General Information
I-5
Thermo Scientific Orion 2120XP Calcium Hardness Analyzer User Guide
Principles of
Calibration
Calibration procedures for analytical instruments are important and must
be performed carefully. The calibration procedure used in the Thermo
Scientific Orion 2120XP is a variation of Double Known Addition (DKA)
using advanced electrode and flow cell technology in combination with the
passive diffusion system. This method has the distinct advantages of being
fast, easy, and accurate.
Before calibration begins, the diverter valve is pushed in to divert flow from
the measure drain, allowing the flow cell to fill.
At the beginning of the DKA calibration the actual concentration in the
sample is unknown. The analyzer measures the potential (E
s
) and stores
this value in the microprocessor. A known amount of standard 1 solution
is added to the flow cell, which increases the concentration (C
s
) with a
corresponding known amount (dC
1
). During this process, air is pumped
into the flow cell, thoroughly mixing sample and standard in a closed-
loop system. The new potential (E
1
) is measured and stored automatically
when stability is reached. Adding standard 2, preferably 10 times more
concentrated than standard 1, increases the concentration (dC
2
) in the
sample reservoir. Again, the new potential (E
2
) is measured and stored
when the reading is stable. Now, we have the following three unknowns:
E
s
=E
o
+ S(T
s
/298.15) log (C
s
/C
iso
)
E
1
=E
o
+ S(T
1
/298.15) log [(C
s
+ dC
1
)/C
iso
]
E
2
=E
o
+ S(T
2
/298.15) log [(C
s
+ dC
1
+ dC
2
)/C
iso
]
S is the Slope at 25 °C (298.15 K)
T is the temperature in Kelvin, measured when the potential E is measured.
E
s
, E
1
, E
2
have been determined during the calibration procedure. The
microprocessor solves these three equations, to obtain the values of S
and E
o
. The calibration result is stored for use during online monitoring
to convert the measured potential and temperature in the sample into
concentration values in either ppm or ppb.
When the calibration is complete the flow cell drains as the sample flow
returns. The flow cell volume returns to the measurement level. After
allowing approximately 30 minutes for concentrated calibration solution
to be flushed from the system, the 2120XP analyzer can begin sample
measurement again.
In addition to Double-Known Addition (DKA), the 2120XP analyzer also
allows the operator the ability to perform an offline calibration.
Double Known
Addition (DKA)
Figure I-2
Flow Cell Volume for DKA
