Precipitation Titration

Precipitation Titration: These titrations are based on precipitation reactions.  The best known is the Mohr method, using Ag+ as a titrant in chlorides (or bromides) determination. Titration with thiocyanates – the so-called Volhard method – can be used for Ag+ determination or indirect determination (through back titration) of chlorides. There are also other less known precipitation titrations, for example, sulfuric acid can be used to determine Ba2+ and ferrocyanide to determine Zn2+

Changing the property of the solution is the concentration of the precipitated substance. As its concentration changes by many orders of magnitude, and is almost always smaller than 1, we use a negative logarithmic scale, similar to that used in the pH definition. 

To detect the endpoint of the titration we use substances that react with the excess titrant creating strongly coloured compounds. These can be for example red silver chromate that starts to precipitate after all chloride has been removed from the solution, or iron (III) thiocyanate complex, created after all silver was precipitated as silver thiocyanate. 

End Point Detection 

Compared to other types of titration – complexometric, potentiometric and acid-base – precipitation titrations do not have a set of universal indicators, that you can select from when designing a new method. Each precipitation titration method has its own, specific way of endpoint detection. The closest to being universal is Fajan’s adsorption indicators, but even these are very limited in their applications. 

Probably the most popular precipitation titration – determination of chlorides by Mohr method – uses red silver chromate for the endpoint detection. At the beginning of the titration, we add a small amount of CrO42− to the solution. As long as there are still chlorides present, Ag+ concentration is too low for the silver chromate to precipitate. After the equivalence point, Ag+ concentration goes up and chromate precipitates, making the solution red. 

This is an interesting case to discuss, as it is relatively easy to estimate necessary concentrations. We have two weakly soluble salts, silver chloride: 

KAgCl = [Ag+] [Cl] = 10−10

and silver chromate 

KAg2CrO4 = [Ag+2] [CrO42−] = 2.10−12 

Reaction taking place during titration is 

Ag+ + Cl → AgCl  

At the equivalence point, 

[Ag+] = [Cl] = √KAgCl = √10−10 = 10−5

At this Ag+ concentration, we should start to see precipitating silver chromate. That means we need [CrO42−] = KAg2CrO4 / [Ag+2] = 0.02 M. 

0.02 M chromate solution is so strongly coloured, that the red precipitate is not easily visible and the endpoint is hard to spot. Thus it is necessary to use a lower concentration of chromate, and experiments show that a concentration of about 0.004 M is acceptable – the titrated solution is still yellowish, but the red colour is hard to miss. However, as the chromate concentration is lower than optimal, question is, how much excess silver do we have to add to the solution before we will be able to see the colour change. This excess will mean a positive error in the determination. 

If the concentration of chromate is 0.004 M, the concentration of Ag+ when the precipitation starts must be 

[Ag+] = KAg2CrO4 / [CrO42−] = 2.10−12/0.004 

That is 1.2 × 10−5 M more than the equivalence concentration. Assuming the final volume of the titrated solution is about 100 ml and the titrant concentration is about 0.1 M, that means we need to add 

0.100 litre × 1.2 × 10−5/0.1 M = 0.012 ml 

excess of the Ag+ solution. This is well below errors from all other sources – please remember, that 50 ml class A burette has an accuracy of 0.05 ml. In practice even smaller concentrations of chromate will still not change error much, besides, it is easy to check errors with a blind test. 

Note that in the discussion above we have assumed no protonation of CrO42−. That is not necessarily correct – pKa2 for chromic acid is 6.5, so even in a neutral solution substantial part of the acid is in the form of HCrO4. Luckily, even at these lower concentrations of CrO42− errors are so small we can safely ignore them, as long as pH does not differ much from neutral. 

However, due to the chromate protonation, the Mohr method will not work in the acidified solutions. That is where the Volhard method comes in handy. Again this method uses a specific indicator. Volhard method is based on the back titration – we add a known amount of Ag+ to the sample containing chlorides, once the chlorides precipitate we titrate excess Ag+ with thiocyanate solution in the presence of Fe3+. Once all Ag+ is precipitated excess SCN creates FeSCN2+ complex with a strong wine colour. 

Much more interesting is the case of Zn2+ titration with ferrocyanide: 

Zn2+ + Fe(CN)64− → Zn2Fe(CN)6 

To detect the endpoint we use a rather unexpected indicator – diphenylamine. Diphenylamine is a redox indicator. Before titration, we add to the solution some small amount of ferricyanide. Zinc ferricyanide is not so weakly soluble, so it does not interfere with the main precipitation reaction. However, its presence means we have a well-known redox system – Fe(CN)64−  /Fe(CN)63−  in the solution. As long as we are before the equivalence point, the concentration of ferrocyanide is very low and redox potential, given by the Nernst equation: 

E = 0.36   0.059 log [(FeCN)6−3 / (FeCN)6−4

is relatively high. Once we are past the equivalence point, the concentration of the ferrocyanide goes up, the potential goes down, and this difference can be easily detected thanks to the diphenylamine colour change. 

Adsorption indicators, mentioned earlier, are rarely used but are worth a few words. These are organic molecules, either basic (like rhodamine) or acidic (like fluorescein or eosin).  Depending on the titration stage (before or after the equivalence point) surface of the precipitate is slightly charged. For example, in the Mohr titration, we precipitate AgCl. AgCl has a very large surface due to its colloidal nature. Before the equivalence point, there is an excess of chlorides in the solution, and they tend to adsorb on the precipitate surface, charging it negative. After the equivalence point, there is an excess of Ag+ in the solution and the situation changes – the surface becomes charged positively. This charge on the surface attracts the organic indicator molecules. Here comes the most important part – these indicators have different colours when they are free in the solution and when they are adsorbed on the precipitate, which helps detect the endpoint. 

Titration curve 

According to the general guidelines, we will calculate concentration before the equivalence point assuming the titrant was a limiting reagent – thus the concentration of a titrated substance is that of unreacted excess. Usually, that is already the answer, however, sometimes, instead of calculating the concentration of a titrated substance, we may want to calculate the concentration of the titrant. To do so it is enough to put the concentration of excess titrated substance into solubility product and to solve for unknown concentration of titrant.  After the equivalence point situation reverses – if what we are looking for is a concentration of titrant, we simply calculate it from dilution of added titrant excess, if what we are looking for is a concentration of titrated substance – we put the concentration of excess titrant into solubility product and we solve for unknown. 

pAg in the 0.1 M AgNO3 solution titrated with 0.1 M HCl if 6 ml of titrant were added to the 20 ml sample. 

According to the reaction equation 

Ag+ + (aq) + Cl (aq) → AgCl (s) 

Silver ions react with chloride 1: 1. We started with 0.020 L × 0.1 M = 0.002 moles (2 mmoles) of silver, and added to it 0.006 L × 0.1 M = 0.0006 moles (0.6 mmole) of chlorides.  Thus, we are left with 2 − 0.6 = 1.4 mmole of silver in 24 ml of solution. 


Methods based on the precipitation of silver compounds are called collectively argentometric methods. They are most often used for the determination of chloride ions, but they can be used also for other halides (bromide, iodide) and some pseudohalides (thiocyanate). 

It is also possible to use indirect argentometric methods for the determination of anions, that create insoluble salts with Ag+ (for example, phosphate PO43−, arsenate AsO43− and chromate CrO42−). These methods are based on back titration of excess silver with a standardized thiocyanate solution. 

Solution used 

The two most important solutions used in argentometric methods are the solution of silver nitrate and the solution of potassium thiocyanate. 

Silver nitrate solution

 Silver nitrate solution of known concentration can be prepared using pure solid AgNO3, after drying it. A most popular solution is that of 0.1 M concentration, although for the determination of a small number of chlorides more diluted solutions can be used (0.02 M).  However, the use of diluted solutions should be preceded by a thorough analysis of possible titration errors. This is especially important in the case of Mohr titration, where some excess silver must be added before red silver dichromate precipitates and signals endpoint. 

Silver nitrate solutions slowly decompose when exposed to light, so they should be kept in dark bottles. 

Potassium thiocyanate solution 

Potassium thiocyanate is not used as a standard substance. Its solutions are prepared by dissolving solid KSCN and standardized against a solution of silver nitrate of known concentration. The usually used solution is 0.1 M.

Potassium Chromate Solution 

5% w/w potassium chromate solution is used as an indicator in titration with silver nitrate. 

Nitric acid 1 + 1 

About 40% solution of nitric acid, used for acidification of chloride solution in Volhard method. 1 1 means simply 1 volume of the acid and 1 volume of water. 

Ammonium Ferric Sulfate Solution 

10% w/w ammonium ferric sulfate (FeNH4(SO4)2, iron alum) is a convenient source of Fe3+ cations, used for the detection of excess thiocyanate. This solution is acidified with nitric acid to avoid iron (III) hydroxide precipitation. 

Standardization of solution 

0.1 M silver nitrate standardization against sodium chloride 

Silver nitrate solutions of known concentration can be prepared from a known mass of dried AgNO3. However, if we do not have access to the high purity reagent, or if we have a solution of unknown concentration, we can easily standardize it against sodium chloride. 

Reaction taking place during titration is 

AgNO3 + NaCl → AgCl ↓ + NaNO3 

Procedure to follow: 

  • Weigh exactly about 0.15-0.20 g of dried sodium chloride into a 250 ml Erlenmeyer flask.
  • Add about 100 ml of distilled water, and dissolve.
  • Add 1 ml 5% w/w potassium chromate solution.
  • Titrate with AgNO3 solution till the first colour changes. 

0.1 M potassium thiocyanate standardization against silver nitrate solution 

Potassium thiocyanate solution has to be standardized, as it is not possible to prepare and dry KSCN pure enough so that it can be used as a standard substance for solution preparation. The easiest method is standardization requires the standardized solution of silver nitrate. As KSCN solution is used for back titration of excess AgNO3 when we need to standardize KSCN solution we usually have standardized silver nitrate solution readily. 

Reaction taking place is 

AgNO3 + KSCN → AgSCN↓ + KNO3  

Procedure to follow: 

  • Pipette 25 ml aliquot of about 0.1 M AgNO3 solution into 250 ml Erlenmeyer flask.
  • Add 50 ml of distilled water.
  • Add 1 ml of 10% FeNH4(SO4)2 solution.
  • Titrate with potassium thiocyanate till the first visible colour changes.

Mohr’s Method 

Mohr’s method of determination of chlorides by titration with silver nitrate is one of the oldest titration methods still in use – it was researched and published by Karl Friedrich Mohr in 1856. 

The idea behind is very simple – chlorides are titrated with silver nitrate solution in the presence of chromate anions. An endpoint is signalled by the appearance of red silver chromate. 

The intense yellow colour of chromate may make detection of the first signs of the formation of red silver chromate precipitation difficult. As some excess silver must be added before precipitate starts to form, if the concentration of titrant is below 0.1 M, we may expect a significant positive error. To correct this error we can determine a blank, titrating a solution of the indicator potassium chromate with standard silver nitrate solution. To make the result more realistic we can add a small amount of chloride-free calcium carbonate to the solution to imitate the white silver precipitate. 

Solution during titration should be close to neutral. At low pH silver chromate solubility grows due to the protonation of chromate anions, in high pH silver starts to react with hydroxide anions, precipitating in the form of AgOH and Ag2O. Both processes interfere with the determination of accuracy. 

The same approach can be used for the determination of bromides. Other halides and pseudohalides, like I and SCN, behave very similarly in the solution, but their precipitate tends to adsorb chromate anions making endpoint detection difficult. 


Reaction taking place during titration is 

Ag+ + Cl → AgCl(s) 

Sample size 

Assuming 0.1 M titrant concentration and 50 ml burette, the aliquot taken for titration should contain about 0.12-0.16 g chloride anion (3.5-4.5 millimoles). 

Endpoint detection 

Before titration small amount of sodium or potassium chromate is added to the solution, making it slightly yellow. During titration, as long as chlorides are present, the concentration of Ag+ is too low for silver chromate formation. Near equivalence point concentration of silver cations rapidly grows, allowing precipitation of intensively red silver chromate which signals endpoint. 

Solutions used 

To perform titration we will need titrant – 0.1 M silver nitrate solution, indicator – potassium chromate solution, and some amount of distilled water to dilute the sample.


  • Pipette aliquot of chlorides solution into 250 ml Erlenmeyer flask.
  • Dilute with distilled water to about 100 ml.
  • Add 1 ml of 5% potassium chromate solution.
  • Titrate with silver nitrate solution till the first colour change. 

Result calculation 

According to the reaction equation 

Ag+ + Cl → AgCl 

Silver nitrate reacts with chloride anion on a 1: 1 basis. That makes calculation especially easy – when we calculate several moles of AgNO3 used it will be already several moles of Cl titrated.   

Volhard’s Method 

It is not always possible to use the Mohr method to determine the concentration of chlorides.  For example, the Mohr method requires a neutral solution, but in many cases solution has to be acidic, to prevent precipitation of metal hydroxides (like in the presence of Fe3+). In such cases, we can use the Volhard method, which is not sensitive to low pH. 

In the Volhard method chlorides are first precipitated with excess silver nitrate, then excess silver is titrated with potassium (or sodium) thiocyanate. To detect the endpoint we use Fe3+ cations, which easily react with the thiocyanate, creating distinct wine red complex. 

There is a problem though. Silver thiocyanate solubility is slightly lower than the solubility of silver chloride, and during titration, thiocyanate can replace chlorides in the existing precipitate: 

AgCl(s) + SCN → AgSCN(s) + Cl

To avoid problems we can filtrate precipitated AgCl before titration. However, there exists a much simpler and easier procedure that gives the same result. Before titration, we add some small volume of a heavy organic liquid that is not miscible with water (like nitrobenzene, chloroform or carbon tetrachloride). These liquids are better at wetting precipitate than water. Once the precipitate is covered with non-polar liquid, it is separated from the water and unable to dissolve. 

Precipitate solubility is not a problem during the determination of I and Br, as both AgBr and AgI have much lower solubilities than AgSCN. 


There are two reactions, as this is a back titration. First, we precipitate chlorides from the solution: 

Ag+ + Cl → AgCl(s) 

Then, during titration, the reaction taking place is: 

Ag+ + SCN → AgSCN(s)

Sample size 

In back titrations, the sample size is more difficult to calculate than during normal, direct titrations. For best accuracy excess silver should be titrated with about 40-45 ml of titrant (assuming – as we usually do – that we are using 50 ml burette). However, that usually means we should use the relatively large initial volume of silver solution. Assuming we will start with 50 ml of pipetted silver nitrate and we will titrate excess with about 25 ml of thiocyanate solution, and finally assuming both solutions used are 0.1 M, the aliquot taken for titration should contain about 0.09 g chloride anion (2.5 millimoles). 

Note, that silver nitrate can be added not using a single volume pipette but from the burette.  If the amount of chlorides is approximately known, this way it is possible to control the excess of silver nitrate and volume of the thiocyanate titrant. 

Endpoint detection 

An endpoint is detected with the use of iron (III) thiocyanate complex, which has a very distinct and strong wine colour. 

Solutions used 

To perform titration we will need 0.1 M silver nitrate solution to precipitate chlorides, titrant – 0.1 M potassium thiocyanate solution, nitric acid (1  + 1) to acidify solution, ammonium ferric sulfate solution that will be used for endpoint detection, nitrobenzene, and some amount of distilled water to dilute sample. 


  • Pipette aliquot of chlorides solution into 250 ml Erlenmeyer flask.
  • Add 5 ml of 1  + 1 nitric acid.
  • Dilute with distilled water to about 100 ml.
  • Add 50 ml of 0.1 M silver nitrate solution.
  • Add 3 ml of nitrobenzene.
  • Add 1 ml of iron alum solution.
  • Shake the content for about 1 minute to flocculate the precipitate.
  • Titrate with thiocyanate solution till the first colour change. 

Result calculation 

As in every back titration, to calculate the amount of substance we have to subtract the amount of titrated excess from the initial amount of reactant used. In the case of argentometry calculations are easy, as all substances are used to react on a 1: 1 basis. 

First, we have to calculate the number of moles of silver nitrate initially added to the chlorides sample. Assuming it was 50 ml of 0.1 M solution, it contained 5 millimoles of silver. Then, the excess was titrated according to the reaction equation: 

Ag+ + SCN → AgSCN(s) 

Thus the amount of excess silver is C × V, and the amount of Cl is 0.005 − C × V moles.

Make sure you also check our other amazing Article on : Non Aqueous Titration
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