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If you perform titrations on a regular basis, then you’ve certainly heard about standardization of the titrant. When carrying out a standardization you determine the titer, which is a correction factor for your titrant concentration, as it is normally not exactly the value written on the reagent bottle label. In this blog entry, I want to give you some valuable information about why standardization is important and how to determine the titer.
Please note this blog entry will not deal with the standardization of Karl Fischer titrants. Read the following article for KF titrants.
Titer determination in Karl Fischer titration
Click on one of the following topics to jump directly to that section:
What is the titer factor?
Titration is an absolute method (or primary method), meaning it is of utmost importance to know the exact concentration of the titrant you are using for your results to be accurate and repeatable by other analysts. This is why you need to carry out a standardization.
Usually the difference between the nominal concentration (e.g., 0.1 mol/L) and the absolute concentration (e.g., 0.0998 mol/L) is given by a dimensionless factor (e.g., 0.0998). The absolute concentration is obtained by multiplying the nominal concentration with this factor, which is usually called «titer». In some cases, it is the absolute concentration which is called «titer».
Over the following sections I will discuss the essentials of standardization, regardless if you use the word «titer» for the correction factor or for the absolute concentration.
Why should you standardize your titrant?
Knowing the exact titrant concentration is important for correct titration results. This is especially true for self-made titrants, but this is also an important step for commercially available titrants. Titrants can age over time, and thus their concentrations will change.
For example: alkaline titrants, such as NaOH or KOH, will absorb CO2 from ambient air, or iodine-rich solutions will release iodine. Therefore, standardization will give you more security to obtain the correct results for your titrations.
What can I do to prevent changes to the titer factor?
This depends on which titrant you use for the analysis. The easiest thing to consider is the bottle you plan to store your titrant inside. Some titrants are light-sensitive, and should be stored in dark brown or opaque glass bottles. Others may react with glass, and are best stored in plastic bottles.
Titrants best stored in brown glass bottles:
- Iodine (I2)
- Potassium permanganate (KMnO4)
- Silver nitrate (AgNO3)
Titrants best stored in plastic bottles:
- Aqueous bases (e.g., NaOH, KOH)
- Non-aqueous bases (e.g., TBAOH)
Another preventive measure is the use of absorber or adsorber material filled into a tube which is connected to the ventilation part of your buret. This is especially important for titrants which react with CO2 or water from the air.
Use soda lime to absorb CO2 and a molecular sieve for moisture. Even if your titrant is not sensitive, it is still recommended to fill the tube with cotton, which will prevent the entry of dust into the bottle.
The image here shows an example of an absorber tube filled with soda lime attached to a buret for NaOH. This will avoid the solution losing strength due to carbon dioxide in the ambient air.
Titrants for which soda lime for CO2 absorption should be used:
- Aqueous and non-aqueous bases (e.g., NaOH, KOH, TBAOH)
- Sodium thiosulfate (Na2S2O3)
Titrants for which molecular sieve for moisture adsorption should be used:
- Perchloric acid (HClO4) in glacial acetic acid
How often should I standardize my titrant?
This question cannot be answered with a general number. Frequency of titrant standardization depends on multiple factors, such as titrant stability, the number of titrations per day/week/month, and the required accuracy for your results.
You should always carry out a standardization when you open a titrant bottle for the first time.
The following table is a guideline which should help you to select the frequency for standardizing your titrants. If you are unsure about the stability of your titrant, carry out frequent standardizations (e.g., daily) over a longer period of time until you are able to establish a standardization frequency based on your obtained titer data. The obtained data will show you how much your titer changes over time, and you can then select a suitable determination frequency. Newer software offers the possibility of monitoring your titer. This will help you as well during this task.
| Standardization frequency per titrant stability type | Daily | 2-3x per week | 1x per week | less than 1x per week |
|---|---|---|---|---|
| Stable (e.g., HCl, EDTA) | Weekly and fresh bottle | Weekly | Weekly | Before every use |
| Unstable (e.g., NaOH, I2) | Daily and fresh bottle | Before every use | Before every use | Before every use |
Stable titrants:
- Aqueous acids (e.g., HCl, H2SO4)
- EDTA
- Silver nitrate (AgNO3)
- Sodium thiosulfate (Na2S2O3)
- Cationic and anionic surfactants
Unstable titrants:
- Aqueous and non-aqueous bases (e.g., NaOH, KOH, TBAOH)
- Non-aqueous acids (e.g., HClO4)
- Iodine (I2)
- Potassium permanganate (KMnO4)
How to determine the titer
The titer is determined using a primary standard or an already standardized titrant. In either case, be sure to carry out the standardization at the same temperature as the sample titration, as the temperature influences the density of the titrant. Titrants expand at higher temperatures, and thus their titer factor decreases.
Describing the titer determination for every titrant would be beyond the scope of this blog. I will therefore only describe the titer determination procedure here for both cases – using a primary standard or an already standardized titrant – in a general way. If you want to know more about which primary standard is recommended for which titrant, then check out our corresponding Application Bulletin.
If you are using a primary standard, dry it at a suitable temperature for a few hours. Allow it to cool down in a desiccator until the substance reaches room temperature, then weigh out an appropriate amount of dried standard for the titration. The weight of the standard depends on the titrant concentration and on the buret volume. I recommend a standard weight which leads to an equivalence point at approximately 50% of the buret volume. If your weight is less than 100 mg, I recommend preparing a standard solution with your primary standard, as otherwise the weighing error becomes too large.
After you have weighed out your standard or pipetted your standard solution into a beaker, add enough diluent (solvent or water) to immerse the measuring and reference part of the sensor, and start the titration.
If you are using an already standardized titrant, the procedure is a bit simpler. Don’t forget: this titrant should be freshly standardized with a primary standard. Accurately pipette an appropriate amount of standardized titrant into a titration beaker. Add enough diluent (solvent or water) to immerse the measuring and reference part of the sensor, and start the titration.
Shifting gears: What are primary standards?
Primary standards fulfill several criteria which makes them ideal for the standardization of titrants. Primary standards are of:
- high purity and stability
- low hygroscopy (to minimize weight changes)
- high molecular weight (to minimize weighing errors)
Additionally, they are traceable to standard reference materials (e.g., NIST traceable).
How to calculate the titer factor
After you’ve finished the titrations for the standardization, now it’s time to calculate the titer factor. Again, the formula for the calculation differs slightly depending on whether you have used a solid, dry primary standard or a standard solution / standardized titrant.
For a solid, dry primary standard use the following formula:
mSTD: Weight of primary standard in mg
p: Purity of the primary standard in %
100: Conversion factor for purity
MSTD: Molecular weight of primary standard in g/mol
VEP: Volume at the equivalence point in mL
cTitrant: Nominal titrant concentration in mol/L
s: Stoichiometric factor
For a standard solution / standardized titrant use the following formula:
VSTD: Volume of standard solution / standardized titrant in mL
cSTD: Absolute concentration of standard solution / standardized titrant in mol/L
VEP: Volume at the equivalence point in mL
cTitrant: Nominal titrant concentration in mol/L
s: Stoichiometric factor
Modern titrators are capable of automatically calculating the titer factor and saving the result together with other relevant titrant data such as concentration and sample name, further improving the data security in your lab.
To summarize:
Standardization of the titrant is not so difficult, just keep in mind to:
- Carry out standardization regularly — even for ready-made titrants to improve result accuracy of your results.
- Use dry primary standards or freshly standardized titrants.
- Carry out the standardization at the same temperature as the sample titration.
If you want to learn more about how you can improve your titration, have a look at our article below where you can find several practical tips.
Your knowledge take-aways
Monograph Practical aspects of modern titration
Introduction; sample preparation; Basic principles of titrimetric analysis; Performing titrations Authors: Dr. W. Richter; Dr. U. Tinner
