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Continuous determination of cobalt and pyrophosphate in plating bath (EDTA titration)

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Continuous determination of cobalt and pyrophosphate in plating bath (EDTA titration)

Classification:
Industry insight
Author:
2019/07/23 17:01

In the gold cobalt cyanide plating bath of pyrophosphate, the mass concentration of potassium pyrophosphate is 100 g/L (K4P2O7 3H2O); the mass concentration of cobalt pyrophosphate is 1.3-4.0g/L, i.e. Rho Co: 0.21-0.65g/L; and C (P2O74-): 0.26mol/L.

A Brief Introduction to One Method

The cobalt in plating bath was titrated by EDTA coordination, and the pyrophosphate ion of cobalt pyrophosphate was free. Then zinc ion and pyrophosphate ion formed zinc pyrophosphate precipitation at pH 3~4:

2Zn2++P2O74-Zn2P2O7_

The excess zinc ion was titrated with EDTA standard titration solution, and the total pyrophosphate content was determined.

Two reagents

(1) Ammonia water: concentrated, (1+1).

(2) Hydrochloric acid: (1+1).

_o-phenanthroline: 2g/L.

(5) Sodium potassium tartrate: 100g/L.

_HAc-NaAc buffer solution: pH 5.5.

_Acetic acid: 1 mol/L.

_Zinc acetate standard solution: 0.2 mol/L.

_EDTA standard titration solution: 0.01 mol/L, 0.05 mol/L.

P-nitrophenol indicator: 95% ethanol solution of 1g/L.

_Malachite green indicator: 1g/L.

_Xylenol orange indicator: 2g/L.

Three steps of determination

(1) Determination of cobalt

(1) In 250 mL conical bottle, adding 1 mL plating solution (V1/mL), 10 mL water and (1+1) 5 mL hydrochloric acid, heating slowly at low temperature until about 30 sec, then cooling to room temperature.

(2) In the test solution, add 80 mL of water, 2 g/L 5 mL of o-phenanthroline, 100 g/L potassium sodium tartrate 5 mL and 2 drops of p-nitrophenol indicator, then add (1+1) hydrochloric acid to fade out, and add 4 drops. Add 5 ml HAc-NaAc buffer solution with pH 5.5, heat slowly to about 60 C at low temperature, add 2-3 drops of malachite green indicator and 5-6 drops of xylenol orange indicator, make the test solution purple-red, titrate immediately with 0.01 mol/LED TA standard solution, the test solution mutates to bright green end point (V2/mL).

(2) Determination of pyrophosphate

(1) In the test solution after the determination of_cobalt, the precise pH test paper is used to adjust the pH value of the test solution so that the pH value is 3.8-4.0. Accurately adding 0.2 mol/L zinc acetate standard solution 5mL (V0/mL), zinc pyrophosphate precipitation is formed and boiled (conical bottle shaking constantly, in order to avoid the test solution boiling and splashing out).

(2) After cooling, filter with quantitative filter paper (with a little pulp inside), wash the conical bottle and precipitate 4-5 times with warm hot water, about 5mL each time.

(3) Collect filtrate and detergent in 250 mL conical bottle, add 2 drops of p-nitrophenol indicator, adjust (1+1) ammonia water to yellow test solution, add (1+1) hydrochloric acid to fade yellow, and add 4 drops. Add 5 ml HAc-NaAc buffer solution with pH 5.5, 2-3 drops of malachite green indicator and 5-6 drops of xylenol orange indicator to make the test solution purple-red, titrate immediately with 0.05 mol/LED TA standard solution, and the test solution mutates to bright green end point (V3/mL).

Four Computations

The mass concentration of cobalt in rho-Co-plating bath, g/L;

Total concentration of pyrophosphate in plating solution, g/L;

C - When cobalt is determined, the concentration of EDTA standard titration solution is mol/L.

V2 - When cobalt is determined, the titration volume of EDTA standard titration solution is mL.

The concentration of c1-zinc acetate standard solution, mol/L;

V0 - Volume of standard solution of zinc acetate, mL;

C 2 - When pyrophosphate is determined, the concentration of EDTA standard titration solution is mol/L.

V3 - When pyrophosphate is determined, the titration volume of EDTA standard titration solution is mL.

173.9 - relative molar mass of pyrophosphate ion (P2O74-), g/mol;

2 - The ratio of zinc ion to pyrophosphate ion number in zinc pyrophosphate;

 

V1 - Sampling volume of plating solution, mL;

58.93 - Relative atomic weight of cobalt, 1.

Five points for attention

(1) When cobalt is determined by EDTA coordination titration, if the end point is not easy to distinguish, excessive EDTA standard titration solution can be added, and 0.02mol/LZn2+ standard titration solution can be used for reverse titration.

(2) When EDTA is contained in the bath formulation, the steps of oxidative decomposition of EDTA should be added in the "three determination steps (1) determination of cobalt". That is to say, 1 mL (V/mL), 5 mL (1+1) hydrochloric acid and 5 mL (1+1) nitric acid should be added to the 250 mL conical bottle and heated to boil until the yellow smoke of nitrogen oxides has been exhausted and cooled to room temperature.

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