Determination of Turbidity (Opalescence)
note—The determination of turbidity may be performed by visual comparison (
Procedure A), or instrumentally using a suitable ratio turbidimeter (
Procedure B). For a discussion of turbidimetry, see
Spectrophotometry and Light-Scattering
851
. Instrumental assessment of clarity provides a more discriminatory test that does not depend on the visual acuity of the analyst.
Hydrazine Sulfate Solution—
Dissolve 1.0 g of hydrazine sulfate, in water and dilute with water to 100.0 mL. Allow to stand for 4 to 6 hours.
Hexamethylenetetramine Solution—
Dissolve 2.5 g of hexmethylenetetramine in 25.0 mL of water in a 100 mL glass–stoppered flask.
Opalescence Stock Suspension—
Add 25.0 mL of Hydrazine Sulfate Solution to the Hexamethylenetetramine Solution in the flask. Mix, and allow to stand for 24 hours. This suspension is stable for 2 months, provided it is stored in a glass container free from surface defects. The suspension must not adhere to the glass and must be well mixed before use.
Opalescence Standard Suspension—
Prepare a suspension by diluting 15.0 mL of the Opalescence Stock Suspension with water to 1000.0 mL. Opalescence Standard Suspension is stable for about 24 hours after preparation.
Reference Suspensions—
Prepare according to
Table 2. Mix and shake before use.
[note—Stabilized formazin suspensions that can be used to prepare stable, diluted turbidity standards are available commercially and may be used after comparison with the standards prepared as described.
]
Table 2
|
Reference Suspension A |
Reference Suspension B |
Reference Suspension C |
Reference Suspension D |
Standard of Opalescence |
5.0 mL |
10.0 mL |
30.0 mL |
50.0 mL |
Water |
95.0 mL |
90.0 mL |
70.0 mL |
50.0 mL |
Nephelometric turbidity units |
3 NTU |
6 NTU |
18 NTU |
30 NTU |
Procedure A: Visual Comparison—
Use identical test tubes made of colorless, transparent, neutral glass with a flat base and an internal diameter of 15 to 25 mm. Fill one tube to a depth of 40 mm with
Solution S, one tube to the same depth with water, and four others to the same depth with
Reference Suspensions A, B, C, and
D. Compare the solutions in diffuse daylight 5 minutes after preparation of the
Reference Suspensions, viewing vertically against a black background. The light conditions shall be such that
Reference Suspension A can be readily distinguished from water and that
Reference Suspension B can be readily distinguished from
Reference Suspension A.
Requirement—
Solution S is not more opalescent than
Reference Suspension B for Type I closures, and not more opalescent than
Reference Suspension C for Type II closures.
Solution S is considered clear if its clarity is the same as that of water when examined as described above, or if its opalescence is not more pronounced than that of
Reference Suspension A (refer to
Table 3).
Procedure B: Instrumental Comparison—
Measure the turbidity of the
Reference Suspensions in a suitable calibrated turbidimeter (see
Spectrophotometry and Light Scattering
851
). The blank should be run and the results corrected for the blank.
Reference Suspensions A, B, C, and
D represent 3, 6, 18 and 30 Nephelometric Turbidity Units (NTU), respectively. Measure the turbidity of
Solution S using the calibrated turbidimeter.
Requirement—
The turbidity of
Solution S is not greater than that for
Reference Suspension B (6 NTU FTU) for Type I closures, and is not greater than that for
Reference Suspension C (18 NTU FTU) for Type II closures (refer to
Table 3).
Table 3
|
Comparison Method |
|
Opalescence Requirements |
Procedure A (Visual) |
Procedure B (Instrumental) |
Type I closures |
no more opalescent that Suspension B |
no more than 6 NTU |
Type II closures |
no more opalescent than Suspension C |
no more than 18 NTU |
Determination of Color
Color Standard—
Prepare a solution by diluting 3.0 mL of
Matching Fluid O (see
Color and Achromicity
631
) with 97.0 mL of diluted hydrochloric acid.
Procedure—
Use identical tubes made of colorless, transparent, neutral glass with a flat base and an internal diameter of 15 to 25 mm. Fill one tube to a depth of 40 mm with Solution S, and the second with Color Standard. Compare the liquids in diffuse daylight, viewing vertically against a white background.
Requirement—
Solution S is not more intensely colored than the Color Standard.