Determination of Free Alkalinity Content in Laundry Powders —Titrimetric Method

A.1 Principle

Titrate the laundry powders sample solution with the hydrochloric acid standard titration solution to a certain pH. Calculate the content of free alkalinity, expressed as equimolar sodium hydroxide according to the consumption of hydrochloric acid solution.

A.2 Reagents

A.2.1 Hydrochloric acid, 0.05 mol/L standard titration solution, prepared and calibrated as specified in 4.3 of QB/T 2739-2005.

A.2.2 Mixed phosphate buffer solution, pH=6.86 (25 ℃).Dissolve the mixed phosphate commercialize+d available in a 150 mL beaker with water recently boiled and cooled. Transfer the solution to a 250 mL one-mark volumetric flask, while washing the bag and combining the liquid. Dilute to the mark and mix.

A.2.3 Sodium tetraborate buffer solution, pH=9.18 (25 ℃).Dissolve the sodium tetraborate decahydrate commercialized available in a 150 mL beaker with water recently boiled and cooled. Transfer the solution to a 250 mL one-mark volumetric flask,while washing the bag and combining the liquid.Dilute to the mark and mix.

A.3 Apparatus

Ordinary laboratory apparatus and

A.3.1 pH-meter (or acidimeter), graduation 0.02 pH unit

A.3.2 Glass electrode and calomel electrode. The glass electrode shall be soaked in water for 24 hours before use.

A.3.3 Magnetic stirrer.

A.4 Procedure

A.4.1 Test portion

Weigh, to the nearest 0.001 g, about 8 g of laboratory sample in a 400 mL beaker. Add about 250 mL of water recently boiled and cooled,and place on a magnetic stirrer (A.3.3) while stirring for 10 minutes. Transfer the solution to a 2 L one-mark volumetric flask, and dilute to the mark.

A.4.2 Calibration of pH-meter

Start the pH-meter (A.3.1), and warm it up for 30 minutes. Calibrate the apparatus according to the specifications with mixed phosphate buffer solution(A.2.2) and sodium tetraborate buffer solution (A.2.3) in turn. Recalibrate the pH-meter for every sample.

A.4.3 Titration

Pipette 50.0 mL of test solution (A.4.1) in a 100 mL beaker. Titrate the solution with the hydrochloric acid standard titration solution (A.2.1) while stirring with a magnetic stirrer. Determine the pH with pH-meter(A.3.1). When the pH stabilizes at 9 for 10 seconds,which is considered as the titration endpoint, record the volume of the hydrochloric acid standard solution consumed.

A.5 Method of calculation

The free alkalinity content X in laundry powders,expressed as a percentage by mass of NaOH, is given by formula (A.1)

where ,

V is the volume, in milliliters, of the hydrochloric acid standard solution used for the titration.

c is the actual concentration, expressed in moles of HCl per litre, of the hydrochloric acid standard titration solution used.

m is the mass, in grams, of the test portion.

Calculate the arithmetic average of the parallel determination results, keep one decimal place, as the result.

A.6 Precision

The maximum difference found between the results of two determinations carried out simultaneously or in rapid succession on the same sample, by the same analyst using the same apparatus, shall not exceed 0.2 % (95 %confidence intervals).