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Volumetric Titer Determination and Performance Check

You are here:   Metrohm USA   >>   Support   >>   FAQs   >>   Titration - Volumetric


 

Introduction

Proper standardization of Karl Fischer volumetric reagent serves as a foundation for valid and analytically correct water content analysis. This is true for two reasons. First, it allows for the most accurate and precise performance from an autotitrator. Using an autotitrator is by far the safest and most accurate way to perform KF analysis. Second, it allows you to have assurances that the instrument is working properly before samples are run. If a titer determination followed by a performance check produce results that meet the specifications as documented below, you can be confident that the instrument is ready to generate reliable data.

Chemistry behind the analysis

To perform the test you do not need to know the details of the KF reaction. It is presented here only as a reference.

The ingredients that make up the KF reagent are as follows:

Methanol: CH3OH (or any suitable alcohol)
Sulfur dioxide: SO2
Organic base: RN (originally pyridine but now replaced by other bases, commonly imidazole)
Iodine: I2
Water (from sample): H2O

The first step of the reaction is the formation of a sulfite intermediate, which then reacts with water and iodine to give the final products:

CH3OH + RN + SO2 = [RNH]SO3CH3 ,
H2O + I2 + [RNH]SO3CH3 + 2RN = [RNH]SO4CH3 + [RNH]I .

The important lesson here is that for each unit of water that reacts, one unit of iodine is consumed, under the conditions of the reaction. By monitoring the amount of iodine used in the analysis, the amount of water can be quantified. All the ingredients listed must be present for the reaction to meet the stoichiometry outlined above. The alcohol is especially critical since it acts as a solvent as well as a reactant.

Instrumentation

  • Titrator: Metrohm 841 KF Titrando® with the 803 Titration Stand featuring the 800 Dosino® for “top down” dosing—this system is recommended with one of the following buret sizes: 5 ml, 10 ml, 20 ml or 50 ml.
A word about Titrando: Titrando is a modern titration system developed by Metrohm®. From years of troubleshooting KF titration problems, it has been determined that most problems are related to a few specific areas as defined below in “Common KF problems… .” Titrando has been specifically designed to monitor quality-critical parameters and to alert you before results are negatively affected. Titrando is a “top-of-the-line” instrument best suited for users who require maximum accuracy, precision and ruggedness.
  • Alternate titrator: Metrohm 758 KFD Titrino® with the 703 Titration Stand and the 806 Exchange Unit for “bottom up” piston dosing—this system is recommended with one of the following buret sizes: 5 ml, 10 ml, 20 ml or 50 ml.
A word about Titrando: Titrando is a modern titration system developed by Metrohm®. From years of troubleshooting KF titration problems, it has been determined that most problems are related to a few specific areas as defined below in “Common KF problems… .” Titrando has been specifically designed to monitor quality-critical parameters and to alert you before results are negatively affected. Titrando is a “top-of-the-line” instrument best suited for users who require maximum accuracy, precision and ruggedness.
  • Syringe: 5 ml–10 ml disposable syringe with a metal tip. A bottle with a bulb dropper or a transfer pipette will work well too.
  • Analytical balance: 4-place, with optional RS-232 or USB communication capabilities.

Optional accessories

  • Metrohm Tiamo™ titration software: Software to support most Metrohm titrators (both Titrando and Titrino). It provides storage for methods and results, and it facilitates communication with networks and LIMS systems.
  • Metrohm Sample Processors: Automation for KF analysis.
  • Metrohm 825 Lab Link: For connecting Titrando titrators to a network without using a computer.

Reagents

  • Hydranal® Composite 5 (Brinkmann Catalog No. 019100014)
    or
  • Hydranal Composite 2 (Brinkmann Catalog No. 019100065)
  • Laboratory grade or deionized water
  • Hydranal Standard 10.0 (10 mg water per gram of standard) (Brinkmann Catalog No. 019101061)

Titer determination

The KF reagent is not stable; it degrades on exposure to air, heat and light. The concentration or “titer value” must be checked frequently.

The standard used for titer determination is simply laboratory grade or deionized water. Titer is expressed as the “water equivalence,” i.e., the amount of water titrated by 1 ml of KF reagent.

All Metrohm Karl Fischer titrators have the basic methods for KF analysis built into their internal memory. The method for titer determination is listed as “H2Otiter.” This method calculates the result as mg H2O per ml of titrant dispensed. The result is stored and used with methods that report water content in samples.

Procedure

Preparing the cell

A Metrohm titrator will dry the cell and measure a drift after “Start” is pressed once. Drift is the rate of addition of KF reagent needed to keep the cell at the dry point; it is measured in microliter/minute. With a Metrohm autotitrator and a brand new or newly cleaned cell, a stable drift should be obtainable within 15 minutes. An acceptable stable drift level is 10 µl/min or less. If the autotitrator maintains this drift for 5 minutes, standardization may commence. These times will be much shorter once the KF cell has been in use.

Injecting the standard

  1. Place the syringe filled with the standard on the balance and tare. The balance should be stable with the following reading: 0.0000 g.
  2. Confirm that the drift on the autotitrator is appropriate—less than 10 µl/min.
  3. Press “Start” on the autotitrator. The display will change, prompting you to add the sample and enter the sample size.
  4. Remove the syringe from the balance and inject the standard into the cell.
    Note: The key here is to have water droplets fall directly into the solvent and not touch the walls of the vessel, the electrode or anything else. The syringe tip does not have to be beneath the solvent line, but this does ensure that the water does not end up on the electrode or vessel walls. Ideally, the syringe tip should be inserted through the septum to prevent atmospheric moisture from entering the cell. However, practical experience has proven that this is not absolutely necessary. If you choose to use the bulb dropper or transfer pipette, simply remove the septum assembly, drop the water in carefully and replace the assembly.
  5. After the water has been dispensed, place the syringe back on the balance to obtain an accurate differential sample weight. The weight will be negative, but the autotitrator takes this into account when calculating the titer. Either type the weight in or, preferably, have the balance connected directly to the autotitrator, and the weight can then be transferred directly. Once the weight is entered, the analysis will begin.

What sample size should I use?

Water is typically used to standardize Karl Fischer titrant. The sample size is a quality-critical parameter that is determined by two key factors: the theoretical concentration of the titrant and the buret size. Metrohm recommends that a range of 20%–90% of the buret volume should be dispensed for maximum accuracy and precision. Based on this fact, the following applies:

Sample size of water

Buret volume (ml) Suggested sample size (mg)
for titer = 2 mg/ml
Suggested sample size (mg)
for titer = 5 mg/ml
5 5 – 8 10 – 20
10 10 – 15 10 – 20
20 10 – 20 20 – 50
50 20 – 50 50 – 100

What is the acceptance criteria for a titer determination?

A titer determination as described above should be done 3–5 consecutive times with the statistics on the instrument turned on. The percent relative standard deviation (% RSD) should be used as the acceptance criteria. A % RSD of 1% or less is generally considered acceptable. When working with a titer value around 5.000 mg/ml, it is very easy. It becomes more challenging when working with 2.000 mg/ml. Ultimately, the acceptable % RSD should be developed through a statistical history of a specific titration system taking into account the environment in which it exists. Among other things, relative humidity, stability of the balance, expertise of the end user and quality of the reagents used all contribute to the % RSD.

Performance check

Once a successful titer determination has been done, the system should be further checked by running a standard with a known, certified value. The method to be used for this analysis is preloaded into all Metrohm instruments, and it is listed as “KF.” The “KF” method will report the result in % water. However, the instrument can be freely programmed to calculate results in any unit.

Procedure

Follow the procedure for “Preparing the cell” and “Injecting the standard” that is found above in the “Titer determination” section. Instead of using water for the titer determination, a certified water standard—such as Hydranal Standard 10.0—should be used.

What sample size should I use?

The following chart shows the proper sample size of Hydranal Standard 10.0 required to produce the maximum accuracy and precision:

Sample size of Hydranal Standard 10.0

Buret volume (ml) Suggested sample size (mg)
for titer = 2 mg/ml
Suggested sample size (mg)
for titer = 5 mg/ml
5 200 – 500 500 – 1,000
10 500 – 750 1,000 – 2,000
20 1,000 – 2,000 2,000 – 2,500
50 2,000 – 3,000 2,500 – 4,000

What is the acceptance criteria for a performance check?

The standard deviation on the Hydranal Standard certificate of analysis can be used as the acceptance criteria. For example, a Standard 10.0 with a certified value of 10.04 ± 0.01 should return a value of 10.03 to 10.05 during a performance check.

Common KF problems and their remedies

  1. Problem:
    The drift on the titrator (using a cell with fresh solvent) will not drop to less than 10 µl/min or cannot consistently maintain that level of stability.

    Remedy:
    The biggest challenge in KF titration is that water is everywhere! Particularly when humidity is high, a stable drift cannot be maintained unless the KF cell is in perfect working condition. Here is what to check when you have this problem:
    • Change the molecular sieve in the desiccant tube on the titration cell and on the titrant bottle. The sieves are what allow the cell to breathe, which is required, while keeping atmospheric water out. If the sieves are exhausted, atmospheric moisture will cause the drift to rise.
    • Be certain that all o-rings are in place and functioning. The electrode, desiccant tube, septum assembly and evacuation tubing all have o-rings underneath the screw nipples that hold them in place. Gently pull on each component to check the o-ring. For example, if you pull on the electrode and you feel resistance from the o-ring, it is in good shape. If you pull and you feel no resistance from the o-ring, either tighten the screw nipple or replace the o-ring. The electrode and desiccant tube can be tightened by hand, but the other smaller screw nipples must be tightened with the torque wrench provided (3 inches long, beige).
  2. Problem:
    The drift on the titrator (using a cell with fresh solvent) goes to 0 µl/min or a negative value.

    Remedy:

    In most cases, a negative drift indicates a dirty indicator electrode. Simply do the following.
  3. Soak the electrode in a solution of 1:1 concentrated nitric acid and water. An hour is usually sufficient, but if the electrode looks bad, soak it overnight. Before the electrode is put back into service, be certain of the following:

    • All acidic media is removed by rinsing with water
    • The electrode is rinsed with methanol and wiped clean
    • he poles on the indicator electrode are parallel
  4. Problem:
    The % RSD is above 1% after 3–5 consecutive titer determinations.

    Remedy:
    Successful standardization of a titrant is much like a “validation” of the system. If any aspect of the system is not functioning, it will show up in your titer values. This is precisely why every lab should develop acceptance criteria based on historical data. If something changes with time, it can be detected before samples are run. There are many things that can contribute to high % RSD. Here are the most common ones.
  5. Error due to expired reagent: KF reagent is unstable. As the reagent degrades, products of that degradation can interfere with the KF reaction. It is quality critical that the reagent being used is valid as per manufacturer’s specification.

    Error due to electrode: Metrohm makes high-quality electrodes that are designed to be rugged. However, with any titration system, electrodes play a major role in accurate and precise results. To be certain that the electrode is in good working condition, perform an acid wash regularly. The frequency of cleaning is dependent on the type of samples run. Acid wash: Soak platinum tips in 1:1 concentrated nitric acid and water for one hour and rinse with water, then methanol and wipe clean.

    Error in sampling due to balance: Without question, it is quality critical to have a balance directly next to the titrator. Even if it is on the next bench, carrying the syringe over any distance causes errors.

    Error in sampling due to user: KF analysis is technique-dependent without question. However, anybody can master it! If poor results are obtained, try again paying close attention to technique. Every new analyst must spend ample time getting familiar with the system. Automated systems with Metrohm Sample Processors can limit user intervention and, therefore, reduce errors in sampling due to user.

  6. Problem:
    I have tried everything and I still need help!

    Remedy:
    The people at Brinkmann Analytical Systems in partnership with Metrohm are dedicated to providing you with outstanding technical support and applications development. If you need special attention, please contact our Applications Laboratory or your local Analytical Systems sales representative. Everyone at Analytical Systems is well trained in titration and is happy to help you.

 

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