PH MEASUREMENTS WITH THE DrDAQ

PRECISION pH MEASUREMENT & RECORDING WITH THE DrDAQ


INTRODUCTION

The DrDAQ is equipped with a quality, high impedance BNC input port which accepts a standard pH electrode. The stability and accuracy of this hardware, coupled with the parameter calculation capability of Picolog, affords a pH measurement accuracy on par with dedicated, but very costly laboratory equipment.

The biggest difficulty with pH measurement depends on the standard probe voltage output variation with temperature. Picotech provides a basic temperature compensation scheme (PICO COMPENSATION) based on their own DD100 temperature probe. By connecting this probe to the EXT 1 input of the DrDAQ and by placing the probe in the sample to be measured along with the pH electrode, Picolog automatically reads the temperature of the sample and corrects the pH reading accordingly. This correction system is limited by the accuracy (100 Kohm NTC resistor) by the mass of the temperature probe and by the accuracy of the pH table written into the software.

In the range 10 to 30°C, tested temperature correction accuracy at pH < 7 is approx. +/- 0.06 pH units, but for pH >8 error can be as high as 0.12 pH units. This means that a sample with a pH of 10.18 could be read with a value of pH = 10.06 (actual measured error with a 10.01 standard buffer) as shown in the table of measurements below.



This series describes an alternative temperature compensation scheme (MATH COMPENSATION) which provides accuracies in the order of +/- 0.03 pH units in the temperature range 10°C to 30°C and can use any type of temperature sensor.

To understand the principle of operation of any low cost, BNC terminated, commonly available pH electrode, we must think of a battery cell electrode immersed in the medium to be measured. The more acidic the medium, the higher the voltage of the cell, conversely, the more basic the medium, the lower the voltage. As later described in detail, the pH value calculation is based on this voltage with an inverse relationship. Considering that the ions must travel trough a glass barrier, the cell current is infinitesimal and therefore the electrode output is at an extremely high impedance.

The electrode voltage output is a measure of H+ activity, which increases, all other conditions being equal, with specimen temperature. Therefore, without compensation, the electrode will output a higher voltage (lower pH, higher acidity) with an increase in temperature and vice-versa. No temperature compensation can be “perfect” (at any pH level and at any temperature) therefore we can have under-compensation: pH still decreases with temperature, or over-compensation: pH increases with increasing temperature. The standard reference temperature for all pH measurements and for commercially available pH buffers is + 25°C.

Considering that the electrode output (at +25°C) swings trough zero volts, between -0.414V (pH=14) and +0.414V (pH=0) the DrDAQ is capable of reading positive and negative minute input voltages (for example 5.92 mV, pH = 6.0 at 25°C) at a very high impedance in the order of 250 MOhm, while using just one + 5V supply, a remarkable design job for such a low cost package!

The mV versus pH curve is a straight line having the value of k = 59.16 mV/pH @ + 25°C. The pH is calculated from the electrode output voltage Ve with the following expression:

pH = 7- (Ve/k) = 7- (Ve/59.16) [1]

The coefficient k can be calculated at any temperature T (Celsius) with the following expression:

k = (0.1984 * T) + 54.2 [2]

The enclosed EXCEL file: “pH electrode temperature dependence” shows how [2] has been derived and allows calculation of actual electrode output voltage at any pH and at any temperature: just change any of the green pH values to the desired figure and the corresponding cell voltages will appear in the column below.

PRECISION pH MEASUREMENT & RECORDING WITH THE DrDAQ


PICO TYPE COMPENSATION

The simplest way to do the pH measurement is to plug the pH electrode into the BNC socket of the DrDAQ, calibrate with at least one buffer solution with a pH near the expected pH of the sample, select the pH input in Picolog and display/record the pH value. This simple procedure is accurate only if both buffer and sample solutions are at or near to 25°C: e.g. 23 to 27°C. During this measurement the EXT 1 input must not be used and it is best to place a 1 KOhm resistor between pins 1 & 2 and a 3.3 KOhm resistor between pins 2 & 3. The reason why is explained below.

With different temperatures of buffer or sample solutions Pico Type Compensation can be used within the accuracy limits described in the INTRODUCTION. As already mentioned, a DD100 NTC type temperature sensor must be plugged into EXT 1 and its body, immersed in the sample, senses the temperature to obtain automatic correction of the measured pH.

As soon as the pH electrode is plugged into the BNC input, Picolog establishes a special connection with EXT 1 and reads the temperature values measured by the DD100. Inside the DD100 we have a 100 KOhm NTC for temperature measurement between pins 1 & 2 and a fixed 330 KOhm resistor beween pins 2 & 3. Picolog reads the value of 330 KOhm and “understands” it must read the NTC temperature dependent resistance values and use them to correct the pH reading by means of an internal scaling table built into the software.

The DrDAQ exploits the internal 100 KOhm pull-up resistor (Pin 1 of EXT 1) to feed the NTC resistor inside the probe, then reads the voltage across the NTC. The open circuit voltage at Pin 1 is derived from the PC USB voltage and has a value of approx. 2.5V. Note that the DD100 does not use the +5.0V present on Pin 4 of EXT 1.

Data for the NTC probe is as follows:
TEMP.,°C..................Pin 1, V.....................NTC Resistance, Ohm

5............................1.822........................268,732
10...........................1.687........................207,503
15...........................1.543........................161,233
20...........................1.395........................126,244
25...........................1.246.........................99,362
30...........................1.100.........................78,571
35...........................0.965.........................62,866
40...........................0.840.........................50,602
45...........................0.725.........................25,217


Typical performance of this compensation method is illustrated in the enclosed screens & .plw files. Compensation accuracy is acceptable for acidic values of pH, but deteriorates above pH = 8.

PRECISION pH MEASUREMENT & RECORDING WITH THE DrDAQ

MATH TYPE COMPENSATION

With Math Compensation we can use any temperature sensor capable of an output voltage in the range 0 – 2.5 V into an impedance value of 1 kOhm or less (like the LM35 IC) connected to EXT 1. In a separate post construction & electrical details of low mass temperature probes (NTC or LM35 IC) will be given. The 1 KOhm resistor swamps to ground the voltage caused by the internal EXT 1 pull-up resistor. Of course no resistor is necessary if the DrDAQ Buffer module is used:(topic14829.html)

Supposing we are using an LM35 IC as the temperature sensor, The DrDAQ/Picolog must be set-up as follows.

1. Modify the plw.ini file. Find this file following the path: MY COMPUTER – SYSTEM (C) – DOCUMENTS & SETTINGS – YOUR USER NAME – LOCAL - APPLICATION DATA – PICO TECHNOLOGY. Here you find a 42XXXXXX-XXXX-XXXX-XXXXXXXXXXXX folder containing the plw.ini file as listed below and add: “[Options]" and "TempComp=0” (as in red below).

[Window]
Recorder=0,1,-1,-1,-1,-1,496,1009,3,265
[General]
FirstTime=No
[Preferences]
BeepOnReading=No
BeepOnLamp=No
BeepOnAlarm=No
[Converter]
Type=1000
Port=0
Selection
[Options]
TempComp=0

This modification removes the software connection between EXT 1 and the pH compensation.

2. Place a 2.2 KOhm resistor between pins 2 & 3 of EXT 1. This makes sure the dangerous DETECT pin does no mischief.
3. If EXT. 2 and EXT. 3 are not used, just terminate inputs with a 1 KOhm resistor between pins 1 & 2.
4. Place another 1 kOhm resistor between 1 & 2 of EXT 1 and connect the LM35: output to Pin 1, ground to Pin 2 & +5V to Pin 4. Or, if smaller mass is needed, remove the 1 kOhm from 1 & 2 & connect the NTC sensor and write the Scaling Table (see post #4 following).
5. Connect any standard pH electrode probe to the BNC pH input. The DrDAQ will read the pH value at the fixed reference temperature of 25°C, e.g. will use the standard expression:

pH = 7-(Ve/ 59.16).

Now that the hardware and the internal software are in working order, we must see how the MATH COMPENSATION works.

A) The voltage at EXT 1 (corrected by a scaling expression, as described further on) is the temperature T of the sample at that moment.
B) The figure given by P = pH/Redox is the raw, uncompensated pH value at whatever temperature the sample is at that moment.
C) By knowing T and using [2] (see previous post) we can calculate the value of parameter k. Example: suppose T = 31°C, k = (0.1984 * 31) + 54.2 = 60.35.
D) Now we can calculate the raw cell output voltage Ve at the pH/Redox input with the expression:

Ve = (7 – P) * k [3]
Example: suppose P = ph/Redox = 4.66,
Ve = (7 – 4.66) * 60.35 = 141.22 mV. This is the cell voltage for the unknown true pH at T = 31°C.

E) But we can also calculate the cell voltage Ve’ for the unknown true pH at 25°C:

V’e = (7 – P) * 59.16, V’e = (7 – 4.66) * 59.16 = 138.43 mV.

F) The voltage differential: Vd = Ve – V’e gives us the voltage error between the pH/Redox reading and the unknown true pH reading, but we must now note an extremely important point. At pH < 7, cell voltage is positive, at pH=7, cell voltage Ve = 0 and at pH > 7 cell voltage is negative. Therefore we must follow the rule:
- For pH < 7, Vd = Ve – V’e
- For pH > 7, Vd = V’e - Ve
In our example pH < 7, therefore Vd = 141.22 – 138.43 = 2.79 mV.

G) From the voltage differential we calculate the pH differential dpH, e.g. the pH error due to different temperature, to be corrected.
dpH = Vd/k [4]
In our example: dpH = 2.79/60.35 = 0.046 pH units. This is the error caused in the electrode output for a sample temperature at 31°C instead of 25°C.

H) Therefore the true pH = P + dpH = 4.66 + 0.046 = 4.706.

The enclosed file Math pHXX.plw implements the above theory for pH < 7. For pH > 7 the VOLTAGE DIFFERENTIAL calculated parameter must be inverted as described in F) above.

PRECISION pH MEASUREMENT & RECORDING WITH THE DrDAQ


The Picolog calculation described in the previous post can be verified with the "pH Electrode Temperature Dependence" Excel file uploaded in a previous post.

1) In the TEMP column D, in place of "30" write "31" °C.
2) In the mV@pH column G, in place of "4.00" write "4.706".
3) In column G, in correspondence to 31°C will appear the value: 138.44 mV, as calculated by the Picolog procedure.

BUILDING SENSORS FOR pH TEMPERATURE COMPENSATION

One of the advantages of the MATH COMPENSATION method is that all individual steps on the calculation of the TRUE pH can be monitored from the Picolog console. Another advantage is that different sensors apart from the DD100 can be used. In this post we shall examine a low mass NTC sensor and an LM35 IC sensor.

Keeping in mind that current flowing inside the measurement sample is minute and that impedance is in the order of hundreds of MegaOhms, not only the temperature sensor body must be non-metallic, but the insulation between its outside surface (in contact with the sample under measurement) and the internal components of the sensor must be as near infinity as possible, otherwise the presence of the temperature sensor will alter the pH measurement. The enclosed photos illustrate the physical construction of the NTC sensor. After the components are soldered together and to the interconnecting wires, they are covered with high quality, 300°C, gasket type silicone compound. When dry the sensor assembly is smeared with two component epoxy resin and placed inside a rigid silicone pipe having a length equal to that of the pH electrode, so that the two sensors can be tied together, if necessary. Only a short section of the silicone pipe is filled with epoxy, in order to keep the mass low, for fast response. When in use care must be taken not to fill the open end of the pipe with extraneous liquids. The two sides of the NTC are connected to Pin 1 and Pin 2 of EXT. 1. The 2.2 KOhm resistor is connected between pins 2 & 3.

The scaling relationship to convert the voltage across the NTC into temperature can be a scaling equation:

(3.1169*X*X) - (43.645*X) + 74.594 or a scaling table:

Vs……………......°C
1.8340………….. 5
1.6830………….. 10
1.5340………….. 15
1.3890………….. 20
1.2460………….. 25
1.1100………….. 30
0.9750………….. 35
0.8430………….. 40
0.7150………….. 45

The complete LM35 sensor is also shown in the enclosed photo. Here a slightly larger silicone pipe must be used, due to the size of this IC. Three wires are used: LM35 output to Pin 1, ground to Pin 2 and +5V to pin 4 of EXT 1. Additionally one 1 KOhm resistor between Pins 1& 2 and one 2.2 KOhm resistor between Pins 2&3 must be connected at EXT 1.

The scaling equation for the LM35 sensor is simply:

X * 100

Of course more accurate calibration of both temperature sensors can be easily accomplished by comparison with a mercury thermometer.

PH ELECTRODES - CARE & LIMITATIONS

Low cost pH electrodes such as the Picolog DD011 and others freely available on Internet require handling precautions and have basic limitations. Here is a limited list.

1. Testing them at 25°C ambient with a neutral (pH = 7) buffer, their output is not zero, but can be anywhere between -30 & +30 mV. Therefore calibration with a fresh pH = 7 buffer, plus calibration with a buffer having a pH near the expected sample pH is good practice.
2. The electrodes under consideration are filled with a gel electrolyte and therefore they normally have a useful lifetime of 12 months for reliable measurements. After they are best discarded.
3. The electrodes must be placed and stored in a vertical position as soon as they are unpacked. If they are accidentally placed upside down, they can develop air bubbles that cause unstable operation. This problem can be eliminated by shacking them like re-setting an old mercury thermometer.
4. Never let the electrode dry up. Always keep the tip immersed in the KCI solution, or in a pH = 4 buffer, at a storage temperature preferably never to go outside the range: 15 to 30°C.
5. Reliable measuring range is up to pH = 11. Above this figure the known problem of “Sodium Error” may produce erroneously lower readings.
6. The electrodes may be damaged by measuring samples containing sulphur. Measurement of sulphur compounds, if cannot be avoided, must be done as quickly as possible, immediately followed by through washing.
7. Carefully wash the electrode in distilled water after every measurement, wipe the outside housing, but do not touch and/or wipe the glass electrode.
8. Before doing a measurement, make sure that the pH electrode, the temperature sensor and the test sample are at the same temperature: this will avoid errors due to temperature shifts of the test components.
9. Always remember that, at the end of the day, even with the proposed Math Compensation and with all possible care, your measurement will have an uncertainty of +/- 0.03 pH units for compensation error, plus +/- 0.03 pH units for other factors, so realistically you will be looking at a total possible error of +/- 0.06 pH units.
Accuracy is always much better at low pH’s below 7 and deteriorates going into the high pH values.

ELECTRODE CALIBRATION

It is good practice to calibrate the electrode before doing pH measurements. For reliable calibration it is essential to do it with a stable ambient temperature of 25°C and to have all components, probes, buffers, etc. at the same temperature within +/- 2°C. Calibration is done without temperature compensation and the step by step procedure/example is as follows.

1. The EXT 1 input must not be used: place a 1 KOhm resistor between pins 1 & 2 and a 3.3 KOhm resistor between pins 2 & 3.
2. With DrDAQ connected, turn Picolog on, open the pH/Redox channel, short the pH/Redox BNC socket and check for a reading of pH = 7.00. A different reading indicates an internal calibration problem with the DrDAQ which must be corrected. You need two calibration buffers, one neutral and one near the wanted measurement range.
3. Neutral buffer solution: pH = 7.01.
4. Example: sample to be measured in the pH range 3 - 5. Use buffer solution 4.01.
5. Connect a temperature probe to EXT 2.
6. Remove electrode from storage solution, rinse in distilled water, shake dry & wipe external body. Do not touch glass electrode.
6. Connect the pH electrode to the BNC socket.
7. Place both electrode & temperature probe into the 7.01 buffer, let stabilize reading for 3' minimum and note reading. Example: pH = 6.78.
8. Wash & dry again electrode & temperature probe.
9.Place both electrode & temperature probe into the 4.01 buffer, let stabilize reading for 3' minimum and note reading. Example: pH = 3.92.
10. in Picolog open the pH/redox channel, go to "OPTIONS", select "TABLE SCALING" & write the following simple table:

3.92 4.01
6.78 7.01

11. System is now ready to take measurements with no compensation or with whichever compensation you have selected.