In our current digital age the nature around us is still analog. Very often we would like to know about the physical properties of nature. A big market, we call it CAEM – Chemical Analysis & Environmental Monitoring, has the challenge to transfer non electrical signals into the digital world.
pH measures the acidity or basicity of an aqueous solution. Solutions with pH less than 7 are said to be acidic and solutions with pH greater than 7 are basic in nature. The pH is all about the molar concentration of hydrogen and hydroxide ions in an aqueous solution. The most common ways to measure pH involve the use of a pH sensitive glass electrode, a reference electrode and a pH meter. Alternative methods also exist such as the use of Indicators or Colorimeters.
Applications:
Almost all processes containing water have a need for pH measurement. All human beings rely on internal mechanisms to maintain the pH level of their blood. Commodities such as wheat and corn, will grow best if the soil they are planted in is maintained at an optimal pH. In plating plants, pH control is used to ensure the luster of chrome on various products from nuts and bolts to toasters and automobile bumpers. The pH of wastewater leaving manufacturing plants and wastewater purification plants, as well as drinkable water from municipal drinking water plants, must be within a specific pH “window”.
Why is temperature compensation needed for accurate pH measurement results?
Glass electrodes used in the measurement of pH are sensitive to the hydrogen ion activity in a solution. Consequently, the concentration of hydrogen ion is not the only factor influencing the pH of a solution. The concentration of other chemicals in the solution, or the ionic strength of the solution, is also a major influence in the measurement of pH. Hence, the activity of the hydrogen ion will vary depending on the concentration of other ions in the substance as well as the ambient temperature as when excited (high temp) the activity is more than what it would be under normal room temperatures.
The general mathematical description of electrode behavior is given by the Nernst Equation:
E = E0 – (2.3RT /nF)log(a)
Where:
E = total potential (in millivolts) between two electrodes
E0 = standard potential of the ion
R = universal gas constant (8.31447 Joules/mol-Kelvin)
T = absolute temperature (in Kelvin)
n = charge of the ion(1, in case of hydrogen)
F = Faraday constant (96485.34 Coulombs/mol)
a= activity of the ion
2.3RT/nF is called the Nernst factor.
The changing of activity ‘a’ of about 10 represents a pH-value changing of 1.
The above equation can be modified for any temperature (since pH is defined as the negative algorithm of the hydrogen ion activity):
E = E0 + (1.98 x 10-4) T[K] pH
However the Nernst factor will change when temperature changes. The change in electrode output vs temperature is linear which can be compensated in the pH meter. The linear function for temperature vs pH change can be expressed as 0.003pH error/pH unit/°C
(1) CN0326 : a completely isolated low power pH sensor signal conditioner and digitizer with automatic temperature compensation for high accuracy (0.5%).
(2) CN0179 is a 4 mA-to-20 mA current loop transmitter for communication between a process control system and its actuator. Besides being cost effective, this circuit offers industry’s low power solution.
(3) CN0336 is a completely isolated 12-bit, 300kSPS data acquisition system utilizing only three active devices. The small footprint of the circuit makes this combination an industry-leading solution for 4 mA to 20 mA data acquisition systems where the accuracy, speed, cost, and size play a critical role.
SEM32 Board
The SEM32 Board has been developed by Semitron in cooperation with Microchip to address the market need of having a very flexible development board based on the low cost 32-bit PIC32MX controller family.
The data processing of the sensed/measured raw data is done by the 32-bit controller of the Microchip PIC32MX family. The controller has a “MIPS M4K“ core.
ADI has a range of solutions as follows:
(1) Sense & Display: SEM32+CN0326+PMOD OLED: Users can connect the Pmod CN0326 from Analog Devices and the Digilent Pmod OLED to the SEM32 board. To offer the possibility of connecting a second PMOD to the SEM32 board, a second Pmod connector has been soldered on to the Protoboard area.
The picture above shows a pH measurement with a pH sensor and PT1000 temperature sensor. Here we measure the pH value of a fluid with 4,01 pH.
(2) Sense-Transmit & Receive-Display
In this demo we are using two SEM32 boards – one to sense the pH/temperature and to transmit it and the other to receiving and display the results. The PIC32MX Controller reads in the pH and temperature value from Pmod CN0326.The next step is to perform the temperature compensation on the received pH value. Once compensated, the pH value is sent from PIC32 MX controller via the current loop transmitter Pmod CN0179. A PMOD form factor enables ease of connectivity with the processor interface.
The second SEM32 board receives the current using the Pmod CN0336. A current between 4 to 20mA represents the pH value between 0 and 10 pH. The Pmod OLED displays the pH value calculated by the PIC32MX controller.
Our Offer: pH/temp made easy kit
We also offer an all-in-one kit to our customers as a ready-to-go solution for their application prototyping needs. This allows the customer to repeat the above showed demos and realise out how easy can chemical analysis and current loop communication be made with parts from Analog Devices and Microchip.
The pH/Temp kit provides the customer a way to avoid the hassle of ordering different parts from different vendors and sites. All the customer is required to do is to open the box, put the things together and have a working solution ready to use.
About the Authors:
Vidushi Kshatri works as technical marketing engineer at Analog Devices. She comes with a background in Electronics & Communications Engineering and has worked in R&D for FPGA/DSP design in the past.
Franz Kraus works as FAE at Semitron W. Roeck. He comes with a background in General Electrical engineering and has worked as software and hardware developer (assembler, C, analog and digital technology) in the past.
