RHOSONICS
11/9/95
Application Note
SODIUM HYDROXIDE
Measurement of sodium hydroxide concentration with
Rhosonics Model 8100 using ultrasound
Industries:
• Chemicals production
• Detergents
• Pulp and paper
• Textiles
• Semiconductors
Instrumentation:
• Rhosonics Model 8100
• MP-130 series probes
Documentatie\Application notes\Naoh-app
Contents
1 Introduction
2 Discussion
3 Instrumentation
4 Conclusion
2
Application Note
1. Introduction
Sodium hydroxide or caustic soda, as it is also frequently called, is manufactured as an
electrolytic co-product of chlorine. It is used in several industries either in concentrated form
or in dilutions down to 400 ppm. In practice, the product can be measured with high accuracy
in a well-equipped laboratory using off-line density meters. When the product must be
measured inline with the same degree of accuracy, other methods are required. Inline sensor
systems, like conductivity sensors, refractive index meters or coriolis meters, simply lack
sensitivity for specific applications, or do not seem to respond anymore at larger
concentrations of NaOH. Chemical methods, like titration analyzers
In this application note, the Rhosonics Model 8100 Ultrasonic Inline analyzer is discussed,
which is ideally suited for the entire range of NaOH concentrations, and with an extremely
high accuracy.
2. Discussion
During the initial evaluation of this application, samples with concentrations of 0 to 50%
NaOH by weight were tested at temperatures between 20 and 40 °C.
Soon it became clear, that the change in sound speed of NaOH is extremely high and
predictable. In the range of 0 to 25 wt%, the change is almost 30 m/s per wt% concentration.
As the graph in figure 1 is showing,
the sound speed is continuously
increasing with concentration, and
Sound velocity vs. sodium hydroxide
will continue to do so until saturation
concentration.
has been reached.
NaOH app note Rhosonics 8100.DOC
2.75
2.50
2.25
t = 30° C
2.00
1.75
Concentration (% by weight)
50
40
30
20
10
1.50
0
Sound velocity (x1000 m/s)
3.00
Since the resolution of the
Rhosonics Model 8100 is ± 0.03
m/s, the resolution of the actual
concentration analysis in the very low
end of the range is ± 0.0002 wt%
NaOH. The temperature of the liquid
has a small influence on the analysis,
however the Model 8100
simultaneously measures the
temperature of the NaOH and
automatically compensates over the
full temperature range.
3
Parameter
Sound velocity
Temperature
NaOH 0 to 1%
NaOH 0 to 15%
NaOH 15 to 30%
NaOH 30 to 50%
Application Note
Accuracy
0.03 m/s
0.03 °C
0.003 %
0.005 %
0.01 %
0.02 to 0. 03 %
Taking all factors in consideration, the accuracy of the ultrasonic analysis is better than
0.005% in the range of 0 to 15%. In the range of 15 to 30% the Model 8100 is better than
0.01% NaOH. In terms of density the accuracy is as high as ± 0.00001 to 0.00002 kg/l.
Note that the accuracy figures as given above are based on a factory calibration and field
calibration after installation. This field calibration is a simple procedure, only needed after the
installation and start-up in the target location.
3. Instrumentation
For NaOH, the Model 8100 is well suited in combination with an MP-130 series probe.
The Model 8100 converts the
measured sound velocity and
temperature into a highly accurate and
reliable concentration signal. It is
possible to select a specific
measuring range of interest, and to
make a field adjustment in order to
meet the highest possible accuracy.
As standard, the MP-130 probe is
equipped with a DIN or an ANSI flange
(DN40 or ANSI -2”) and is fully made of
AISI-316 stainless steel. The probe is
optimized for installation in the actual
process line and is not sensitive to drift
or wear.
Figure 3. Rhosonics Probe Model MP-130. With
2”-150 LBS flange as standard, Ex-proof version.
NaOH app note Rhosonics 8100.DOC
4
Application Note
Alternatively, the MP-54 probe or the recently
introduced MP-48 may be chosen, as it is more costeffective for applications that do not call for explosionproof equipment. This probe is mounted inside a
special pre-fabricated pipe section, ensuring minimal
obstruction of the flow of the liquid.
For applications, demanding for the extraction of the
sample out of the product line, the MP-42 sensor with
cell is the most suitable solution. The cell is equipped
with Swagelok connectors for convenient integration is
the process line.
Since all probes are factory-calibrated, there is no
need for calibration on-site. All liquid-data are factory
stored and can be easily user-modified, without the
need for special skills or tools.
Figure 4. Rhosonics Probe Model MP-42.
With its 2” Tri-Clover fitting, it is ideally suited for fitting
in bypass-systems, samples systems and sanitary
piping.
4. Conclusion
The Rhosonics Model 8100 accurately measures the concentration of sodium hydroxide
IN-LINE. In addition, the analyzer is maintenance-free and able to measure the concentration
throughout a large range. Therefore, the Rhosonics analyzer is the ideal choice for sodium
hydroxide applications.
For more information, contact:
Stationsplein 62, NL-3743 KM Baarn
Phone: +31 35 543 1998 Fax: +31 35 543 1902
E-mail: [email protected] Website: www.rhosonics.nl
NaOH app note Rhosonics 8100.DOC