KF titrator (coulometric) · Calibration procedure

Water standard recovery

Injects a certified liquid water standard and checks that the coulometer finds the certified amount of water.

Illustration of KF titrator (coulometric)

PURPOSE

Why this is checked

A coulometer needs no titre because the charge is an absolute measure. What can still go wrong is the cell chemistry, the endpoint, the generator electrode or the weighing. Recovering a certified water standard checks the whole system end to end.

Frequency

Typically daily or before each series of samples, after changing reagent and after maintenance, as defined in your SOP.

Learn how the KF titrator (coulometric) works

YOU WILL NEED

Standards & tools

Certified liquid water standard for coulometry, for example 1.0 mg/g (0.1%) or 0.1 mg/g, with certificate
Calibrated analytical balance (readability 0.1 mg or better)
Dry glass syringe with a long needle

BEFORE YOU START

Prerequisites

Fresh anolyte (and catholyte, if used) in the cell, with enough remaining capacity
Drift low and stable, for example below 10 µg/min
Standard ampoule at room temperature and opened just before use

PROCEDURE

Step by step

1Open the ampoule, rinse the syringe with a small amount of standard and discard it.
2Draw up about 1 mL of standard, wipe the needle and weigh the filled syringe.
3Inject the standard through the septum below the anolyte surface and start the determination.
4Reweigh the empty syringe and enter the injected mass (difference).
5Repeat to obtain three determinations from the same ampoule.
6Calculate the recovery of each and the mean.

RECORD

Example observation table

Injection	Mass injected (g)	Expected water (µg)	Water found (µg)	Recovery (%)
1	1.0123	1012.3	1004.6	99.24
2	0.9876	987.6	982.9	99.52
3	1.0045	1004.5	1001.2	99.67

CALCULATION

How to calculate the result

Recovery (%) = Water found (µg) ÷ [Mass injected (g) × Certified content (µg/g)] × 100

Certified content = 1.00 mg/g = 1000 µg/g
Injection 1: expected = 1.0123 × 1000 = 1012.3 µg
Recovery = 1004.6 ÷ 1012.3 × 100 = 99.24%
Mean of three = 99.48%; %RSD = 0.22%

ACCEPTANCE CRITERIA

Pass if

Mean recovery typically within 97.0–103.0% for a 1.0 mg/g standard (wider limits are often used for 0.1 mg/g standards), or the limits in your SOP.
%RSD of replicate recoveries within the SOP limit, for example not more than 2%.

IF IT FAILS

What to do

Check drift, septum and joints, then repeat with a fresh ampoule.
Check the syringe weighing technique and the balance.
Replace anolyte and catholyte and clean the generator electrode if recovery remains low.
Do not test samples until a valid recovery is obtained; assess samples tested since the last passing check.

Drift check

In LimsPulse · Calibration Management

LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.

See how