Methods to be Reviewed by OMB in 2016

Jaudzems:

ournal of

AOAC I

nternational

ol.

98, N

5, 2015

1393

(a)

Check and maintenance of the combined silver

electrode

.—Rinse electrode with deionized water and wipe

before use. Renew the electrolyte periodically permanufacturer’s

recommendations. If fat sticks to the electrodes during a series

of analyses, then eliminate it by briefly immersing the electrode

in acetone. Follow manufacturer’s recommendations for the

storage of electrodes when not in use.

(

Note

: In place of the combined silver electrode, separate

silver and reference electrodes may also be used.)

(b)

Automated titration

.—Prior to first use, check the

system linearity by use of a range of required volumes

(e.g., 0.1–15 mL) of 0.1 M NaCl solution. Additionally, prior

to each use check the system suitability by preparing three

calibration check samples using 5.0 mL 0.1 M NaCl solution.

H. Extraction and Analysis

(a)

Weigh an appropriate aliquot RTF or reconstituted

powder (e.g., 25 g) into a suitable beaker (e.g., 150 mL,

manual or semiautomatic procedure) or the autosampler

titrator cups (automatic titration). For adult nutritionals with

a high chloride content, weigh a smaller test portion, e.g., 5 g

reconstituted or RTF product.

(b)

Add 50 mL 2% (v/v) nitric acid solution as well as

a magnetic stirring rod. Place the autosampler cup on a

magnetic stirrer and stir until mixed or finely suspended.

(c)

The pH of the test solution should be below 1.5. In case

of doubt, check by means of a pH meter and, if necessary, add

more 2% (v/v) nitric acid solution.

(d)

Under continuous stirring, titrate the sample solution

automatically with 0.1 M AgNO

solution up to the end

potential. Record the volume of 0.1 M AgNO

solution

consumed. If performing manual titrations, plot a graph

of the variation of potential difference as a function of the

quantity of the titrant added, continuing the addition of the

titrant beyond the presumed equivalence point. The end point

of the titration corresponds to the point at which the potential

changes most rapidly (

see

Figure

2015.08

(e)

Special case: determination of very low amounts

of chloride

.—When determining chloride amounts below

20 mg/100 g, for greater accuracy and precision it is preferable

to use a 0.01 MAgNO

solution for the titration. Determine the

titer of this solution by means of a 0.01 M NaCl solution.

I. Calculations

Calculate chloride content (w) in mg/100 g RTF or

reconstituted sample using the equation:

= A × Mw ×

× 100

where A = volume (mL) of 0.1 M or 0.01 M AgNO

solution used for titration; Mw = atomic weight of chloride

(= 35.45 g/mol);

= exact molar concentration of the AgNO

solution (0.1000 or 0.0100); m = mass of the test portion, in g;

and F = dilution factor for preparation of reconstituted powder

or concentrate.

Results

The validation study was conducted in accordance with the

SPIFAN SLV guidelines (3).

Figure 2015.08. Typical titration curve obtained from an automatic titrator.

Candidates for 2016 Method of the Year

102