pH can be defined as a scale by which an acidic and alkaline solution can be distinguished. Acidic solutions have pH value less than 7 whereas alkaline solutions have pH value more than 7. The pH value of a neutral solution is 7. Mathematically, pH can be defined as the negative logarithm of the hydrogen ion concentration.
pH = -log10(H+)
A pH meter is a device used to measure the pH level a given solution. It was not a device commonly used for domestic purposes. However, the rise in number of people exposed to the concept of pH and its importance has increased its demand as a household tool. Apart from this, it is widely used for medical applications, research, agricultural and industrial purposes.
2. Basic Principles Involved
The basic components of a simple pH meter are
Ion-selective electrode (ISE): This is an electrode which is sensitive to the H+ ion concentration of a given solution.
Reference electrode: An electrode whose properties are known to us. The most commonly used reference is Ag/Ag Cl due to its non-toxicity.
Voltmeter: To measure the potential difference between the electrodes.
Figure 1 - Basic setup of a pH meter.
From: http://www.seafriends.org.nz/dda/ph.htm .
The ISE provides an electric potential which varies directly with the H+ ion concentration of the solution. The reference electrode provides a constant potential without responding to the ion concentration changes in the solution. The potential difference between the electrodes is measured by the voltmeter. Thus, the potential difference between the electrodes varies directly with the change in H+ ion concentration in the solution. It is related by the Nernst equation as follows.
E = E0 - 2.3 (RT/n F) log10 (H+)
Where
E = total potential (in mV) developed between the electrodes
E0 = asymmetry potential or standard potential of the electrode at H+ = 1mol /l
R = Gas constant
T = temperature °K
n = number of ions transferred
F = Faraday constant
H+= activity of the hydrogen ion in solution
The term 2.3(RT/n F) is known as the Nernst slope and its value becomes 0.591 at temperature 278 K and we have n=1 as we are considering H+ ions here. Thus, we reduce the above equation to
E-E0 = 0.0591 pH
Figure 2 - Slope of an ideal electrode at temperature 278 K
From: www.reagecon.com/techpapers/phfaqv4.pdf
For an ideal electrode, a change in potential of 59.1mV implies change in 1 unit of pH value. Also, the asymmetry potential will be 0mV for an ideal electrode at pH and 278 K. However, the existence of an ideal electrode is practically not possible and thus we calibrate the pH meter. When we calibrate the pH meter, the actual slope and asymmetry potential of the real electrode is found out and the pH meter readings are adjusted accordingly to deliver accurate results. These are the basic principles involved in a pH meter's working. An important factor that affects the Nernst slope is the temperature of operation. As the temperature varies, the asymmetry point also varies. To null this effect, we use an ATC (Automatic Temperature Compensator) before it is processed further.
The research department would find the following link useful. http://www.reagecon.com/techpapers/effectsoftemperatureonphv4.pdf . It provides an in-depth study of the effect of temperature on pH.
3. Digital pH meters
In digital pH meters the ISE, reference electrode and voltmeter are clubbed together into a single unit and it is known as a pH probe. The pH probe senses the change in ion concentration and converts it into electric potential as explained in the previous section. The output of the pH probe is then fed to the amplifier and further to the analog to digital converter which is then displayed.
Display
Amplifier and ADC
pH probe
Figure 3 - Flowchart showing the processing in a digital pH meter.
A simple pH meter circuit is shown below and will serve the purpose of understanding the concept. An operational amplifier with high input impedance for example TL081, TL082 or LMC6001 can be used to amplify the probe output. The high input impedance requirement comes in due to the high resistance offered from the pH probe.
Figure 4 - Circuit diagram of a simple digital pH meter
From: http://www.66pacific.com/ph/simplest_ph.aspx
4. Types Of pH meters
There are mainly 4 types of pH meters are available in the market. The following information has been taken from an article from http://www.articlesnatch.com/Article/A-Guide-On-How-To-Buy-Ph-Meter/1263937. It is a classification that is based on the application point of view. It was posted by a random user of the website. It is a brilliant classification and GoodSense would definitely find it useful.
The first one is the Pen or Tester type pH meter. As the name says, this type of pH meter has the shape similar to that of a pen. It is intended to serve domestic and industrial needs and is quick and easy to use. It is the basic form of pH meters available and thus does not have a wide variety of designs and functions to choose from.
Figure 5 - Pen type pH meter
From: http://www.oka.com.tw/Product-20097916355.html
The second one is the Portable type pH meter. This particular type is used mainly used for agricultural purposes. It is also used for domestic purposes but not as commonly as the tester type. It is preferred over the tester type pH meter due to its accuracy and wide variety of designs and features available to tackle different environmental situations.
Figure 6 - Portable type pH meter
From: https://www.instrumentchoice.com
The third one is the Benchtop type pH meter. This is used mainly for research purposes due to its ability to provide accurate outputs. It also has advanced calibration features and computer interfacing for data collection which makes it apt for laboratory usage. This type of pH meter is the most highly priced than all the other types of pH meters due to the extra features and increased cost of manufacturing.
Figure 7 - Bench top type pH meter
From: http://www.benmeadows.com
The last one is the In-Line type pH meter which is manufactured to serve industrial needs. They are made to hold up to rough environments. They are often used to check processes and to transfer pH reading to other devices.
Figure 8 - In-Line type pH meter
From: http://www.jencostore.com
5. Market Analysis
A thorough analysis of the recent demand in pH meters around the world has been done by the market research firm 'Frost and Sullivan'. They published the report on 19 Feb, 2010. It provides an in-depth analysis of the market behavior of pH meters. The cost of the report is US $ 6000. The report will be very useful for GoodSense to understand the market behavior of pH meters and to whether extend their contribution to the field of pH meters. Few pages of the report were spotted at the website http://www.reportbuyer.com from where some useful data was extracted for this report.
Figure 9 - Revenue from pH meter sale from 2005 to 2009 and the expected revenue from 2010 to 2013
From: http://www.reportbuyer.com
The revenue of various pH meter manufacturers around the world were taken into account while the report was made. There is a decrease in revenue in the year 2009 compared to 2007 and 2008 due to the economic position. But, it is expected to increase in the coming years as per the analysis. The water & waste treatment and pharmaceuticals & biotechnology fields have been the major application throughout. Cost plays a major role here and has been the factor that influenced the most of the customers.
Figure 10 - Revenue from various fields of application from 2002 to 2009
From: http://www.reportbuyer.com
The North American region and Europe has yielded a bulk amount from the sale of pH meters whereas Asian region and rest of the world account a less percentage. But the Asian pacific region is the target for most of the manufacturers as they growing drug and pharmaceutical industries and environmental research labs.
Figure 11 - Percentage revenue by geographic regions, 2009
Thermo Fischer Scientific had the major revenue share in 2009. They acquired it mainly from sale of benchtop and portable pH meters. Right behind them were Hach company and Nova Analytics.
Figure 12 - Revenue market share of leading pH meter manufacturers in percentage.
From: http://www.reportbuyer.com .
6. Product Of Prime Focus
As GoodSense is a company that is stepping to the field of pH meters, it will be better to start with a type of pH meter that is commonly used. It is important for the company that it gains public acceptance. Thus, a pen type or tester type pH meter would be a good idea as it used commonly for household purposes and thus helps in gaining popularity in an extremely competitive field. This type of pH meter is easy to manufacture and a research lab can be set up after sometime which is essential to compete with the pH meter manufacturing giants like 'Thermo Fischer Scientific'. GoodSense can then expand their expertise to the other type of pH meters in course of time.
The basic principle is same as explained earlier in section 3. As GoodSense already specializes in process instrumentation, the manufacture of a pH sensor will not be a trouble. The ATC is placed after pH sensor so that the variation in the final pH value due to change in temperature is nullified. The flow diagram below shows the steps involved in the processing of the analog signal from the pH probe.
Digital Display
Dual Slope ADC
Differential Amplifier
pH sensor output
Differential Amplifier
The output of the pH sensor is a signal that is difficult to be processed by the ADC. Thus the need for signal amplification becomes crucial. Hence, we employ a differential amplifier to serve the purpose. A1 and A2 are operational amplifiers with high input impedance which is used to handle the negligible current from the pH sensor. Operational amplifier A3 cancels out the solution potential that is added to the reference electrode output as well as the pH sensor output and produces a signal equal to the pH potential alone. The case is similar to cancelling out the noise using a differential amplifier. This process is advantageous in a way that the pH indicant can be placed at a considerable distance away from the electrodes and has less maintenance requirements. The final amplified pH signal is available at A4.
Figure 13 - Differential pH amplifier
From: Electronic Instrumentation by H.S. Kalsi, Courtesy - books.google.com
Dual Slope ADC
We have the amplified analog signal now which has to be converted to a digital signal in order to be displayed. A dual slope ADC is used for this. Although the dual slope ADC has the disadvantage of having a low input bandwidth and high conversion time, it is employed as it can reject high as well low frequencies. Also, the accuracy of measurement is not something that GoodSense would like to compromise.
Figure 14 - Dual Slope ADC
From: Lecture notes 6 - Dr. Mark Pitter, University of Nottingham
Operational Amplifier TL082
Figure 15 - Operational Amplifier TL082 pin diagram
From: http://www.national.com/ds/TL/TL082.pdf
The operational amplifier used has low supply current. Hence high input impedance is an essential. Therefore, we use the TL082 op-amp which satisfies the requirement and is commonly used in pH related circuits.
7. Cost Estimation
Components Used
Total Price per component(approx.)
pH sensor
250 GBP
Amplifier and ADC
150 GBP
Display
40 GBP
Others (casing, etc.)
50 GBP
Total
490 GBP
Number of units manufactured per year = 30000(approx.)
Cost of manufacturing product = 14700000 GBP
Overhead charges = 10000000 GBP
Total Cost = 24700000 GBP (approx.)
8. Time Calculation
Component Collection : 20 days
PCB Design : 15 days
Casing/Structure Design : 10 days
Assembling : 5 days
Check and shipping : 2 days
Total Number of days : 52 days
