Chemical Oxygen Demand (Cod)

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CHEMICAL OXYGEN DEMAND (COD)

Chemical oxygen demand (COD) is used as a measure of

Chemical oxygen demand (COD) is used as a measure of oxygen requirement of aoxygen requirement of a sample that is susceptible to oxidation by

sample that is susceptible to oxidation by strong chemical oxidant. The dichromatestrong chemical oxidant. The dichromate reflux method is preferred over

reflux method is preferred over procedures using other oxidants (eg potassiumprocedures using other oxidants (eg potassium permanganate) because of its superior oxidizing ability, applicability to

permanganate) because of its superior oxidizing ability, applicability to a wide varietya wide variety of samples and ease of manipulation. Oxidation of

of samples and ease of manipulation. Oxidation of most organic compounds is 95-most organic compounds is 95-100% of the theoretical value.

100% of the theoretical value.

Dichromate Reflux Technique

Standard Method.

Equipmen

t

Required

1.

1. 500-millilitre (ml) Erlenmeyer flask with standard (24/40) tapered glass joints500-millilitre (ml) Erlenmeyer flask with standard (24/40) tapered glass joints

2.

2. Friedrichs reflux condensers (12-inch) with standard (24/40) tapered Friedrichs reflux condensers (12-inch) with standard (24/40) tapered glassglass joints

joints

3.

3. Electric hot plate or six-unit heating shelf Electric hot plate or six-unit heating shelf

4.

4. Volumetric pipettes (10, 25, and 50ml Volumetric pipettes (10, 25, and 50ml capacity)capacity)

5.

5. Burette, 50 ml - 0.1 ml accuracyBurette, 50 ml - 0.1 ml accuracy

6.

6. Burette stand and clampBurette stand and clamp

7.

7. Analytical balance, accuracy 0.001gram (g)Analytical balance, accuracy 0.001gram (g)

8.

8. SpatulaSpatula

9.

9. Volumetric flasks (1,000ml capacity)Volumetric flasks (1,000ml capacity)

10.

10.Boiling beads, glassBoiling beads, glass

11.

11.Magnetic stirrer and stirring barsMagnetic stirrer and stirring bars

Chemicals Required

1.

1. Potassium dichromate (KPotassium dichromate (K22Cr Cr 22OO77) 0.25N) 0.25N

2.

2. Sulphuric acid (HSulphuric acid (H22SOSO44) / silver sulphate (Ag) / silver sulphate (Ag22SOSO44) solution) solution

3.

3. Mercuric sulphate (HgSOMercuric sulphate (HgSO44) crystals) crystals

4.

4. Ferrous ammonium sulphate (FAS) [Fe(NHFerrous ammonium sulphate (FAS) [Fe(NH44))22(SO(SO44))22], approximately 0.01N], approximately 0.01N

5.

5. Ferroin indicator (1, 10-phenanthroline and ferrous Ferroin indicator (1, 10-phenanthroline and ferrous ammonium sulphate)ammonium sulphate) Caution:

Caution: In carrying out the In carrying out the following procedures, use proper safety measures,following procedures, use proper safety measures, including protective clothing, eye protection, and a

including protective clothing, eye protection, and a fume hood. Reagents containingfume hood. Reagents containing heavy metals (HgSO

heavy metals (HgSO44 and Agand Ag22SOSO44) should be disposed of as ) should be disposed of as toxic wastes.toxic wastes.

Chemical Preparation

1.

1. Dissolve 12.259g of oven-dried (primary standard grade dried at Dissolve 12.259g of oven-dried (primary standard grade dried at 103103oo_{C to a}_{C to a}

constant weight) potassium dichromate in distilled water and dilute to 1 litre constant weight) potassium dichromate in distilled water and dilute to 1 litre volume in a volumetric flask.

volume in a volumetric flask.

2.

2. Add 22g of reagent grade silver sulphate to a 4kg bottle of concentratedAdd 22g of reagent grade silver sulphate to a 4kg bottle of concentrated sulphuric acid (H

sulphuric acid (H22SOSO44) and mix until the silver sulphate goes into solution.) and mix until the silver sulphate goes into solution.

3.

3. Use 1 g of mercuric sulphate (HgSOUse 1 g of mercuric sulphate (HgSO44) to complex 100 mg chloride ) to complex 100 mg chloride (2,000(2,000

mg/l). mg/l).

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4.

4. Dissolve 1.485g of 1,10-phenanthroline monohydrate and 0.695g of ferrousDissolve 1.485g of 1,10-phenanthroline monohydrate and 0.695g of ferrous ammonium sulphate heptahydrate in distilled water and dilute to

ammonium sulphate heptahydrate in distilled water and dilute to

approximately 100ml. (Alternatively, this indicator may be purchased as approximately 100ml. (Alternatively, this indicator may be purchased as Ferroin Indicator from most scientific suppliers.)

Ferroin Indicator from most scientific suppliers.)

5.

5. Dissolve 39g reagent grade ferrous ammonium sulphate hexahydrate inDissolve 39g reagent grade ferrous ammonium sulphate hexahydrate in distilled water. Add 20ml of concentrated sulphuric acid (H

distilled water. Add 20ml of concentrated sulphuric acid (H22SOSO44). Cool and). Cool and

dilute to exactly 1 litre in a volumetric flask using distilled water.

dilute to exactly 1 litre in a volumetric flask using distilled water. The ferrousThe ferrous ammonium sulfate (FAS) titrant must be standardized daily by

ammonium sulfate (FAS) titrant must be standardized daily by the followingthe following procedure:

procedure:

Dilute 10ml of standard potassium dichromate (K

Dilute 10ml of standard potassium dichromate (K22Cr Cr 22OO77) solution to 100ml) solution to 100ml

with distilled water.

with distilled water. Slowly add 30ml of concentrated sulphuric acid and coolSlowly add 30ml of concentrated sulphuric acid and cool to room temperature. Titrate with ferrous ammonium sulphate titrant, using 2 to room temperature. Titrate with ferrous ammonium sulphate titrant, using 2 to 3 drops (0.10 to 0.15 ml) of

to 3 drops (0.10 to 0.15 ml) of Ferroin indicator.Ferroin indicator. Normality of FAS = (ml K

Normality of FAS = (ml K22Cr Cr 22OO77)(0.25))(0.25)

ml FAS required ml FAS required

The deterioration of FAS can be decreased if it is stored in a

The deterioration of FAS can be decreased if it is stored in a dark bottle.dark bottle.

Procedure

1.

1. Place a 50ml sample or an aliquot diluted to 50ml in a 500ml refluxing flask.Place a 50ml sample or an aliquot diluted to 50ml in a 500ml refluxing flask. The blank is prepared using 50ml of distilled water. This is a precise

The blank is prepared using 50ml of distilled water. This is a precise measurement and a 50ml volumetric pipette should be used.

measurement and a 50ml volumetric pipette should be used. Refer to CODRefer to COD

Range and Sample Size below for dilution.

2.

2. Add 5 to 7 glass boiling beads.Add 5 to 7 glass boiling beads.

3.

3. Add 1g of mercuric sulphate (HgSOAdd 1g of mercuric sulphate (HgSO44), 5ml of concentrated sulphuric acid /), 5ml of concentrated sulphuric acid /

silver sulphate solution, and mix until the HgSO

silver sulphate solution, and mix until the HgSO44 is in solution. The function of is in solution. The function of

the mercuric sulphate is to bi

the mercuric sulphate is to bind or complex chlorides. One gram may not nd or complex chlorides. One gram may not bebe required if the chloride concentration is low.

required if the chloride concentration is low. (Caution: Always add acid slowly(Caution: Always add acid slowly down the side of the flask while mixing to avoid overheating. It may be

down the side of the flask while mixing to avoid overheating. It may be necessary to use gloves because of the heat

necessary to use gloves because of the heat generated.)generated.)

4.

4. Accurately add 25ml of 0.25 N Accurately add 25ml of 0.25 N potassium dichromate (Kpotassium dichromate (K22Cr Cr 22OO77) and mix.) and mix.

5.

5. Add while mixing, an additional 70ml Add while mixing, an additional 70ml of concentrated sulphuric acid-silver of concentrated sulphuric acid-silver sulphate solution.

sulphate solution.

6.

6. After thorough mixing, attach the flask to the reflux condenser, apply heat,After thorough mixing, attach the flask to the reflux condenser, apply heat, and reflux for 2

and reflux for 2 hours. Refluxing time can be decreased depending on thehours. Refluxing time can be decreased depending on the ease of oxidation of organic materials.

ease of oxidation of organic materials. This time may be determined byThis time may be determined by refluxing for periods from 15

refluxing for periods from 15 minutes to 2 hours and comparing the rminutes to 2 hours and comparing the results.esults.

7.

7. A reagent blank containing 50ml of distilled water treated with the sameA reagent blank containing 50ml of distilled water treated with the same reagent as the sample should be refluxed wi

reagent as the sample should be refluxed with each set of th each set of samples.samples.

8.

8. Cool the apparatus to room temperature after Cool the apparatus to room temperature after the refluxing period. Washthe refluxing period. Wash down the interior of

down the interior of the condenser and flask twice with the condenser and flask twice with approximately 25mlapproximately 25ml portions of distilled water.

portions of distilled water.

9.

9. Remove flask from the condenser and dilute to Remove flask from the condenser and dilute to a final volume of a final volume of approximately 350ml with distilled water.

approximately 350ml with distilled water.

10.

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11.

11.Place flask on a magnetic stirrer and rapidly titrate with 0.1 N Place flask on a magnetic stirrer and rapidly titrate with 0.1 N ferrousferrous ammonium sulphate to the first red-brown endpoint.

ammonium sulphate to the first red-brown endpoint. Caution:

Caution: Use care in titration. The endpoint is very sharp and may beUse care in titration. The endpoint is very sharp and may be reached rapidly.

reached rapidly.

Formula to determine COD:

COD (mg/l) = (a-b)(N) x

COD (mg/l) = (a-b)(N) x 8,0008,000 // sample size (ml)sample size (ml) Where:

Where:

a = ml Fe(NH4)2(SO4)2 used for blank a = ml Fe(NH4)2(SO4)2 used for blank b = ml Fe(NH4)2(SO4)2 used for sample b = ml Fe(NH4)2(SO4)2 used for sample N = normality of FAS titrant (

N = normality of FAS titrant (Fe(NHFe(NH44))22(SO(SO44))22))

ml sample = the actual

ml sample = the actual volume of sample used before dilutionvolume of sample used before dilution

Sources of Error

1.

1. The largest error is The largest error is caused by using a nonhomogeneous sample. Every effortcaused by using a nonhomogeneous sample. Every effort should be made to blend and

should be made to blend and mix the sample so that solids are mix the sample so that solids are never never excluded from any aliquot.

excluded from any aliquot.

2.

2. Always use the largest sample practical and use Always use the largest sample practical and use the largest glassware that isthe largest glassware that is in keeping with good laboratory practice.

in keeping with good laboratory practice.

3.

3. Use volumetric flasks and volumetric pipettes with a Use volumetric flasks and volumetric pipettes with a large bore.large bore.

4.

4. The KThe K22Cr Cr 22OO77 oxidizing agent must be precisely measured. Use a volumetricoxidizing agent must be precisely measured. Use a volumetric

pipette and use the same one

pipette and use the same one each time if possible.each time if possible.

5.

5. When titrating, be certain that the burette is clean and free of When titrating, be certain that the burette is clean and free of air bubbles.air bubbles.

6.

6. Always read the bottom of Always read the bottom of the meniscus and position the meniscus at eyethe meniscus and position the meniscus at eye level.

level.

COD Range and Sample Size

COD Range COD Range (mg/l) (mg/l) 50-800 800 100-1500 1500 240-3700 3700 480-7500 7500 1200-18800 18800 2400-3700 3700 40000-375000 375000 Volume of Volume of Sample (ml) Sample (ml) 5500 2255 1100 55 22 11 00..11

All samples high in solids should be blended for 2 minutes at high speed and stirred All samples high in solids should be blended for 2 minutes at high speed and stirred when an aliquot is taken.

when an aliquot is taken. Sample volumes less than 25ml should not be pipettedSample volumes less than 25ml should not be pipetted directly, but serially diluted and then a portion of the

directly, but serially diluted and then a portion of the diluent removed:diluent removed:

1.

1. 500ml of sample diluted to 1,000 ml = 0.5 ml sample/ml of diluent, .: 50 ml of 500ml of sample diluted to 1,000 ml = 0.5 ml sample/ml of diluent, .: 50 ml of diluent = 25 ml of sample.

diluent = 25 ml of sample.

2.

2. 100 ml of sample diluted to 1,000 ml = 0.1 ml sample/ml diluent, .: 50 ml of 100 ml of sample diluted to 1,000 ml = 0.1 ml sample/ml diluent, .: 50 ml of diluent = 5 ml of sample.

diluent = 5 ml of sample.

Elimination of Interference

One gram of mercuric sulphate (HgSO

One gram of mercuric sulphate (HgSO44) will complex 100mg of chloride in a 50ml) will complex 100mg of chloride in a 50ml

sample (2,000 mg/l). For samples higher in

sample (2,000 mg/l). For samples higher in chloride more HgSOchloride more HgSO44 should be used inshould be used in

the ratio of 10:1 HgSO the ratio of 10:1 HgSO44..

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Interference from nitrites can be prevented by the addition of 10:1 ratio of

Interference from nitrites can be prevented by the addition of 10:1 ratio of sulfamicsulfamic acid:nitrite. The addition of the silver sulphate (AgSO

acid:nitrite. The addition of the silver sulphate (AgSO44) concentrated sulphuric acid) concentrated sulphuric acid

(H

(H22SOSO44) refluxing acid will ) refluxing acid will aid in the aid in the oxidation of some organic nitrogen compounds,oxidation of some organic nitrogen compounds,

but aromatic hydrocarbons and pyridine are not oxidized

but aromatic hydrocarbons and pyridine are not oxidized to any appreciable amount.to any appreciable amount.

Chemical oxygen demand

From Wikipedia, the free encyclopedia From Wikipedia, the free encyclopedia Jump to:

Jump to: navigationnavigation,, searchsearch In

In environmental chemistryenvironmental chemistry, the, the chemical oxygen demandchemical oxygen demand ((CODCOD) test is commonly used to) test is commonly used to

indirectly measure the amount of

indirectly measure the amount of organic compoundsorganic compounds ininwater water . Most applications of COD. Most applications of COD

determine the amount of

determine the amount of organicorganic pollutants pollutantsfound infound in surface water surface water (e.g.(e.g.lakeslakes andandriversrivers),),

making COD a useful measure of

making COD a useful measure of water qualitywater quality. It is expressed in milligrams per liter (. It is expressed in milligrams per liter (mgmg//LL),),

which indicates the

which indicates the massmass of oxygen consumed per liter of of oxygen consumed per liter of solutionsolution. Older references may. Older references may

express the units as

express the units as parts per million parts per million (ppm).(ppm).

Overview

The basis for the COD test is that nearly all organic compounds can be fully oxidized to

carbon dioxide

carbon dioxide with a strongwith a strong oxidizing agentoxidizing agent under under acidicacidic conditions. The amount of conditions. The amount of oxygenoxygen required to oxidize an organic compound to carbon dioxide,

required to oxidize an organic compound to carbon dioxide, ammoniaammonia,, and water is given by:and water is given by:

This expression does not include the oxygen demand c

This expression does not include the oxygen demand caused by the oxidation of ammoniaaused by the oxidation of ammonia

into nitrate. The process of ammonia being converted into nitrate is referred to as

nitrification.

nitrification. The following is the correct equation for the oxidation of ammonia into nitrate.The following is the correct equation for the oxidation of ammonia into nitrate.

The second equation should be applied after the first one to include oxidation due to

nitrification if the oxygen demand from nitrification must be known. Dichromate does not

oxidize ammonia into nitrate, so this nitrification can be safely ignored in the standard

chemical oxygen demand test.

The

The International Organization for StandardizationInternational Organization for Standardizationdescribes a standard method for describes a standard method for

measuring chemical oxygen demand in

measuring chemical oxygen demand in ISO 6060ISO 6060 [1][1]..

History

For many years, the strong

For many years, the strong oxidizing agentoxidizing agent potassium permanganate potassium permanganate ((K K MnMnOO44) was used for ) was used for

measuring chemical oxygen demand. Measurements were called

measuring chemical oxygen demand. Measurements were called oxygen consumed oxygen consumed fromfrom

permanganate, rather than the

permanganate, rather than the oxygen demand oxygen demand of organic substances. Potassiumof organic substances. Potassium

permanaganate's effectiveness at oxidizing organic compounds varied widely, and in many

cases

cases biochemical oxygen demand biochemical oxygen demand (BOD) measurements were often much greater than(BOD) measurements were often much greater than

results from COD measurements. This indicated that potassium permanganate was not able to

effectively oxidize all organic compounds in water, rendering it a relatively poor oxidizing

agent for determining COD.

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Since then, other oxidizing agents such as

Since then, other oxidizing agents such as cerceric ic sulfsulfateate,, potassium iodate potassium iodate,, andand potassium potassium dichromate

dichromate have been used to determine COD. Of these, potassium dichromate (have been used to determine COD. Of these, potassium dichromate (K K 22Cr Cr 22OO77) has) has

been shown to be the most effective: it is relatively cheap, easy to

been shown to be the most effective: it is relatively cheap, easy to purify purify, and is able to nearly, and is able to nearly

completely oxidize almost all organic compounds.

In these methods, a fixed volume with a known excess amount of the oxidant is added to a

sample of the solution being analyzed. After a refluxing digestion step, the initial

concentration of organic substances in the sample is calculated from a titrimetric or

spectrophotometric determination of the oxidant still remaining in the sample.

Using potassium dichromate

Potassium dichromate

Potassium dichromate is a strong oxidizing agent under acidic conditions. (Acidity is usuallyis a strong oxidizing agent under acidic conditions. (Acidity is usually

achieved by the addition of

achieved by the addition of sulfuric acidsulfuric acid..) The reaction of potassium dichromate with organic) The reaction of potassium dichromate with organic

compounds is given by:

where

where d = 2n/3 + a/6 - b/3 - c/2d = 2n/3 + a/6 - b/3 - c/2. Most commonly, a 0.25. Most commonly, a 0.25 N N solution of potassium dichromatesolution of potassium dichromate

is used for COD determination, although for samples with COD below 50 mg/L, a lower

concentration of potassium dichromate is preferred.

In the process of oxidizing the organic substances found in the water sample, potassium

dichromate is reduced (since in all

dichromate is reduced (since in all redoxredoxreactions, one reagent is oxidized and the other isreactions, one reagent is oxidized and the other is

reduced), forming Cr

reduced), forming Cr 3+3+_{. The amount of Cr}_{. The amount of Cr}3+3+_{is determined after oxidization is complete, and is}_{is determined after oxidization is complete, and is}

used as an indirect measure of the organic contents of the water sample.

Blanks

Because COD measures the oxygen demand of organic compounds in a sample of water, it is

important that no outside organic material be accidentally added to the sample to be

measured. To control for this, a so-called blank sample is required in the determination of

COD (and

COD (and BODBOD, for that matter). A blank sample is created by adding all reagents (e.g. acid, for that matter). A blank sample is created by adding all reagents (e.g. acid

and oxidizing agent) to a volume of

and oxidizing agent) to a volume of distilled water distilled water . COD is measured for both the water and. COD is measured for both the water and

blank samples, and the two are compared. The oxygen demand in the blank sample is

subtracted from the COD for the original sample to ensure a true measurement of organic

matter.

Measurement of excess

For all organic matter to be completely oxidized, an excess amount of potassium dichromate

(or any oxidizing agent) must be present. Once oxidation is complete, the amount of excess

potassium dichromate must be measured to ensure that the amount of Cr

potassium dichromate must be measured to ensure that the amount of Cr 3+3+ _{can be determined}_{can be determined}

with accuracy. To do so, the excess potassium dichromate is

with accuracy. To do so, the excess potassium dichromate is titratedtitrated withwith ferrous ammoniumferrous ammonium sulfate

sulfate(FAS) until all of the excess oxidizing agent has been reduced to Cr (FAS) until all of the excess oxidizing agent has been reduced to Cr 3+3+_{. Typically, the}_{. Typically, the}

oxidation-reduction indicator

oxidation-reduction indicator FerroinFerroin is added during this titration step as well. Once all theis added during this titration step as well. Once all the

excess dichromate has been reduced, the Ferroin indicator changes from blue-green to

reddish-brown. The amount of ferrous ammonium sulfate

reddish-brown. The amount of ferrous ammonium sulfate added is equivalent to added is equivalent to the amountthe amount

of excess potassium dichromate added to the original sample. and also we can determine

COD by boiling the water sample and we can determine CO2 ratio by the infra-red analyzer

Preparation

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A solution of 1.485 g

1,10-A solution of 1.485 g 1,10- phenanthroline phenanthroline monohydrate is added to a solution of monohydrate is added to a solution of 695 mg695 mg

FeSO

FeSO44·7H·7H22O in water, and the resulting red solution is diluted to 100 mL.O in water, and the resulting red solution is diluted to 100 mL.

Calculations

The following formula is used to calculate COD:

where

where bb is the volume of FAS used in the blank sample,is the volume of FAS used in the blank sample, s sis the volume of FAS in theis the volume of FAS in the

original sample, and

original sample, and nn is the normality of FAS. If milliliters are used consistently for volumeis the normality of FAS. If milliliters are used consistently for volume

measurements, the result of the COD calculation is given in mg/L.

The COD can also be estimated from the concentration of oxidizable compound in the

sample, based on its stoichiometric reaction with oxygen to yield CO

sample, based on its stoichiometric reaction with oxygen to yield CO22 (assume all C goes to(assume all C goes to

CO

CO22), H), H22O (assume all H goes to HO (assume all H goes to H22O), and NHO), and NH33(assume all N goes to NH(assume all N goes to NH33), using the), using the

following formula:

COD = (C/FW)(RMO)(32) COD = (C/FW)(RMO)(32)

Where C = Concentration of oxidizable compound in the sample, Where C = Concentration of oxidizable compound in the sample, FW = Formula weight of the oxidizable compound in the sample, FW = Formula weight of the oxidizable compound in the sample, RMO = Ratio of the # of moles of oxygen to # of moles of oxidizable RMO = Ratio of the # of moles of oxygen to # of moles of oxidizable compound in their reaction to CO

compound in their reaction to CO22, water, and ammonia, water, and ammonia

For example, if a sample has 500 wppm of phenol:

C C66HH55OH + 7OOH + 7O22 → 6CO→ 6CO22 + 3H+ 3H22OO COD = (500/94)(7)(32) = 1191 wppm COD = (500/94)(7)(32) = 1191 wppm

Inorganic interference

Some samples of water contain high levels of oxidizable inorganic materials which may

interfere with the determination of COD. Because of its high concentration in most

wastewater

wastewater ,, chloridechloride is often the most serious source of interference. Its reaction withis often the most serious source of interference. Its reaction with

potassium dichromate follows the equation:

Prior to the addition of other reagents,

Prior to the addition of other reagents, mercuric sulfatemercuric sulfatecan be added to the sample tocan be added to the sample to

eliminate chloride interference.

The following table lists a number of other inorganic substances that may cause interference.

The table also lists chemicals that may be used to eliminate such interference, and the

compounds formed when the inorganic molecule is eliminated.

Inorganic Inorganic molecule molecule Eliminated Eliminated by

by Elimination formsElimination forms Chloride

Chloride MercuricMercuric sulfate sulfate Mercuric chloride Mercuric chloride complex complex

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Nitrite

Nitrite SulfamicSulfamic acid

acid NN22 gasgas Ferrous iron Ferrous iron -- --Sulfides Sulfides --

--Government regulation

Government regulation

Many governments impose strict regulations regarding the maximum chemical oxygen

demand allowed in wastewater before they can be returned to the environment. For example,

in

in SwitzerlandSwitzerland, a maximum oxygen demand between , a maximum oxygen demand between 200 and 1000 mg/L must be reached200 and 1000 mg/L must be reached

before wastewater or

before wastewater or industrial water industrial water can be returned to the environmentcan be returned to the environment [2][2]..

PREPARATION OF SOLUTIONS PREPARATION OF SOLUTIONS Stock Iron Standard Solution, 10 ppm Stock Iron Standard Solution, 10 ppm

Primary standard solid ferrous ammonium sulfate hexahydrate, (NH

Primary standard solid ferrous ammonium sulfate hexahydrate, (NH44))22(SO(SO44))22 6H6H22O, 392.13O, 392.13

g/mol, is available on the side shelves for preparation of the standard iron solution. g/mol, is available on the side shelves for preparation of the standard iron solution. 1. Tap a

1. Tap a small small amount of the solid ferrous ammonium sulfate onto a sheet of glassineamount of the solid ferrous ammonium sulfate onto a sheet of glassine weighing

weighing

paper that has been folded in the middle. Zero your balance.

paper that has been folded in the middle. Zero your balance. Accurately Accurately weigh about 0.07 gweigh about 0.07 g of pure dry ferrous ammonium sulfate (to

of pure dry ferrous ammonium sulfate (to

+

0.1 mg) onto a folded sheet of glassine paper or 0.1 mg) onto a folded sheet of glassine paper or into a small, plastic weighing boat.

into a small, plastic weighing boat.

2. Transfer the ferrous ammonium sulfate

2. Transfer the ferrous ammonium sulfate quantitativelyquantitatively into a 1-L volumetric flask, carefullyinto a 1-L volumetric flask, carefully squirting down the weighing boat and the neck of the flask to ensure a quantitative transfer. squirting down the weighing boat and the neck of the flask to ensure a quantitative transfer. Add about 100-200 mL of distilled water.

Add about 100-200 mL of distilled water. Dissolve the solid completely before diluting to Dissolve the solid completely before diluting to volume

volume

3. Pipet 2.5 mL of concentrated sulfuric acid into the flask, rinse the neck of the flask down, 3. Pipet 2.5 mL of concentrated sulfuric acid into the flask, rinse the neck of the flask down, and mix carefully with swirling.

and mix carefully with swirling. [Be very careful when using concentrated H[Be very careful when using concentrated H22SOSO44; it is; it is

quite caustic.]

quite caustic.] Dilute the solution to the mark. Calculate the iron concentration of theDilute the solution to the mark. Calculate the iron concentration of the solution in

solution in



g of iron per mL (ppm) and in molar (M) units.g of iron per mL (ppm) and in molar (M) units.

Because this solution is used to calibrate absorbances and prepare a calibration curve, it Because this solution is used to calibrate absorbances and prepare a calibration curve, it must be prepared very

must be prepared very carefully and accurately. The results of carefully and accurately. The results of the entire experimentthe entire experiment rest

rest

on preparing this solution

on preparing this solution accurately.accurately.

The iron solution must be prepared daily, so there is no point in saving the solution to re-use The iron solution must be prepared daily, so there is no point in saving the solution to re-use it if

it if

you end up needing to re-do the experiment. You will need to prepare another standard you end up needing to re-do the experiment. You will need to prepare another standard solution.

solution.

Iron Standard Calibration Solutions Iron Standard Calibration Solutions

1. Into each of five 100-mL volumetric flasks, pipet 1, 5, 10, 20, and 35 mL of the standard 1. Into each of five 100-mL volumetric flasks, pipet 1, 5, 10, 20, and 35 mL of the standard iron

iron

solution, respectively. Use a combination of

solution, respectively. Use a combination of 1-, 5-, 10-, and 25-mL volumetric 1-, 5-, 10-, and 25-mL volumetric pipets. Thepipets. The 1- and 5-mL pipets are located in the drawer marked for the experiment.

1- and 5-mL pipets are located in the drawer marked for the experiment. 2. Pour about 50 mL of distilled water into a 6

2. Pour about 50 mL of distilled water into a 6ththflask to serve as the “blank” (i.e. zero ironflask to serve as the “blank” (i.e. zero iron

concentration). concentration).

3. Obtain the unknown sample from the Teaching assistants and treat it in the same manner as 3. Obtain the unknown sample from the Teaching assistants and treat it in the same manner as the standards, as indicated below.

the standards, as indicated below.

4. Line all seven 100-mL volumetric flasks in this order: The blank, those with 1-35 mL of 4. Line all seven 100-mL volumetric flasks in this order: The blank, those with 1-35 mL of iron

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stock standard solution added, and your unknown sample. To each

stock standard solution added, and your unknown sample. To each flask (including theflask (including the distilled water “blank” and the unknowns), pipet in order –

distilled water “blank” and the unknowns), pipet in order – a. 1 mL of

a. 1 mL of the hydroxylamine solution,the hydroxylamine solution, b. 10 mL of the

b. 10 mL of the 1,10-phenanthroline solution, and1,10-phenanthroline solution, and c. 8 mL of the sodium acetate solution.

c. 8 mL of the sodium acetate solution.

Note that the “blank” solution must have all the reagents in it except for any ferrous Note that the “blank” solution must have all the reagents in it except for any ferrous

ammonium sulfate. ammonium sulfate.

5. Swirl each flask to mix the contents, then carefully dilute each solution to the 100-mL 5. Swirl each flask to mix the contents, then carefully dilute each solution to the 100-mL mark

mark

and mix thoroughly. and mix thoroughly.

6. Allow the solutions to stand for 10 minutes to fully develop the color. Mix well again. Fill 6. Allow the solutions to stand for 10 minutes to fully develop the color. Mix well again. Fill each of seven clean, dry plastic cuvettes about two-thirds full with each of the seven

each of seven clean, dry plastic cuvettes about two-thirds full with each of the seven

solutions, keeping them in the same order. (If the insides of the cuvettes are wet or spotted, solutions, keeping them in the same order. (If the insides of the cuvettes are wet or spotted, rinse them out twice with the appropriate solution first.)

rinse them out twice with the appropriate solution first.) Stock Reagent Solutions

Stock Reagent Solutions

The sodium acetate buffer (1.2 M), the 1,10-phenanthroline solution (1 g/L), and the The sodium acetate buffer (1.2 M), the 1,10-phenanthroline solution (1 g/L), and the hydroxylamine hydrochloride solution (100 g/L) are prepared

hydroxylamine hydrochloride solution (100 g/L) are prepared by the Teaching Assistants andby the Teaching Assistants and should be available for your use.