42-37
NITROGEN, AMMONIA, continued
INTERFERENCES
A solution containing a mixture of 500 mg/L CaCO
3
and 500 mg/L Mg
as CaCO
3
does not interfere. If the hardness concentration exceeds these
concentration, extra Mineral Stabilizer should be added.
Iron and sulfide interfere by causing a turbidity with Nessler Reagent.
Residual chlorine must be removed by addition of sodium arsenite
solution. Use two drops to remove each mg/L Cl from a 250 mL
sample. Sodium thiosulfate can be used in place of sodium arsenite.
See Sampling and Storage Section.
Less common interferences, such as glycine, various aliphatic and
aromatic amines, organic chloramines, acetone, aldehydes and alcohols
may cause greenish or other off colors or turbidity. It may be necessary
to distill the sample if these compounds are present.
Seawater samples may be analyzed by addition of 1.0 mL (27 drops) of
Mineral Stabilizer to the sample before analysis. This will complex the
high magnesium concentrations found in sea water, but the sensitivity of
the test will be reduced by 30 percent due to the high chloride
concentration. For best results, perform a calibration, using standards
spiked to the equivalent chloride concentration, or distill the sample as
described below.
DISTILLATION
a) Measure 250 mL of sample into a 250-mL graduated cylinder and
pour into a 400-mL beaker. Destroy chlorine, if necessary, by adding 2
drops of Sodium Arsenite Solution per mg/L Cl
2
.
b) Add 25 mL of Borate Buffer Solution and mix. Adjust the pH to
about 9.5 with 1 N sodium hydroxide solution. Use a pH meter.
c) Set up the general purpose distillation apparatus as shown in the
Hach Distillation Apparatus Manual. Pour the solution into the
distillation flask. Add a stir bar.
d) Use a graduated cylinder to measure 25 mL of demineralized water
into a 250-mL erlenmeyer flask. Add the contents of one Boric Acid
Powder Pillow. Mix thoroughly. Place the flask under the still drip tube.
Elevate so the end of the tube is immersed in the solution.
To Next Page
Loading...
