Automated water content determination with the 874 Oven Sample

Application Bulletin 280/3 e

Automated water content determination with the 874 OSP

Page 1 of

Application Bulletin 280/3 e

Automated water content determination with the 874 Oven

Sample Processor

Branch

All branches

Keywords

Titration; Karl Fischer titration; coulometric; gas extraction;

oven technique; automation; 874 Oven Sample Processor;

water standard; sodium tartrate

Summary

In principle the gas extraction or oven technique can be used

with all types of samples which release the contained water

when the samples are heated. The oven technique is

essential whenever direct Karl Fischer titration is impossible

because the sample contains interfering components or, due

to its consistency, is difficult to place in the titration vessel.

This Application Bulletin shows examples from food,

pharmaceutical, plastics and petrochemical industry to

describe automated water content determination using the

oven technique in combination with coulometric KF titration.

Instruments

874 Oven Sample Processor

KF Titrator (coulometric)

Electrodes

Double Pt wire electrode (for coulometry)

Generator electrode with or without diaphragm (for

coulometry)*

* For measurements of samples with absolute water contents

< 100 µg, the generator electrode with diaphragm is recommended.

Reagents

For the coulometric technique, special reagents suitable for

the oven technique are available.

Due to the constant gas flow during measurements, methanol

contained in the reagents evaporates. With the generator

electrode without diaphragm a decreasing methanol content

can lead to false high results, with recoveries of up to 110%.

The methanol loss should therefore be compensated

regularly to avoid too high results.

Coulometric reagents have a limited water capacity. The

capacity is equivalent to the amount of sulfur dioxide.

Decreasing sulfur dioxide concentrations (lower than half of

the initial concentration) lead to longer determination times.

The reagent should be completely replaced. The monitoring

of the reagent capacity and the exchange of the reagent can

be done by the titration system and the software.

Standards

Standards (with different water contents) suitable for the oven

technique are commercially available.

Carrier gas

The carrier gas transports the released water into the titration

vessel. In principle it is not important what gas is used.

However, due to oxidation reactions with oxygen, the thermal

stability of organic substances at higher temperatures is often

poor, when using dry air as carrier gas. Inert gases, usually

nitrogen, offer more flexibility and reliability.

For solids with a water content below 1% we recommend a

gas flow rate of 50 mL/min. Gas flow rates of up to 100

mL/min can be used for solids with higher water contents.

Higher flow rates do usually not lead to shorter determination

times.

Measurements in liquid samples (oil, fuel, solvents,…) should

be carried out using gas flow rates of 80 mL/min or higher.

Furthermore it is recommended that the inlet needle is

immersed in the sample and the gas is allowed to flow

through the sample.

General

The combination of the oven technique with coulometric Karl

Fischer titration is ideal for samples with low water content.

Foodstuff, pharmaceutical products, plastics or mineral oil

products can be analyzed fully automated and accurately.

In accordance with the gas extraction principle the water is

driven out of the heated sample by a stream of dry carrier gas

and transferred to the titration vessel, where the water

content is determined.

Application Bulletin 280/3 e

Automated water content determination with the 874 OSP

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Preparation

Sample

The sample is thoroughly mixed. Make sure the sample does

not change its water content during the mixing. The optimal

sample size depends on the water content of the sample.

Unfortunately there is no equation describing the relation

between water content and sample size. But there is a rule of

thumb: the higher the water content, the smaller the sample

size and vice versa. Sample weights that are too low have a

negative effect on the measuring accuracy (balance error).

The upper limit for the sample size is defined by the volume

of the sample vial. Theoretically the maximum sample weight

is also limited by the water capacity of the reagent. Usually

the capacity is approximately 1000 mg H

O per 100 mL

reagent (please contact reagent manufacturer for more

information). In other words, with one sample this capacity is

never used.

The absolute amount of water transferred to the titration

vessel is recommended to be in the range of 300 to 5000 µg.

If the absolute amount of water for a sample cannot be

reduced (e.g. smaller sample size …) also larger amounts of

water (> 5000 µg) can be determined using the oven

technique. Please be aware that in such cases the

determination times will increase.

The appropriate amounts of sample are weighed into the

sample vials and the vials are sealed with a septum cap.

Previous to usage, the vials and caps should be conditioned

for at least 24 hours at ambient air.

Table 2 on pages 5 and 6 shows recommended sample

weights for a choice of samples.

Instrument and software

The 874 Oven Sample Processor and the KF Titrator are

connected to a PC. The tiamo™ software is started. After the

instruments are recognized and therefore visible in the device

list, the work position for the conditioning vial, the tower

(sample positions) and the shift position are defined. The

tiamo™ software includes several preprogrammed methods

which can be loaded and used with an 874 system.

Depending on the KF Titrator, the methods need to be

adapted with the right instruments. For the analysis of sample

series, the following sequence of methods is recommended:

• systemprep

• blank value

• water content

The method “systemprep” is run once before a sample series.

This makes sure that the system of tubes is purged and ready

for the sample determinations. Afterwards the blank values of

empty sample vials are determined. Three replicas of the

blank value determination are recommended. Finally the

analysis of the water content of the sample takes place.

Normally a flow rate of the carrier gas (air, nitrogen or a

different inert gas) of 40 to 60 mL/min is sufficient. The flow

rate can be adjusted up to 150 mL/min to make sure that the

gas flow is high enough to transfer the released water as fast

as possible into the titration vessel. Generally the

determination time for liquid samples decreases if the gas

flow is increased. Avoid high gas flow rates when analyzing

solid samples which could swirl up.

The vials for conditioning, system preparation, determination

of the blank and the sample vials are placed on the rack of

the 874 Oven Sample Processor. For coulometric KF

titrations, the titration cell is filled with 150 mL of reagent and

then conditioned.

Temperature gradient

For samples whose temperature behavior is unknown, a so-

called temperature gradient is run (available temperature

range: 50 to 250 °C). The required method “temp gradient” is

a preprogrammed method included in the tiamo™ software.

This method uses a heating rate of 2 °C/min to heat up

samples from 50 to 250 °C. Figure 1 shows a theoretical

temperature gradient where the sample is heated from 50 to

250 °C in 100 min.

Fig. 1: Diagram showing a theoretical temperature gradient.

While a temperature gradient is being run it is possible to

record both, the amount of water released and the drift as a

function of time (see Fig. 2). The software tiamo™ offers the

possibility to correlate the amount of water released and the

drift with the oven temperature. This allows statements about

the kinetics of the water release as a function of the

temperature.

Application Bulletin 280/3 e

Automated water content determination with the 874 OSP

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Fig. 2: Temperature gradient (2 °C/min) of sodium tartrate

dihydrate showing the amount of released water and the

associated drift value as a function of the temperature

The temperature gradient of sodium tartrate (Fig. 2) shows

that the surface water and parts of the crystal water are

released starting at 50 °C up to approximately 120 °C. At

120 °C both, the amount of water which is released and the

drift increase again. Up to 140 °C the remaining water of

crystallization is released. After the water of crystallization

has been released, the drift decreases to its basic value of

approximately 10 µg/min. A sign of decomposition is the drift

value, which starts to increase at around 220 °C. Also the

color change of the sodium tartrate after running the

temperature gradient suggests decomposition (see figure 3).

Fig. 3: Sodium tartrate dihydrate before (left) and after (right)

running a temperature gradient.

The temperature curve can be used to determine the optimal

oven temperature for extracting the water from the sample.

This temperature should be high enough for the water to be

extracted completely without any decomposition of the

sample. The determination time should be kept as short as

possible. Therefore the oven temperature should be chosen

as high as possible, but approximately 20 °C below the start

of decomposition. This procedure works for many different

samples and can be used for a fast assessment of the oven

temperature.

However, for sodium tartrate dihydrate the manufacturer

recommends an oven temperature of 160 °C. According to

the temperature gradient in figure 2, an oven temperature of

200°C could be used. The reason for this discrepancy is the

fact that for some samples a heating rate of 2 °C is too high.

In such cases the heating rate may be reduced to find the

optimal oven temperature for the sample analysis. Figure 4

shows the temperature gradient of sodium tartrate dihydrate

using a low heating rate of 0.07 °C/min.

Fig. 4: Temperature gradient (0.07 °C/min) of sodium tartrate

dihydrate.

A closer look at the drift values between 170 and 220 °C

shows that the drift starts to increase already around 190 °C

(see figure 5). Therefore an oven temperature of 160 °C is

used for the analysis of sodium tartrate dihydrate.

Fig. 5: Temperature gradient (0.07 °C/min) of sodium tartrate

dihydrate between 170 and 220 °C.

Please note that the time to record a temperature gradient

increases if the heating rate is reduced. The temperature

gradient in figure 4 took more than 24 hours to finish.

Analysis

Each analysis consists of the following steps:

• Conditioning of the complete titration system (titration

cell and tubing)

Application Bulletin 280/3 e

Automated water content determination with the 874 OSP

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• Extraction of the water from the sample

• Transport of the water into the titration cell

• Karl Fischer titration

• Calculation of the result

Conditioning

Conditioning or titrating the titration cell to dryness is carried

out under stirring in the so-called conditioning position. This

conditioning step must be carried out before every

determination. The oven is heated up to the defined

temperature. After the temperature is reached and the

titration system is conditioned, the first measurement can

start.

Carrier gas dried with molecular sieve is used to transport the

released water into the titration cell, where the determination

of the water content takes place. By means of a double hollow

needle (figure 6), the gas is led through the sample vial. The

length of the needle can be adjusted depending on the

sample which is analyzed. Generally it is recommended to

use a short immersion depth of the needle for solid samples

(needle holder 6.2049.050). This prevents the needle from

being blocked by the sample. In addition swirling of powdery

samples can be avoided. For liquid samples the needle can

be immersed in the sample (needle holder 6.2049.040). In

this way the gas flows through the sample, which mixes the

sample and leads to faster extraction of the water.

Fig. 6: Principle of the oven technique in liquid samples

Water extraction

The dried carrier gas is passed through the sample vial and

transfers the released water into the titration cell. The

temperature of the oven can be varied according to the

temperature stability of the sample.

It is generally advisable to set an extraction time. During the

extraction time no endpoint is accepted, even if the endpoint

criteria are fulfilled. In this way, there is enough time for the

sample to heat up to the defined temperature. We

recommend a time of 2 to 5 min.

Karl Fischer titration

For a coulometric Karl Fischer titration a KF reagent which

contains iodide is used. During the determination the iodide

is oxidized to iodine at the anode of the generator electrode.

The iodine is used for the KF reaction. The coulometric

technique is an absolute method and there is no titer

determination required. The endpoint indication is bi-

voltametric using a double Pt-wire electrode (indicator

electrode) to which a constant alternating current is applied.

This creates a potential difference (voltage) between the two

platinum wires. If even very small amounts of free iodine are

present, the voltage drops suddenly; which indicates the

endpoint of the titration.

Titration sequence

The water content determined by the gas extraction with

subsequent KF titration is made up as follows:

water content

absolute

water content

sample

+ blank value + drift x determination time

During “conditioning” the needle is located in the conditioning

vial, water contained in the system, is removed until a

constant low drift in the range of 1 to 10 µg/min is achieved.

If the automatic drift correction is activated the drift value must

be stable. A stabilizing time can be defined to ensure a stable

drift value. For drift correction, the drift value measured at the

start of the determination is multiplied with the determination

time and subtracted from the found water content at the end

of the determination.

Alternatively the value for drift correction can be determined

using a separate method, which is carried out prior to every

sample series. In a first step, the system is conditioned until

a stable drift is achieved. Depending on the reagent this can

take between approximately 15 min and 2 hours. Then the

consumption of iodine is recorded during 10 min. The

determined mean value is stored and can be used for drift

correction and as stop criterion. It is essential that during the

sample series the system drift does not change (e.g.

interfering additives in sample).

Application Bulletin 280/3 e

Automated water content determination with the 874 OSP

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System preparation means that the whole system is adjusted

to the selected conditions. An empty sample vial is treated in

exactly the same way as the following samples, but the value

is not taken into account. We recommend that this step –

which is at the same time used for checking that the analysis

system is working properly – is carried out before every new

sample series.

Apart from the water in the sample, the sample vial also

contains atmospheric humidity; this makes a blank value

determination necessary. A three-fold determination of the

blank value is normally sufficient. The mean value is stored

as a Common Variable and taken into account in the

calculation of the water content (subtracted). If large sample

sizes are used, it might be necessary to correct the blank

value (see KF Application note AN-K-048).

The system preparation and the blank value determination

must be carried out using the same method parameters as

for the analysis of the samples. In order to do this, an empty

system preparation vial and three empty blank value vials are

placed on the rack of the 874 Oven Sample Processor and

analyzed before the samples.

End of titration

The titration and the gas extraction of the sample is stopped

as soon as the drift value (amount of water per time) falls

below a predefined value. Usually the parameter “relative

drift” is used to stop a determination. The stop drift is

calculated by adding the drift at the start of the determination

and the value entered for the “relative drift”. The endpoint is

reached if the actual drift is smaller than the sum of the two

mentioned values. The higher the chosen value for the

“relative drift” the sooner the determination is stopped and the

more water remains in the sample. The “relative drift” value

should not be higher than 5 µg/min. If high accuracy is

required and for small water contents the “relative drift”

should be reduced (e.g. to 2 µg/min).

Parameters

Table 1: Recommended parameters for the oven technique in

combination with coulometry.

Parameter Setting

General parameters I(pol) 10 µA

Generator current

400 mA or

auto*

Control parameters

EP at

50 mV

Dynamics

70 mV

Max. rate

Maximum

µg/min

Min. rate

15 µg/min

Stop criterion rel. drift

Relative stop drift 5 µg/min

Conditioning

parameters

Start drift 10 µg/min

Stabilizing time 60 s

* 400 mA are used with the generator electrode without diaphragm.

The setting auto is chosen for the generator electrode with

diaphragm.

Troubleshooting

Procedure for poor precision (reproducibility):

• Optimize the titration and control parameters.

• Check whether the sample vials are tightly sealed.

• Drift too high: Switch off the gas flow.

If drift value decreases: Check the needle system,

transfer tube.

If drift value does not change: check titration cell,

septum and/or seals leak, molecular sieve exhausted,

poorly conditioned reagent, ensure thorough mixing.

• Clean electrodes according to the electrode leaflet.

• Check the needle system and possibly clean it with

water and methanol. Dry the components afterwards.

• Check the transfer tube and clean it with water and

methanol to remove condensates and dirt.

• KF reagent contaminated/exhausted: change the

solution, possibly use a different batch.

• Check electrical contacts; defective contacts can lead

to an unstable measuring signal.

• Check if chosen oven temperature is suitable for

sample analysis.

• Balance: too inexact, drafts, temperature equilibrium

not reached, sample weight not optimal/too low.

• Possibly carry out a qualification of the analysis system.

Please contact Metrohm Service for further information.

Example applications

The following table 2 provides an overview of samples

analyzed by the oven technique. In each case the Karl

Fischer water determination was carried out in combination

with a Coulometer using N

as a carrier gas (flow rate

40 mL/min).

Application Bulletin 280/3 e

Automated water content determination with the 874 OSP

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Table 2: Selection of samples which can be analyzed using the

oven technique

Sample

Temp.

[°C]

Sample

weight [g]

Water content

Foodstuff

Lyophilizate 120 0.06 1.4%

Aromas

100

0.03–0.08

2.5-5.4%

Maltodextrin

100

0.03–0.08

4.7-8.8%

Lactose

monohydrate

155 0.06 5.2%

Skimmed milk

powder

90 0.06 4.4%

Whole milk

powder2

90 0.06 3.9%

Sweet whey

powder

90 0.06 4.9%

Glucose

monohydrate

90 0.06 8.9%

Maltose

monohydrate

120 0.06 5.9%

Garlic powder

110

0.2

3.5%

Mineral mixtures

110

0.3

6.0-7.0%

Plastics

Polypropylene

170

3.0

380 µg/g

Polyethylene 115 3.0 40 µg/g

Olefins 180 3.0 100 µg/g

Polyamide 180 0.3 0.8%

Polyoxy-

methylene

(POM)

170 0.3 0.1%

Polystyrene

120

0.05…0.2

200–500 µg/g

Refinery products

Transformer oil

150

3.0

80 µg/g

Mineral oil

120

1–3

10–100 µg/g

Insulating oil 140 3 5 µg/g

Crude oil 140 2

500-1200

µg/g

Additive 120 0.01…0.03 4.4%

Antimony

dialkyl-

thiocarbamate in

crude oil

50–130 3.0 700 µg/g

Pharmaceutical products

Collagens

160

0.07–0.4

10.6-9.2%

Denture cleaner,

effervescent

tablets

70 0.2–1.5 3.8%

Drugs 140 0.04 6.7%

Lyophilizate 150 0.01 5.0%

Others

Sodium tartrate

dihydrate

160 0.02–0.08 15.5%

Potassium

citrate

monohydrate

220 0.03 5.6%

Polyammonium

compounds

220 0.03–0.3 1.0-85.0%

Emulsified fat

compound

220 0.03–0.08 85.0%

Formamidosulfo

nic acid

220 0.2–0.3 < 1.0%

Pigment 100 0.03–0.3 7.9%

Polyol ether 150 0.6–1.3 0.2%

Dibutene

100–

140

0.03 250 µg/g

Lithium cobaltite

100

0.45–1.0

64 µg/g

Building rubble

Surface water

50–60

0.3

0.6%

Bound water

85–140

0.3

1.0%

With the 874 Oven Sample Processor the dissolution of the

sample in KF reagent, which is required in a direct Karl Fischer

titration and which frequently cannot be carried out completely , is

no longer necessary

With the addition of 4 mL methanol as extraction agent (the water

content of the methanol must be determined and included in the

blank value).

With the addition of 4 mL 1,5-pentandiol, extraction time 600 s

(the water content of the 1,5-pentandiol must also be determined

and included in the blank value).

Above 70 °C the contained carbonate decomposes.

Reference

• Metrohm Monograph 8.026.5013 - Water Determination

by Karl Fischer Titration

• AN-K-048 – Sample preparation with the oven

technique – relative blank (available on

www.metrohm.com)

Author

Competence Center Titration

Metrohm International Headquarters