Instrumentation - Acid Concentration
March 25, 2024

Introduction Conductivity Refraction Sound Velocity Recommended Methods Instruments

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Introduction

The on-line continuous measurement of sulphuric acid/oleum is based on the fact that a physical property will change as the concentration changes.  Properties such as conductivity, refractive index and sound velocity are typically used to measure sulphuric acid/oleum concentrations.  The recommended methods of measurement over the whole range of sulphuric acid and oleum are given below.

Conductivity

Electrical conductivity is a property of a fluid that can be used to determine its concentration.  The graph below shows the conductivity (micromhos) at 25ºC for several different fluids, including sulphuric acid from 0 to 100%wt.  The vertical axis ranges from 0 to 900,000 micromhos.

For sulphuric acid, the conductivity varies such that for a given conductivity there could be more than one concentration.  This makes conductivity unsuitable for measurement over the full range of acid concentration (i.e. 0 to 100% H2SO4).  However, conductivity can be used where the concentration will vary over a small range and where the range will not cross over one of the peaks or valleys of the conductivity curve.

Refraction

The refraction of light through sulphuric acid will depend on the temperature and concentration.  This property can be used to determine the concentration of sulphuric acid. 

Energy radiated from a LED passes through the prism surface to be reflected off a mirror to the prism-to-process interface.  The light reaching this interface intersects the same interface over a series of angles chosen to include the critical angle (Ic) for the process being measured.  Light intersecting the interface at an angle greater than the critical angle is refracted into the solution.  Light intersecting the interface at less than the critical angle is reflected up to a mirror and out of the prism up to the charge coupled device (COD) linear array to be scanned.

The image seen by the CCD is composed of simple light and dark periods.   As the refractive index of the process changes, the change in critical angle, changes the ratio of light to dark periods as seen by the CCD.  This time period change is further enhanced and displayed as a reading in refractive index.

Sound Velocity

The velocity of sound in sulphuric acid/oleum can be used to determine the acid concentration.  The first graph illustrates the variation of sound velocity with acid concentration (0 to 100% H2SO4) and second graph illustrates the same relationship except for oleum.

The nature of sound velocity in sulphuric acid makes this property ideal for determining acid concentrations in the range of 60 to 100% H2SO4.   Calibration of the instrument is done by comparison with acid of known concentration.

The velocity of sound in sulphuric acid/oleum is affected by the presence of bubbles and cleanliness of the acid.  The sample must be drawn off in such a manner as to avoid entrainment of bubbles and foreign matter which will affect the accuracy of the measurement.

Recommended Methods

The recommended methods of measurement over the whole range of sulphuric acid and oleum are given in the following table.

Instruments

Type	Range	Manufacturer	Model No.	Remarks
Conductivity and Sound Velocity	-	SensoTech GmbH	LiquiSonic	Accuracy: ± 0.05 wt% Brochure
Conductivity	0-25% H2SO4 35-75% H2SO4 95-100% H2SO4	Foxboro	Trans: 870EC-TF49F Sensor: 871EC-TF2-V Mtg Flg: 51191/EP129A (2")	Temperature compensation required
		Rosemount	Model 222 Toroidal Conductivity	Maximum Temperature: 182ºC Maximum Pressure: 125 psig Connection: 1" or 2" - 150 lb Flange Temperature compensation required
Density	0-90% H2SO4		DMA 5001	Benchtop density meter Accuracy: <0.01% w/w Can be combined with automatic filling device (Xsample 320) Link to Anton Paar
			DMA 35 Ampere	Handheld device for onsite measurement Accuracy: 0.5% w/w Link to Anton Paar
Refraction	-	The Electron-Machine Corp.	MPR E-Scan	Accuracy: ± 1% of span or 0.000075 Refractive Index (RI) whichever is greater Span: 0.0015 RI minimum, 0.132 RI maximum Temperature: up to 150ºC (300ºF) Mounting:
	-		PIOX R721	Range: nD: 1.3 ... 1.7, oBrix: 0 ... 100 Temperature: -20 to 130oC Pressure: PN10, PN16, on request PN40
	-		Products: Probe Process Refractometer PR-43-GP Compact Process Refractometer PR-43-GP, Saunders Body Refractometer PR-23-W Teflon Body Refractometer PR-23-M, Refinery Probe Process Refractometer PR-23-RP.	Range: Measurement range covers 0-100 % by weight (Refractive Index nD 1.3200 – 1.5300).   Remarks: Reliable and accurate product based on digital sensing technology; even large amounts of suspended solids, bubbles or color changes do not affect the measurement or the accuracy. The accuracy is ±0.1 % b.w. and the measurement is automatically temperature compensated. The technology allows configuring low and high concentration alarms. Intrinsically safe and hazardous area certification is provided for hazardous areas. Special wetted parts materials such as AISI 904L stainless steel, Alloy 20, Hastelloy C-276, Nickel 200 and Titanium ASTM B348. Process temperature range: -20°C to 150°C / -4°F to 302°F.
Sound Velocity	60-100% H2SO4	Nusonics	Model 86 Sonic Concentration Monitor Sensor: HSX/T	Accuracy: ± 0.1wt% for 90-100% H2SO4 Range: 500-3000 m/s Temperature: -15 to 185ºC Application Note
Sound Velocity and Density	0-100% H2SO4 0-65% free SO3		DSA 5000	Benchtop sound velocity and density meter Accuracy: 0.02% w/w H2SO4, 0.04-0.1% free SO3 Covers the whole sulfuric acid (0-100%) and Oleum range (0-65%) Can be combined with automatic filling device (Xsample 320) Link to Anton Paar
	0-110% H2SO4		L-Sonic 5100 L-Dens 7400 L-Com 5500	Accuracy up to ± 0.02 wt% One-stop shop: cover entire H2SO4 and Oleum range with one portfolio No reversal points for 90% - 110% H2SO4 Combination of sound velocity and density in one sensor Space and cost saving sensor integrated evaluation unit Special wetted parts such as Hastelloy, Tantalum, 24k Gold or Rhodium Customer specific solutions (e.g. adapters, individual sound fork lengths) Automated temperature compensated measurement In-house formula development for customer specific solutions