chlorides & alkalinity

7/24/2019 Chlorides & Alkalinity

1/8

Precipitation Titration: Determination of Chloride by the Mohr Method

by Dr. Deniz Korkmaz

Introduction

Titration is a process by which the concentration of an unknown substance in solution is

determined by adding measured amounts of a standard solution that reacts with the

unknown. Then the concentration of the unknown can be calculated using the stoichiometry of the reaction and

the number of moles of standard solution needed to reach the so called end point.

Precipitation titrations are based upon reactions that yield ionic compounds of limited

solubility. The most important precipitating reagent is silvernitrate. Titrimetric methods

based upon silvernitrateare sometimes termed argentometric methods. Potassium

chromate can serve as an end point indicator for the argentometric determinationof

chloride, bromide and cyanide ions by reacting with silverions to form a brick-red silver

chromate precipitate in the equivalence point region.

The Mohr method uses chromate ions as an indicator in the titration of chloride ions with a silvernitrate

standard solution. After all the chloride has been precipitated as white silver chloride, the first excess of titrant

results in the formation of a silverchromate precipitate, which signals the end point (1). The reactions are:

By knowing the stoichiometry and moles consumed at the end point, the amount of

chloride in an unknown sample can be determined. This report describes experiments

aimed at determining the concentration of chloride in a solid sample

Discussion

The well known Mohrs method in which alkaline or alkaline earth chloridesreact with

silvernitratein the presence of a few drops of potassium chromate solution as indicator is a simple, direct and

accurate method for chloride determination.

In this experiment, the amount of chloride in an unknown sample was determined by

Mohr titration. The titration was carried out at a pH between 7 and 10 because chromate

ion is the conjugate base of the weak chromic acid (2, 3). Therefore, when the pH is lower than 7, chromate

ion is protonated and the chromic acid form predominates in the

solution. Consequently, in more acidic solutions the chromate ion concentration is too

low to produce the precipitate at the equivalence point. If the pH is above 10, brownishsilverhydroxide forms and masks the end point. A suitable pH was achieved by

saturating the analyte solution with sodium hydrogen carbonate.

Since the solubilities of silverchloride and silverchromate depend on temperature, all

tirations were carried out at about the same temperature. Good stirring during the addition

of the silvernitrateis also required for a sharp and reproducible end point; otherwise,

silverchromate that forms locally before the endpoint can become occluded in the silver

chloride precipitate instead of redissolving.

Standard silvernitratesolution and the silverchloride precipitates formed were protected from light at all

times because silverchloride decomposes according to:2AgCl Ag(s) + Cl ( g)


2/8

The silverion concentration at chemical equivalence in the titration of chloride with

silverions is given by:

[Ag ] =

=

= 1.35 x 10 M+

-5

The chromate ion concentration required to initiate formation of silverchromate under

this condition can be computed from the solubility product constant for silverchromate:4

[CrO ] ==

= 6.6 10 M2-

-3

In principle, an amount of chromate to give this concentration should be added, in order

to develop the red precipitate after the equivalence point. However, a chromate ion

concentration of 6.6 x 10-3 M imparts such an intense yellow color to the solution that

formation of the red silverchromate is not readily detected. Thus, lower concentrations of

chromate ion are generally used. An excess of silvernitrateis therefore required before

precipitation begins. An additional excess of the reagent must also be added to produce

enough silverchromate to be seen over the heavy white precipitate of silverchloride.

These two factors create a positive systematic error in the Mohr method that becomes

significant in magnitude at reagent concentrations lower than about 0.1 M. A correction

for this error was made by a blank determination. In a blank determination, all steps of

the analysis are performed in the absence of the analyte. In this experiment, blank was

determined by titrating a solution of a small amount of chloride free calcium carbonate

and indicator potassium chromate with standard silvernitratesolution. Calcium

carbonate was used to imitate the white silverchloride precipitate. Reagent volumes were

corrected for blank in the calculations.

CHLORIDE ION DETERMINATION

This test measures the soluble chloride Ion concentration in the mud filtrate. The Chloride can come from sodium chloride

calcium chloride or !otassium chloride. Also for the titration to "or# correctl$ the !H of the filtrate needs to be onl$ "ea#l$

basic %!H & '.(). This is the reason for the first ste! in the !rocedure. There are t"o chemical reactions

ta#in* !lace simultaneousl$ durin* the titration+

,. A* %-) - Cl A*CL /. / A*

%-) - CrO0 A*/CrO0

The first reaction the formation of sil1er chloride accounts for the a!!earance of the "hite s!ecs or mil#$ a!!earance durin*

the titration. The formation of the sil1er chromate "hich is red "ill not start until all the chloride Ions are tied u! as sil1er

chloride. The sil1er nitrate "ill then react "ith the chromate from the !otassium chromate indicator to form sil1er chromate.

2o for the abo1e t"o reactions to occur the filtrate needs to be "ea#l$ basic %!H & '.(). Hi*h !H "ill !reci!itate sil1er o3ide.

4ROCED5RE

,.

/.

(.

0.

6. 7.


3/8

4

i

!

e

t

t

e

,

.8 ml of filtrate or sam!le to be tested into titration dish. Add / ( dro!s

of !henol!htahalein indicator to the filtrate.

If a !in# color a!!ears titrate "ith N968 sulfuric acid until the color chan*es from !in# to that of the ori*inal

sam!le. If no !in# color a!!ears the test can be continued.

If needed add /6 68 mls of distilled "ater. This ser1es to dilute the dar# color of a dee!l$ colored filtrate. It does

not influence the test in an$ "a$ if there are no chlorides !resent in the distilled "ater. If an$ "ater other than

distilled "ater is used the chloride in it must be accounted for b$ titration before addin* the filtrate to it.

Add ,8 ,6 dro!s of !otassium chromate indicator to *i1e the filtrate a bri*ht $ello" color.

Add sil1er nitrate from a !i!ette dro!"ise stirrin* continuousl$ "ith a stirrin* rod until the sam!le :ust turns

from $ello" to an oran*ered.

CALC5LATION

,.

/.

(.

4arts !er million %!!m) chloride & mls of sil1er nitrate used 3 ,888 %If 8.8/'/N 2il1er Nitrate lml & ,888

!!m Cl is used)

4arts !er million %!!m) chloride & mls of sil1er nitrate used 3 ,8888 %If 8./'N 2il1er Nitrate lml & ,8888

!!m Cl is used)

!!m %NaCl) & !!m Chloride 3 ,.76

REMAR;2

,. The chloride test ma$ be run on the same sam!les used in the 4f determination if the Mf test "as not

!erformed.

/. A1oid contact "ith sil1er nitrate.


4/8

(.

0.

6.

The end !oint of the reaction is "hen the sil1er chromate is first formed. It is reddish in color.


5/8

O?I Testin* Eui!ment ,0068 Instructions Chloride and Al#alinit$ Determination 4a*e 0 of 6

II. ?ILTRATE AL;ALINIT= %4f Mf)

These tests are desi*ned to measure the al#alinit$ contributions from soluble mud constituents. The 4f and

Mf test to*ether can be used to *et an estimate of the h$dro3ide Ion %OH) the bicarbonate Ion %HCO()

and the carbonate Ion %CO( /) concentration. Also the 4f can be used "ith the 4m to determine the e3cess

amount of lime !resent in a lime base mud.

4ROCED5RE %4f)

,. 4i!ette , ml of filtrate into a clean titration dish. %Do not dilute "ith distilled "ater.)

/. Add more than / ( dro!s of 4henol!hthalein. If no color de1elo!s the 4f is 8 %and !H is less than '.(). If

a !in# color de1elo!s add N968 sulfuric acid throu*h a !i!ette until the color chan*es !in# to that of the ori*inal

sam!le.

CALC5LATION2 %4f)

,. The 4f is the amount of N968 sulfuric acid reuired to dischar*e the !in# color. /.

E3cess lime content lb9bbl & 4m 4f

0

4ROCED5RE %Mf)

,. To the same sam!le used for the 4f determination add / ( dro!s of From cresol *reenmeth$l red

indicator.

/. A*ain add N968 sulfuric acid until the color chan*es from a blue*reen to a li*ht !in# color.

CALC5LATION2 %Mf)

,. Re!ort the Mf al#alinit$ of the filtrate as the total 1olume of N968 sulfuric acid reuired to reach the end !oint

includin* the 1olume reuired to reach the 4f end !oint.

/. An estimation of h$dro3ide Ion bicarbonate Ion and carbonate Ion concentrations can be determined usin* the

follo"in* table.

O?ITE@ ,887


6/8

O?I Testin* Eui!ment ,0068 Instructions Chloride and Al#alinit$ Determination 4a*e 6 of 6

CALC5LATION2 O? AL;ALINIT=

Test Results


7/8

4. Calculate the concentration of chloride ions in the diluted seawater.

5. Calculate the concentration of chloride ions in the original undiluted seawater.

6. Calculate the concentration of sodium chloride in the seawater in molL 1, gL1 and g/100 mL (%).

Additional Notes1. Silvernitratesolution will stain clothes and skin. Any spills should be rinsed with water immediately.

2. Residues containing silverions are usually saved for later recovery of silvermetal. Check this with your

teacher.

3. The Mohr titration should be carried out under conditions of pH 6.5 9. At higher pH silverions may be

removed by precipitation with hydroxide ions, and at low pH chromate ions may be removed by an acid-

base reaction to form hydrogen chromate ions or dichromate ions, affecting the accuracy of the end point.

4. It is a good idea to first carry out a rough titration in order to become familiar with the colour change at

the end point.

5. The Mohr titration is sensitive to the presence of both chloride and bromide ions in solution and

will not be too accurate when there is a significant concentration of bromide present as well as the

chloride. However, in most cases, such as seawater, the bromide concentration will be negligible. For this

reason, the method can also be used to determine either the total concentration of chloride and bromide

in solution, or the concentration of bromide when the chloride concentration is known to be negligible.

http://intro.chem.okstate.edu/ChemSource/Seawater/act2.htm

Activity 2: Determination of the Salinity of Seawater

Introduction

This method involves chemical volumetric analysis. A silver nitrate (AgNO 3 ) solution

of known concentration is used to precipitate out the chlorides in a seawater sample.

The procedure is complicated by the fact that the dense white precipitate, silver

chloride (AgCl), does not settle rapidly. It is impossible to tell when all the chlorides

have been removed from the seawater unless an indicator is used to signal complete

precipitation by a visual color change. The indicator in the analysis is chromate ion.

When all of the chloride ion is exhausted, the chromate ion reacts with silver ions and

produces silver chromate, which is red. The instant a permanent orange tingeappears in the solution (one that doesnt vanish with mixing), the addition of silver

nitrate is stopped. The final solution color should look like that of orange juice.

The chemical reactions are:

The titration is carried out in a 150-mL Erlenmeyer flask, using a white background

and good lighting, but away from direct sunlight. Constant but gentle stirring is

essential, since the silver chloride tends to form clots, which trap some of the

reagents. A magnetic stirrer is very helpful. The silver nitrate is added until the faint

reddish or orange tinge remains after stirring for 45 s. At this point one drop should

give the desired end point. Observable flocs of silver chloride will obscure the endpoint, since the solutions that they contain in their interstices will leak out slowly

and cause the end point to be impermanent. A stirring rod left permanently in the

flask, especially if a magnetic stirrer is not used, will help to break up these curds.

Purpose

1. To determine the chlorinity (and thus the salinity, since they are directly

related) of a sample of seawater.

2. To use the titration technique with a reaction different from an acid/base

reaction.

Safety

Wear protective goggles throughout the laboratory activity. Avoid getting AgNO 3 on

hands since it stains the skin.Procedure
http://intro.chem.okstate.edu/ChemSource/Seawater/act2.htmhttp://intro.chem.okstate.edu/ChemSource/Seawater/act2.htm


8/8

1. With a pipet deliver 5.0mL seawater into a 150-mL Erlenmeyer flask.

2. Using a 10-mL graduated cylinder, add 5.0mL potassium chromate indicator

solution, and begin to add the silver nitrate solution from a buret while

constantly stirring.

3. When the solution turns orange, stop adding silver nitrate. The color should

persist for less than 20 sec. Continue adding silver nitrate dropwise, mixing all

the while, until orange color persists. Record the volume of silver nitrate added.

4. Thoroughly wash your hands before leaving the laboratory.

Data Analysis and Concept Development

From a simple algebraic proportion one can determine the chlorinity of the seawater

because it has been shown experimentally that the salinity is a constant 1.80655

times higher than the chloride level. Thus, the salinity (S) can be calculated (using

the equation S ppt = 1.80655 Cl ppt; the chlorinity (Cl) of the artificial seawater

should be about 19.05 ppt).

1. What is the number of moles of silver used in the titration (AgNO 3 is 0.2184 M)?

2. Determine the number of moles of chloride in the sample.

3. Determine the grams of chloride present in the sample.

4. What is the chlorinity of 10g of sample? (The density of seawater is about

1.05 g / mL 1 ).

5. What is the salinity of the sample in ppt?

chlorides & alkalinity

Documents