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Determination of the Ion Concentration in a Natural Rubber Latex Sample

Determination of the Ion Concentration in a Natural Rubber Latex Sample
B.Sc (Hons, USJ) (Polymer Science and Technology, Chemistry, Physics)
Last Updated On : January 20, 2023
Published Date : January 12, 2023

Experimental way of determining the ion concentration in a natural rubber latex sample

Natural rubber contains a variety of metal cations such as Potassium, Sodium, Calcium, Magnesium, Iron, Copper, and Manganese. Measurement and control of metal ion content in natural rubber latex are highly significant in the NR industry. Because the ions in the latex directly affect the stability of the latex. Generally, ammonia is added to the latex to reduce the metal ion content. Ammonia will form a complex with metal ions and reduce the free ion concentration.

The methodology described below describes a titrimetric method to determine Zinc, Calcium, and Magnesium ion contents present in rubber compounds. These three ions can be estimated by titrating aliquots of an extract of the latex serum with EDTA (Ethylenediaminetetraacetic acid) under various conditions.

At pH 10 and using 'Eriochrome black T (EBT)' as the indicator, all the metal ions in the sample will react. At pH 10 with EBT and Potassium Cyanide (KCN) present, only the Magnesium and Calcium ions will react. Because Zinc ions will form a complex with cyanide.

At pH 12 Potassium cyanide and Murexide indicator, only Magnesium ions will be determined. From the results of the above three titrations’ concentrations of each ion can be determined.

The experimental procedure of determining the ion concentration in a natural rubber latex sample

01. Collecting the latex serum

Nearly 10 grams of latex are weighed into a clean dry beaker. To that beaker containing latex, 2% acetic acid was added to coagulate. Then the coagulum is collected using a glass rod. The coagulate is embedded and squeezed by a glass rod and washed. The total serum is collected along with washing with distilled water. The filtrate and washings are made up to 250 ml by adding distilled water in a graduated flask. This solution is used for the estimation of the metal present.

02. Titration for total metal ions

An amount of 25 ml of the test solution is pipetted out into a titration flask and it is added 5 ml of buffer solution. The sample is diluted up to 100 ml and it is added 5 drops of EBT indicator. This solution is titrated against 0.005 M EDTA solution to a clear-blue endpoint. Titration should be repeated until two similar endpoints are obtained.

03. Titration for Ca2+ and Mg2+ ions

An amount of 25 ml of test solution is added to 5 ml of buffer solution and 5 ml of KCN solution. This sample is diluted up to 100 ml and titrated against 0.005 M EDTA solution to a sky-blue endpoint using the EBT indicator. Titration is repeated until a similar endpoint is obtained.

04. Titration for Ca2+ ions

An amount of 25 ml of test solution is added to 5 ml of KCN and 5 ml of 8M KOH solution. The solution is diluted u to 100 ml, and it is added 0.1g of Patton & Reeder (P&R) indicator (Calconcarboxylic acid). Then the sample is titrated against 0.005 M EDTA solution to a red-violet endpoint. Titration is repeated until a similar endpoint is obtained.

Readings

Let’s assume that the following results have been obtained from the above titrations.

  • Mass of the initial latex sample - 10.000g
  • The concentration of EDTA solution - 0.005 M

Titration 01 - Titration results for finding total metal ions in the latex sample

Titration numberThe volume of the test solution (cm3)The volume of buffer solution (cm3)The volume of EDTA (cm3)
125.005.01.60
225.005.01.50
Table 01: Titration 01 - Titration results for finding total metal ions
  • Color change – pink/purple to clear blue

Titration 02 - Titration results for finding Ca2+ and Mg2+ metal ions in the latex sample

Titration numberThe volume of the test solution (cm3)The volume of buffer solution (cm3)The volume of KCN (cm3)The volume of EDTA (cm3)
125.005.05.01.60
225.005.05.01.50
Table 02: Titration 02 - Titration results for finding Ca2+ and Mg2+ ions
  • Color change – red-purple to dark blue

Titration 03 - Titration results for finding Ca2+ metal ions in the latex sample

Titration numberThe volume of the test solution (cm3)The volume of buffer solution (cm3)The volume of KCN (cm3)The volume of KOH (cm3)The volume of EDTA (cm3)
125.005.05.05.01.60
225.005.05.05.01.50
Table 03: Titration 03 - Titration results for finding Ca2+ ions
  • Color change – pink/red to blue

Calculation

Metal ion percentages of the sample can be calculated as follows.

Finding the total metal ions in the latex sample

Determination of the Ion Concentration in a Natural Rubber Latex Sample eq 01

Each Mg2+, Ca2+, and Zn2+ react with EDTA 1:1 stereochemistry. Therefore, the mols of EDTA are equal to the mols of total metal ions.

Determination of the Ion Concentration in a Natural Rubber Latex Sample eq 02

The total metal ions present in the 25 ml of the test solution is 7.75 * 10-6 mol. The mols of metal ions present in 250 ml of the test solution will be,

Determination of the Ion Concentration in a Natural Rubber Latex Sample eq 03

Finding Ca2+ and Mg2+ metal ions in the latex sample

Determination of the Ion Concentration in a Natural Rubber Latex Sample eq 04

The Mg2+ and Ca2+ ions present in the 25 ml of the test solution are 6.0 * 10-6 mol. The mols of Mg2+ and Ca2+ ions present in 250 ml of the test solution will be,

Determination of the Ion Concentration in a Natural Rubber Latex Sample eq 05

Finding Ca2+ metal ions in the latex sample

Determination of the Ion Concentration in a Natural Rubber Latex Sample eq 06

The Ca2+ ions present in the 25 ml of the test solution is 1.75 * 10-6 mol. The mols of Ca2+ ions present in 250 ml of the test solution will be,

Determination of the Ion Concentration in a Natural Rubber Latex Sample eq 07

Metal ionsThe amount present in 25 ml of test solution
Ca2+, Mg2+, Zn2+7.75 * 10-5 mol
Ca2+, Mg2+6.0 * 10-5 mol
Ca2+1.75 * 10-5 mol
Table 04: The amount of metal ions present in 25 ml of test solution

Determination of the Ion Concentration in a Natural Rubber Latex Sample eq 08

Finding the mass percentages of each metal ion in the latex sample

According to the above results, mass percentages of each metal in the initial latex sample can be calculated as follows.

Ca2+ percentage

Determination of the Ion Concentration in a Natural Rubber Latex Sample eq 09

Mg2+ percentage

Determination of the Ion Concentration in a Natural Rubber Latex Sample eq 10 1

Zn2+ percentage

Determination of the Ion Concentration in a Natural Rubber Latex Sample eq 11

What happens to latex when adding acetic acid to it?

In fresh NR latex, rubber particles are surrounded by minus charges. Due to these minus charges, rubber particles cannot come to each other. They are repulsed by each other. But when adding acetic acid to the latex, minus charges are neutralized. Therefore, rubber particles can interact with each other and they will coagulate.

What happens when divalent ions are present in latex?

As mentioned above rubber particles are surrounded by minus charges. Divalent metal ions like Ca2+, Mg2+, and Zn2+ can bond with these minus-charged sites. Since these ions are divalent, one ion can bond with two minus-charged sites. One metal ion acts as a bridge that interconnects two rubber particles. So, when metal ions are present in latex, the latex will be coagulated.

Coagulation of rubber particles by divalent ion
Figure 01: Coagulation of rubber particles by divalent ion

Why do use EDTA for the titration of metal ions?

EDTA is a versatile chelating agent. It can form four or six bonds with a metal ion and it chelates with both transition metal ions and main group metal ions.

the structure of EDTA
Figure 02: The structure of EDTA

Why do use EBT as the indicator?

A blue dye called Eriochrome Black T is used as the indicator in 1st and 2nd titrations. This indicator forms a complex with Mg2+ ions and produces a pink/purple solution. The indicator is blue when it is not complexed with Mg2+ and the solution is basic.

When titration with EDTA it reacts with divalent ions (Ca2+ and Zn2+). Once all the divalent ions in the solutions have reacted with EDTA, it starts to react with Mg2+ ions that are complex with the EBT indicator. Therefore, the solution becomes blue at the endpoint.

The structure of EBT
Figure 03: The structure of EBT

What are the functions of the Patton and Reeder indicator (Calconcarboxylic acid)?

A blue dye called Patton & Reeder (P&R) has been used as the indicator in titration 3. P&R indicator forms a complex with the Ca2+ ions and turns the medium from blue color to pink. When titration with EDTA, Ca2+ ions form a complex with EDTA. Because the EDTA-Ca2+ complex is more stable than the P&R-Ca2+ complex. As Ca2+ ions are decomplex with the indicator, the solution becomes blue again.

The structure of Patton and Reeder's indicator
Figure 04: The structure of Patton and Reeder's indicator

What is the function of Potassium cyanide (KCN) in titration 2?

When adding KCN, CN- ions form a strong complex with Zn2+ ions. This complex is stronger than the Zn-EDTA complex. Therefore, when titrating, only Ca2+ and Mg2+ ions will react with EDTA.

What is the function of Potassium hydroxide (KOH) in titration 3?

In titration 3, both KCN and KOH are added. CN- forms a strong complex with Zn2+ and OH- will react with Mg2+ forming Mg(OH)2. Therefore, in titration 3, only Ca2+ ions will react with EDTA.


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References and Attributes

Figures:

The cover image was created using an image by Vyacheslav Argenberg, licensed under CC BY 4.0, via Wikimedia Commons


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