, Dissolvine® M carries no hazard warnings and may also qualify for eco labeling.

Table 2: Solubility of several chelates in various media at 25°C

Figure 2: Solubility of chelating agents, expressed as their sodium salt, in water at various pH levels

Density

The density of the liquid can be used as a quick reference for checking the concentration of the material (figure 3). The density of the solid is important for packaging and plays a role in tableting the granules.

Chemical Stability

Like all the Dissolvine® chelating agents,The thermal stability of Dissolvine® M-S has been determined using Thermal Gravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). MGDA-Na3 crystals will lose all crystal water at temperatures around 200°C and start to decompose at temperatures above 300°C.

Solutions of MGDA-Na3 are fully stable at temperatures of up to 170°C for six hours, or at 150°C for one week. This means that MGDA can be a useful biodegradable alternative to EDTA when used for scale prevention or for cleaning boilers.

Figure 3: Density of a solution of MGDA-Na3 plotted against concentration

Acid/base Dissociation Constants

Dissolvine® chelating agents are amino polycarboxylic acids that ionize in water to multiple charged species depending on pH. The ionization constants, or pKa values, for MGDA are shown in table 3. Again we see a close similarity to NTA. The ion species distribution of the MGDA molecule as a function of the pH can be calculated from the pKa values (figure 4).

Chelating Power

Chelating agents are added to products or processes to control the properties of metal ions. For example, chelating agents are used in cleaning and personal care to complex with cations (e.g. Calcium, Magnesium, Fe, etc.) and prevent reactions with other ingredients that often lead to precipitation. In other applications, chelates are used to remove unwanted scale by complexing the scale metal ions.

Table 3: The acid dissociation constants (pKa)* for MGDA, NTA and EDTA

*A.E. Martell, R.M. Smith, NIST Critically selected stability constants of metal complexes (NIST standard reference database 46, Version 7.0, 2003). pKa values: as determined at an ionic strength of 0.1M and at a temperature of 25°C, or if not available at 20°C.

Figure 4: Ionized forms of MGDA as a function of pH

Chelates are used in copper and nickel plating to deliver metal ions in the ideal form for the plating process. For each application, it is important to select a chelating agent that is sufficiently strong to do the job. An indication of the chelates’ strength or affinity for a certain metal ion can be derived from the dissociation constants, stability constants and conditional stability constants.

The stability or equilibrium constant (K), generally expressed as log K, is an indication of the strength of the complex formed between the metal ion and the chelating agent. The higher the log K value, the tighter the bond between the metal ion and the chelating agent, which in turn increases the likelihood that a complex will be formed (table 4).

The pH of the system and the oxidizing nature of the environment can affect the stability and effectiveness of the chelating system. For each metal complex, there is an optimum pH and an active pH range in which the metal complex is stable. The conditional stability constant is an indication of the stability of the complex as a function of the pH (figure 5).

Chelating Capacity

Chelates generally form 1:1 complexes with metal ions. The quantity of chelating agent needed depends both on the concentration of metal ion to be chelated and the molecular weight of the chelate. In general, while a chelate with a high molecular weight will complex a metal ion more strongly than a chelate with a low molecular weight, a larger quantity will be needed. The chelating capacity of Dissolvine® M-40 and Dissolvine® M-S expressed as mg chelate/g MGDA product are compared to NTA and EDTA products in table 5.

The experimentally determined CaCO3 chelating value (CaCV) of Dissolvine® M-40 is 147 mg/g and 297 mg/g Dissolvine® M-S. These measurements were performed using Ca2+ as a titrant and with two different means to detect the endpoint: one with a Ca2+ ion selective electrode and another using carbonate as a precipitation indicator. The found values correspond well with the theoretical CaCV.

Table 4: Stability constants (log K values1) and active pH range for Dissolvine® M-40 / Dissolvine® M-S (MGDA)

1. A.E. Martell, R.M. Smith, NIST Critically selected stability constants of metal complexes (NIST standard reference database 46, Version 7.0, 2003); Log K values as determined at an ionic strength of 0.1M and at a temperature of 25°C or 20°C. Log K for Fe3+ and Mn2+ the figure was extracted from P.T. Anastas, Green Processes, Volume 9: Designing Safer Chemicals.
2. Active pH range: calculated for demineralized water at 0.1 mol/l. Lower pH limit: the conditional stability constant logK’ ≥ 3. Upper pH limit is based on the precipitation of the metal hydroxide; at upper pH limit the fraction chelated ≥ 95%.

Unlike very strong chelates like EDTA and DTPA, the ‘chelating ability’ of MGDA is dependent on the testing conditions (the indicator, temperature and concentration). Besides the theoretical chelating capacity, there is also a practical ‘chelating capacity’. For example, when using Ca ions this practical chelating capacity is often called Ca dispersing power.

To illustrate the strong calcium binding strength of MGDA, experiments have been performed with various chelating agents and the calcium ion indicator Hydroxy Naphthol Blue (HNB), which is used in this experiment as a competitive chelating agent. HNB has a relatively high affinity for calcium and shifts color from blue to red when fully complexed to calcium. As a result, the color of a solution containing calcium ions, HNB and the tested chelate gives a measure for the calcium binding efficiency of the chelate vs. the HNB.

Figure 5: Theoretical curves of the conditional stability constant (log K’) of MGDA for various metal ions as a function of pH (1:1 metal:chelate complex).

In figure 6 the calcium affinity at pH 11–12 for a number of chelates is compared. The key finding is that Dissolvine® M-40 / M-S as well as Dissolvine® GL are very effective for complexing hard water ions.

Another measure of the ability to complex the calcium and magnesium hard water ions, and thus to soften water, is presented in figure 7. It shows a calculated plot of water hardness versus the strength of a builder (log K) in the presence of an equal molar amount of Ca ions and chelates. MGDA is capable of achieving low water hardness levels, while citrate is only capable of providing a medium hardness unless a significantly higher amount is used vs. Ca ion present. The ideal wetting conditions for a fast cleaning process appear only at a low water hardness; a few ppm of Ca.

Table 5: Theoretical chelating capacity expressed as mg of chelated substance /g Dissolvine® M-40, Dissolvine® M-S, EDTA and NTA for several metal ions and CaCO3

Figure 6: The calcium complexing efficiency of various chelating agents in competition with Hydroxy Naphthol Blue (HNB) at pH 11–12

Figure 7: Water hardness reduction in the presence of various chelates versus Log K of the Ca-chelate stability constant.