GB/T 1707-2012Lithopone (English PDF)

6.1 Determination of Barium Sulfate and Total Zinc Content

6.1.1 General

The barium sulfate content is determined using the gravimetric method. The total zinc content can be determined using either Method A (potassium hexacyanoferrate titration) or Method B (Na2EDTA titration). The arbitration test for total zinc content uses Method A (potassium hexacyanoferrate titration).

6.1.2 Determination of barium sulfate and total zinc content - Method A (potassium

hexacyanoferrate titration)

6.1.2.1 Principle

Dissolve zinc sulfide and zinc oxide in the specimen with hydrochloric acid, then add sulfuric acid to maintain a certain acidity and perform solid-liquid separation. Determine the barium sulfate content of the solid using the gravimetric method. For the liquid, titrate the total zinc content with potassium hexacyanoferrate (II) using diphenylamine as an indicator at a pH of 1.5 ~ 3.0.

6.1.2.2 Reagents

Unless otherwise specified, all reagents used shall be of analytical grade, and water with a purity of at least Grade III as specified in GB/T 6682 shall be used.

Warning—Reagents shall be used in accordance with appropriate health and safety regulations.

6.1.2.2.1 Hydrochloric acid. 1+2

Add 1 volume of concentrated hydrochloric acid (approximately 37% by mass, rho~1.19 g/mL) to 2 volumes of water.

6.1.2.2.2 Sulfuric acid. 1+8

Carefully add 1 volume of sulfuric acid (approximately 96% by mass, rho~1.84 g/mL) to 8 volumes of water.

6.1.2.2.3 Concentrated ammonia solution. Density. 0.9 g/mL.

6.1.2.2.4 Ammonia solution. 1+3

Add 1 volume of concentrated ammonia solution to 3 volumes of water.

6.1.2.2.5 Diphenylamine indicator. 50 g/L ethanol solution.

6.1.2.2.6 Congo red test paper.

6.1.2.2.7 Lead acetate test paper.

6.1.2.2.8 Zinc chloride standard solution. Contains approximately 5 g of zinc per liter

[concentration (c) is expressed as grams of zinc per milliliter of solution].

Weigh approximately 5 g of high-purity zinc (accurate to 0.1 mg), dissolve it in 300 mL of hydrochloric acid solution (6.1.2.2.1), and transfer the solution to a 1 000 mL volumetric flask.

Dilute to the mark with water and shake well.

6.1.2.2.9 Potassium hexacyanoferrate (II) standard solution. c[K4Fe(CN)6]~0.05 mol/L [titer

(T1) is expressed as grams of zinc per milliliter of solution].

6.1.2.2.9.1 Preparation

Dissolve 21.0 g of potassium hexacyanoferrate (II), 0.3 g of potassium hexacyanoferrate (III), and 2 g of anhydrous sodium carbonate (for solution stabilization) in water, and transfer the solution to a 1 000 mL volumetric flask. Dilute to the mark with water and shake well.

6.1.2.2.9.2 Standardization

Pipette 25 mL of zinc chloride standard solution (6.1.2.2.8) into a beaker. Add ammonia solution (6.1.2.2.4) until Congo red test paper (6.1.2.2.6) just turns pure red in the solution.

Then carefully add hydrochloric acid (6.1.2.2.1) dropwise to neutralize, adding a few drops in excess until the Congo red test paper turns a stable blue (pH 1.5-3.0).

Dilute with water to 150 mL, heat this solution to boiling, and add 10 drops of diphenylamine indicator (6.1.2.2.5).

Immediately titrate with potassium hexacyanoferrate (II) solution (6.1.2.2.9) until the solution turns a stable yellow or yellow-green color.

Then, back-titrate the solution with zinc chloride standard solution (6.1.2.2.8) until the solution color just turns blue again.

The solution temperature must be maintained above 80°C throughout the titration process.

6.1.2.2.9.3 Calculation of titer

Calculate the titer (T1) of the potassium hexacyanoferrate (II) standard solution according to Formula (1), expressed in grams per milliliter (g/mL).

Where.

c -- Concentration of zinc chloride standard solution, in grams per milliliter (g/mL);

V2 -- Volume of zinc chloride standard solution consumed during back titration, in milliliters (mL);

V1 -- Volume of potassium hexacyanoferrate (II) standard solution consumed during titration, in milliliters (mL).

6.1.2.3 Operating steps

Weigh approximately 0.6 g (accurate to 0.1 mg) of the specimen that has been dried beforehand at (105±2)°C and place it in a beaker. Add 25 mL of hydrochloric acid solution (6.1.2.2.1), immediately cover with a watch glass, and boil until no more hydrogen sulfide is released [tested with lead acetate test paper (6.1.2.2.7)]. Dilute with 100 mL of water, add 5 mL of sulfuric acid solution (6.1.2.2.2), and boil the solution again.

Allow the precipitate to settle, and while still hot, filter the upper layer of solution using slow-speed quantitative filter paper. Transfer the precipitate onto the filter paper, and wash the beaker and filter paper with hot water containing sulfuric acid (approximately 3%) until the washing liquid is free of zinc ions [the washing liquid droplets do not react with potassium hexacyanoferrate (II) solution (6.1.2.2.9)]. Then, while still wet, transfer the precipitate, along with the filter paper, to a pre-weighed porcelain crucible, ashed at low temperature, and ignite at (800±20)°C until constant weight.

Add a slightly excessive amount of ammonia solution (6.1.2.2.3) to the filtrate [test the color change from blue to red using Congo red test paper (6.1.2.2.6)], and then add hydrochloric acid solution (6.1.2.2.1) dropwise until the Congo red test paper (6.1.2.2.6) just turns a stable blue in the solution (pH 1.5-3.0).

Heat the solution to boiling, add 10 drops of diphenylamine indicator (6.1.2.2.5), and immediately titrate the solution according to the method for standardizing potassium hexacyanoferrate (II) solution in 6.1.2.2.9.2.

6.1.2.4 Representation of results

The barium sulfate content (w1) is calculated according to Formula (2) and expressed as a mass fraction (%).

The total zinc content (w2) is calculated according to Formula (3) and expressed as a zinc sulfide mass fraction (%).