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By potassium dichromate return instructions

I、 Methods Summary
Acidification of water samples by adding an excess of potassium dichromate solution back to boil, remaining potassium
dichromate, ferrous ammonium sulfate titration; by the amount of consumption of potassium dichromate can
The water samples obtained COD (Chemical Oxygen Demand, referred to as COD), to indicate that water samples can be
oxidized organics content.

II、Apply scope
This method applies to halogen concentration of less than 2,000 mg/L in surface water, ground water and releasing water
chemical oxygen demand examination.

III、 Interference
(A)Than X-Ding and similar compounds cannot be oxidized to the low COD values
(B)Linear chain of aliphatic compound the volatile is not easy to oxidize. May join the silver sulfate reagent to do for the
catalyst. But note that sulfuric acid precipitation of silver reagent would halogen formation is not easy to oxidation.
(c)Halide ions (X-) of interference, prior joining the mercury sulfate salt to generate wrong way excluded, usually
joined the 0.4 g mercury sulfate in 20 mL water. If the known halogen ions concentration in water samples is less than
2,000 mg/L, as long as maintaining mercury sulfate: halide ion =10:1 ratio (not necessarily in 20 mL water and 0.4 g
mercury sulfate), but when the halide ion concentrations greater than 2,000 when mg/L, this method does not apply.
(D) Nitrite nitrogen in water sample usually little more than 1 or 2 mg/L, in this case the interference of ions can be ignored.
Interference with nitrite, joined 10 mg per 1 mg nitrous nitrogen amino sulfonic acid (Sulfamic Acid) to exclude. Note: blank
water samples shall be included in the same amount of amino sulfonic acid.
(E) The inorganic salt as for example six price chromium ion, the ferrous ion, the Asian manganese ion and the sulfide and
so on will form the disturbance. Above therefore if known includes the disturbance material, should distinguish the quota
and adjust the COD value.
(F)The ammonia nitrogen in the wastes or by contain to release in the nitrogen organic material of the ammonia nitrogen ,
while containing the chlorine ion of Gao density, will be oxidized and cause interference.

(A)Reflow unit: diameter 24/40 the 500 mL or 250 mL round-bottom flask grinding, 30 cm long straight or spherical
condensation pipe.
(B)Heating device.
(C)scales : precision scales to 0.1 mg.

(A)Try water:Go to ion distilled water.
(B)Sulfuric acid mercury:Analytical.
(C)Potassium bichromate standard solution, 0.0417 M: By reagent water dissolution analysis level potassium bichromate of
12.259 g reagent (in 103 ℃ dry 2 hours ) measuring flask in the 1 L, quota to graticule.
(E)Silver sulfate reagent: Joins the 25g silver sulfate in the 2.5L strong sulfuric acid, settles 1 to 2 angel silver sulfates
dissolves completely. The use has  proper market may also.  Ferroin (Ferroin) indicator : dissolve 1.485 g 1,10 - phenanthroline
monohydrate (1,10 - phenanthroline monohydrate, C12H8N2 ‧ Hm2O) and 0.695 g of ferrous sulfate in reagent water, diluted
to 100 mL . The use has  proper market may also. Ferrous ammonium sulfate titration, 0.25M: 98 g ammonium ferrous sulfate
dissolved in the reagent in the water, add 20mL concentrated sulphuric acid, dilute to 1 l after cooling. Calibration before use.
Calibration method: dilute 10 mL 0.0417 m to about 100 mL of standard solution of potassium dichromate, add 30 mL of
concentrated sulphuric acid, cooling to room temperature, add 2 to 3 drops of phenanthroline indicator, to 0.25 m ammonium
ferrous sulfate titration, when the solution from blue green to reddish brown when the end point.

Iron ammonium sulfate of titrate solution volume iron ammonium sulfate titrate
solution molar density = (10×0.25)/consumption

(F)COD standard solution: Dissolves 0.1700 g anhydrous potassium hydrogen phthalate in 1 L measuring flask
(120 ℃ dry overnight) in the reagent water, quota to graticule. Theory of COD value the potassium hydrogen phthalate is
1.176 mg/mg, theory of COD value this solution is 200 mg/L. In has not observed in the microorganism growth situation, this
solution may refrigerate the preservation in the brown bottle to three months.

VI、Sampling and save
glass or plastic bottle collection of about 500 mL of the sample, if not immediately after sampling and analysis, concentrated
sulfuric acid to adjust pH value should be 2, and 4 degrees centigrade refrigerated, save for a period of 7 days.

(A)If COD of water kind is worth is bigger than a 50 mgs/ L:
1.Takes 20 mL to mix the even water sample (, if COD of value the water sample is bigger than time 900 mg/L, should give
suitable dilution) in 250 mL round flasks, joins 0.4 g mercuric sulfate and several grain of zeolite, then joins 2 mL silver sulfate
reagent slowly,  shake mix mercury sulfate dissolved, when mix must cool the round flask to avoid the volatile matter .
2.Add  10.0 mL 0.0417 m potassium dichromate solution after mixing, connecting condensation pipe and into the
cooling water.
3.By Condenser tube top  joined 28 mL sulphuric acid reagent (Note: after shaking mixed, heated so as to avoid acid
spills) reflux temperature for 2 hours (such as known water samples do not need 2 hours when the COD values of up to 2
hours return, can reduce the discretion of reflow time), when you return to top of a small beaker covered in condensing
tubes to prevent pollutants fall into.
4.After the cooling, by  Condenser the top flushing by 30 mL distilled water to  Condenser the endophragm, takes out the
round-bottom flask, joins distilled water of the 30 mL, cools to the room temperature.
5.Joins 2 to 3 drop of Philippine Luo lin indicator, by 0.25 M the iron ammonium sulfate solution titrate to the equivalent point,
at which point solution from blue-green to reddish brown. All samples should better use the isometric  indicator.
6.Meanwhile carries on the blank test by the reagent water.

(B)If COD of value the water sample is lower than 50 mg/L:
1.Should use 0.00417 m standard solution of potassium dichromate and 0.025 m ammonium ferrous sulfate titration solution,
according to seven, (a) from 1 to 6 operations. Operations should be particularly careful, glassware or air of organic matter
in the air will cause error margin.
2.If must further increase the sensitivity, may concentrate the water sample before the backflow digestion, the method is as
follows: In is bigger than water sample in the 20 mL volume, joins all reagents, boils in the round flask, but does not connect
Condenser, causes its volume to drop to 60 mL.  of the mercuric sulfate must regard the concentration ,in the water sample
density of the chloride ion, according to mercuric sulfate: Principle of the chloride ion = 10:1 makes the moderate adjustment.

(C)Potassium hydrogen phthalate standard solution made samples for quality control analysis to assess the quality of
analysis techniques and reagents.

(E)If the water sample volume surpasses 20 mL, then needs the reagent amount used also should the proportion to
increase, but two hour backflow time's standard may also use judgment to reduce, so long as result quite .

VIII、Results processing
Chemistry dosage=[(A-B)×C×8000]/V
A:blank consumption of the volume of Ferrous ammonium sulfate solution volume(mL)   
B:Water consumption of the volume of Ferrous ammonium sulfate solution volume(mL)   
C:Ferrous ammonium sulfate titration of the molar concentration(M)   
V:Water volume(mL)

IX、Quality control
(A)The blank sample, the standard sample and the increase sample must also determine with the sample.
(B)Analysis of repeated every batch of samples at least once.
(C )Each batch of similar matrix or at least one out of every ten samples and concentration of the sample analysis of
adding a known volume of standard solution of samples to check its recovery rate, if recovery rates exceed the control
limit (75zhi125%) need to do  repeat  again.
(D)With similar matrix and concentration of each batch of samples at least every ten samples or analysis of a quality
control sample results with the standard value if the difference in the 85-115% must need to do repeat again.

X、Precision and accuracy
Home single certain laboratory to the quality control sample of 100 mgs/ L through carry on 22 repetition analysis, as follows
show as a result:

Test project Sample
(mg / L)
(mg / L)
Recovery rate
± The standard
Analysis number
of times
COD 100.0 97.4 97.4 ±2.2 22

(A)erican Public Health Association, American Water WorksAssociation & Water Environment Federation.
1992.StandardMethods for the Examination of Water and Wastewater,18thEd., pp. 5 - 6 ~ 5 - 8. APHA,
Washington,D.C., USA .