Potentiometric determination of nitrilotriacetic acid (NTA) and/or ethylenediaminetetraacetic acid (EDTA) in washing agents

Potentiometric determination of nitrilotriacetic acid (NTA) and/or ethylenediamine tetra acetic acid (EDTA) in washing agents

Summary

Application Bulletin No. 76 describes the polarographic determination of small concentrations (1 ... 100 mg/L) of NTA and EDTA in water. Since in certain countries legislation has been introduced requiring that phosphates in washing agents should be replaced NTA and EDTA have become more and more important as complexing agents and builders. This bulletin describes the determination of higher concentrations of NTA and/or EDTA in washing agents by means of potentiometric titration. The ion-selective copper electrode is used as indicator electrode. Other components normally found

in washing agents do not interfere with this determination.

Instruments and accessories

• 702 SET/MET Titrino, 716 DMS Titrino, 736 GP Titrino, 751 GPD Titrino or 785 DMP Titrino or 726 or 796 Titroprocessor with 700 Dosino or 685 Dosimat

• 2.728.0040 Magnetic Stirrer

• 6.3014.213 or 6.3014.223 Exchange Unit

• 6.0502.140 ion-selective copper electrode (Cu ISE) with 6.2104.020 electrode cable

• 6.0726.100 double-junction Ag/AgCl reference electrode (bridge electrolyte KNO3 sat.) with 6.2106.020 electrode cable

Reagents

• Titrant: c(Cu2+) = 0.01 mol/L:

• Dissolve 2.416 g Cu(NO3)2 * 3 H2O in dist. water, add 0.5 mL conc. HNO3 and make up to 1 L with dist. water.

• NTA standard solution, c(NTA) = 0.01 mol/L: Make a slurry with 1.9114 g NTA and dist. water. While stirring, add w(NaOH) = 32% drop by drop until the solution is clear, then make up to 1 L with dist. water.

• EDTA standard solution, c(EDTA) = 0.01 mol/L:

• Make a slurry with 2.9225 g EDTA and dist. water. While stirring, add w(NaOH) = 32% drop by drop until the solution is clear, then make up to 1 L with dist. water.

• Buffer solution pH = 9.6: Dissolve 80 g NH4NO3 in dist. water, add 70 mL w(NH3) = 25% and make up to 1 L with dist. water.

Analysis

• Weigh 0.5 ... 1 g sample into a 100 mL volumetric flask, add 50 mL dist. Water and dissolve as well as possible by heating lightly up to approx. 40 C. After cooling down, carefully fill to the mark with dist. water and mix.

• Pipet 10.0 mL of the mixture (corresponding to 50 … 100 mg of the original sample) into a glass beaker and add 2.0 mL c(NTA) = 0.01 mol/L or c(EDTA) = 0.01 mol/L. After addition of 10 mL buffer solution pH = 9.6 titrate with c(Cu2+) = 0.01 mol/L.

Calculation

1 mL c(Cu2+) = 0.01 mol/L corresponds to 1.9114 mg NTA or 2.9225 mg EDTA

% NTA = (EP1 – C31) * C01 * C03 / C00

% EDTA = (EP1 – C31) * C02 * C03 / C00

EP1 = titrant consumption in mL

C00 = 50 ... 100 (sample mass used in mg original sample)

C01 = 1.9114

C02 = 2.9225

C03 = 100 (conversion factor for %)

C31 = 2.0 [added volume of c(NTA) = 0.01 mol/L or c(EDTA) = 0.01 mol/L in mL]

Remarks

Mixtures of NTA and EDTA are usually determined as a whole. Separation of the two components is only possible if the mixture ratio is most favorable. In that case EDTA is determined first.

If separate determination of the individual components is required and the mixture ratio is unfavorable, please refer to Application Bulletin No. 76.

Answer :

785 DMP Titrino 02287 785.0010

user

U(init) -239 mV DET U

smpl size 88.3 mg

EP1 5.582 ml -165 mV

NTA 1.26 %

stop V reached

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Literature

Metrohm, Application Bulletin No. 143/2 e