Calibration of Metrohm dosing and exchange units

Metrohm Dosing Test

02

Used every day – and always accurate ...

Everyday use

Your Metrohm dosing or exchange unit operates reliably

day in, day out, or even overnight if so configured. It al ­

ways dispenses an exact volume. The cylinder is filled

with titrant before the sequence starts again.

You base your decisions on the results obtained because

you know that you can rely on the values delivered by

your Metrohm titration system.

But are these values absolutely reliable?

Yes they are, provided that your Metrohm dosing or ex ­

change unit is new, or has been regularly serviced. Like

any instrument exposed to chemicals or which has mov­

ing parts, or both in the case of a Metrohm buret, it may

become less accurate over time.

Alcoholic KOH, for example, attacks the glass cylinder

and can thus alter its inside diameter. Intensive use of the

internal buret or just simply aging may also compromise

the performance of the measuring system.

The Metrohm Dosing Test

The Metrohm Dosing Test has been developed to deal

with this situation: your dosing or exchange unit is perio­

dically inspected to verify that its accuracy and precision

are still within the required tolerances.

Furthermore, the Metrohm Dosing Test is carried out by

specially trained Metrohm Service Engineers at your work­

place using recognized standards and procedures.

The Metrohm Dosing Test can be conducted during an ­

nual servicing of the instrument, either as a component of

a Metrohm Care Contract or «on demand» to mi nimize

disruption to your laboratory workflow.

03

... for very demanding requirements

by the corresponding quality certificate. This means you

can rely on the accuracy and precision of your piston

buret right from the start.

The Metrohm Dosing Test checks that these require­

ments are still being fulfilled in your daily operations and

confirms continuous compliance with the respective

standards.

Piston burets have very demanding requirements with

respect to accuracy and precision of the dosed volumes,

as defined in ISO standard 8655­3:2002 «Piston­operated

volumetric apparatus – Part 3: Piston burettes» and in

U.S. Pharmacopeia USP 35­NF 30, General Chapter <31>

«Volumetric Apparatus».

When they leave factory, Metrohm burets fulfill these

strin gent requirements without exception, as confirmed

Maximum permissible errors for motor-driven

piston burets according to ISO 8655-3:2002:

Nominal Maximum permissible errors

volume systematic random

mL ± µL ± µL

≤1 6.0 1.0

2 10 2.0

5 15 5.0

10 20 7.0

20 40 14

50 100 25

Tolerances for burets according to N.I.S.T. Class A

(ASTM E287-02):

Designated

10

volume («micro» 25 50

mL type)

Subdivisions, mL 0.02 0.1 0.1

Limit of error, mL 0.02 0.03 0.05

The Metrohm Dosing Test is based on a gravimetric

method. This means that the dispensed water is weighed

and the volume dosed is then calculated using the spe­

cific density of water. The calculated dosed volume is

sub sequently compared to the displayed volume.

04 The accuracy of the results depends not only on exact

determination of the mass, but on other parameters as

well. These are primarily the temperature of the water

and the ambient air as well as the barometric pressure.

This is why the Metrohm Service Engineers are equipped

with calibrated thermometers and barometers.

One test method for every requirement

Procedure according to ISO 8655-6

In the procedure specified in the standard «ISO 8655­

6:2002 Piston­operated volumetric apparatus – Part 6:

Gravimetric methods for the determination of measure­

ment error», three different volumes are used in the test:

• 10% of the nominal cylinder volume

• 50% of the nominal cylinder volume

• 100% of the nominal cylinder volume

Ten individual measurements are carried out for each of

these three volumes. The test liquid is water conforming

with grade 3 as specified in ISO 3696. The systematic and

the random measurement errors are calculated from the

measured values.

Metrohm Dosing Test, cumulative procedure

The general conditions of the Metrohm Dosing Test cor­

respond to those of standard ISO 8655­6:2002. Details

of the test conditions, the test procedure, and the calcula­

tions are given in Metrohm Application Bulletin 283/1 e.

In the cumulative procedure, the test volumes are dis­

pensed from one cylinder filling without any refilling. The

smallest test volume is exactly 10% of the cylinder vol­

ume, the largest is exactly 100%. The remaining 8 test

volumes are randomly selected from the range between

these two limits.

This procedure ensures that the entire volume range of

the buret is covered. The linearity of the dosed volume

can thus be reliably checked.

05Metrohm Dosing Test, repetitive procedure

In the repetitive procedure of the Metrohm Dosing Test,

one test volume is dispensed and then the cylinder is

refilled. Each dosing thus starts with a cylinder that is

completely full.

Customer-specific Dosing Test

The Customer­specific Dosing Test is carried out in accor­

dance with the procedure and general conditions speci­

fied in standard ISO 8655­6:2002.

However, customers can select the following parameters:

• The selected test volume can be any value between

10% and 100% of the nominal cylinder volume.

• The number of repeated measurements can vary

between 3 and 10, and this number can be varied

for the individual test volumes.

This enables compliance with customer­specific require­

ments, e.g. those specified in an SOP.

Choosing the appropriate test procedure Your advantages

Procedure according to ISO 8655­6 • Official procedure according to an internationally

recognized standard

• Based on ISO 8665­6:2002

Metrohm Dosing Test, cumulative procedure • Checks the entire volume range

• Quickest procedure

• Based on ISO 8665­6:2002

Metrohm Dosing Test, repetitive procedure • Checks the entire volume range

• Simulates practical conditions

• Complies with the requirements of your SOP

Customer­specific Dosing Test • The buret can be checked in the volume range you

need

06

Stay on the safe side

The Dosing Test gives you certainty – the certainty that

your Metrohm buret is dispensing the displayed value.

This in turn gives you assurance that you really can rely

on your results, and that this will not cause problems

during auditing.

Once the Metrohm Dosing Test has been successfully

completed, you will receive the corresponding certificate.

This certificate confirms clearly and traceable that your

buret continues to comply with the requirements as veri­

fied by the test, as well as the reference instruments used

in the test.

In short, you can focus on your work knowing that you

can rely 100 percent on the accuracy of your Metrohm

buret.

07

Good to know

The test includes inspection and maintenance of

your Metrohm buret

Every Metrohm Dosing Test includes inspection and

main tenance of your Metrohm buret.

The Metrohm Service Engineer checks the piston, cylin­

der, valve disc, connecting tubes, and other components

for any signs of damage or wear, and then cleans and

lubricates the parts as necessary.

If the buret no longer complies with the requirements,

the Metrohm Service Engineer can replace the corres­

pon ding components directly on­site – and then repeats

the Dosing Test.

Inspection and maintenance of your drive unit

(Dosino, Titrino, Titrando)

The exchange unit or the dosing unit and the buret drive

form a unit, and the complete system is checked during

the Dosing Test.

The drive unit itself is also a high­precision instrument,

re gardless whether it is a Dosino, a Titrino or even a

Titrando. The piston stroke of 50 mm or 80 mm is di vi ded

into 10,000, respectively, 20,000 increments.

Needless to say, this stroke is directly related to the dosed

volume. If the drive unit deviates from the spe cifi cations,

it directly compromises the result of the Dosing Test.

Inspection and maintenance of your drive is thus worth­

while for several reasons:

• No unpleasant surprises during the Dosing Test.

• Error­free function of the drive unit, not only during

the Dosing Test itself, but also during operation.

• Maximum flexibility in combining the drive unit with

an exchange unit resp. a dosing unit.

Provided that all of your exchange, dosing, and drive

units have been calibrated, any ex change or dosing unit

can be combined with any drive unit.

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