Data Logger

ACID-BASE TITRATIONS

DATA LOGGER

1.0              Introduction to data loggers

The data logger is an invaluable tool to collect and analyze experimental data, having the ability to clearly present real time analysis with sensors and probes able to respond to parameters that are beyond the normal range available from the most traditional equipment. The differences between various data loggers are based on the way that data is recorded and stored.

2.0       Definitions of Data Loggers

The data logger is an electronic device that automatically records, scans and retrieves the data with high speed and greater efficiency during a test or measurement, at any part of the plant with time. The type of information recorded is determined by the user for example, whether temperature, relative humidity, light intensity, voltage, pressure or shock is to be recorded, therefore it can automatically measure electrical output from any type of transducer and log the value. A data logger works with sensors to convert physical phenomena and stimuli into electronic signals such as voltage or current. These electronic signals are then converted into binary data. The binary data is then easily analyzed by software and stored on memory for post process analysis.

3.0       Characteristics of Data Loggers

Data loggers possess the following characteristics:

1.      Modularity: Data loggers can be expanded simply and efficiently whenever required, without any interruption to the working system.

2.      Reliability and Ruggedness: They are designed to operate continuously without interruption even in the worst industrial environments.

3.      Accuracy: The specified accuracy is maintained throughout the period of use.

4.      Management Tool: They provide simple data acquisition, and present the results in handy form.

5.      Easy to use: These communicate with operators in a logical manner, are simple in concept, and therefore easy to understand, operate and expand.

3.1       Operation of data logger:

The ability to take sensor measurements and store the data for future use is, by

definition, a characteristic of a data logger. However, a data-logging application rarely

requires only data acquisition and storage. Inevitably, the ability to analyze and present

the data to determine results and make decisions based on the logged data is needed. A

complete data-logging application typically requires most of the elements stated below:

1.      Acquire

2.      Online analysis

3.      Log

4.      Display

5.      Offline analysis

1.)    Acquire – This step includes your sensors and data logger hardware as well as conversion of physical phenomena into digital signals.

2.)    Online analysis – This step includes any analysis that is likely to be done before

storing the data. A common example of this is converting the voltage measurement to meaningful scientific units, such as degree Celsius. These complex calculations and data compression are completed before logging the data. Every data logging software application should complete this conversion from binary value of voltage and the conversion from voltage to scientific units.

3.)    Log – This step refers to the storage of analyzing data including any formatting

required for the data files.

4.)    Offline Analysis - This step includes any analysis that is to be done after storing the

data. A common example looks for trends in historical data or data reduction.

5.)    Displaying, reporting - This step includes the creation of any reports that are needed to make to present data and displaying the data. However, this can also present data straight from online analysis. This represents the ability to monitor and view the data as acquired and analyzed in addition to simply viewing historical data. As an example, it should have the following components:

·         Hardware to digitize what is to be logged including sensors, signal conditioning, and analog-to-digital conversion hardware.

·         Long-term data storage.

·         Data-logging software for data acquisition, analysis, and presentation.

4.0       Advantages of Data Loggers:

1.) Data Loggers don’t interfere with the users in performing their tasks [6].

2.) They can operate independently of a computer and they are available in various

     shapes and sizes.

3.) The range of data loggers varies from simple channel inputs to multichannel

     devices.

4.1       Applications of Data Loggers:

They can be used in the following applications such as:

1)                            In an unattended recording of weather stations to record parameters like temperature, wind speed / direction, solar radiation and relative humidity.

2)                            For hydrographic recording of water flow, water pH, water conductivity, water level and water depth.

3)                            In the recording of soil moisture levels.

4)                            To record gas pressure and to monitor tank levels.

5)                            During transportation monitoring, troubleshooting, educational science, quality studies, field studies and general research.

6)                            Remote collection of recorded data and alarming or unusual parameters are possible with the help of data loggers where these are connected to modems and cellular phones.

 5.0       Data Logger in Experiment

EXPERIMENT: ACID BASE TITRATION

ENGAGE

Problem Statement: 

How do acid base interacts?

Introduction:

This experiment aims to generate the titration curves of some typical Acid-Base neutralization reactions. The presented simple setup produces titration curves, which are almost identical with those presented in textbooks of analytical chemistry.

A titration curve is a plot showing the changes of pH of the titrated solution versus the volume of the added standard solution (titrant). Acid-Base titration curves can be constructed in several ways. One way is manual recording and plotting of pH values after each manual addition of an aliquot from the titrant solution. Another way is automatic recording and plotting of pH values continuously during automatic addition of the titrant. The last approach is the principle of operation of expensive Automatic Titration equipment. DrDAQ data logger connected to A PC with PicoLog data logging software allows the automatic recording and plotting of pH values. Continuous addition of the titrant solution can be realized by a peristaltic or syringe-type pump, which pumps the solution at a predetermined and fixed rate. A much cheaper alternative is to use an air pump (like that used in a home aquarium). The objectives of this experiment are manifold:

1. To construct acid-base titration curves in a very similar way to that offered by automatic titrators.
2. To learn some of the principles associated with acid-base titration curves by using DrDAQ as an educational tool.
3. To use the generated titration curves to determine the concentration of some analytes in common samples such: as acetic acid in vinegar, and sodium bicarbonate in baking powder.

EMPOWER

Equipment required

1. DrDAQ.
2. Glass combination pH electrode.
3. One beaker (125 ml).
4. Magnetic stirrer-magnet bar
5. Air pump (JUN ACO 9903) (can demonstrate the validity of the experiment) for higher accuracy and reliability a peristaltic or syringe pump is preferred.
6. Tygon Tubing.
7. 1 l glass bottle with tight lid.
8. 0.1 mol/l HCl.
9. 0.1 mol/l NaOH.
10. 0.1 mol/l Na₂CO₃.
11. Vinegar.
12. Graduated cylinder, 25 ml.
13. 5 ml graduated pipette.
14. 25 ml pipette.

DrDAQ

PicoLog data logging software

Figure 1

Experiment set up:

1.      The system is connected as shown in Figure (1). The air pump propels the titrant solution with a fixed and known flow rate:

[Volume (V) of the titrant added after time (t) = flow rate (mL/sec) * time(t) (sec)]

In this way, the amount added of the titrant becomes a linear function of time, the variable which can be recorded with DrDAQ and PicoLog.

The flow rate is kept constant by fixing the following variables:

• The speed of the pump
• The setting of the control tap
• The height of the tube above the level of the air pump (this is not important with other types of pumps)

2.      Make sure that the inlet air stream lies above the solution level in the glass bottle, do not let air bubble in to the titrant solution.

3.      Make sure that there are no air leaks around the Tygon tubing coming in to and out from the glass bottle. It is recommended to use epoxy to seal the tubing in the lid of the bottle.

4.      Use high a stirring rate and position the glass pH electrode as far as possible from the falling drops of the titrant to minimize local concentration of the titrant in the vicinity of the glass pH electrode.

5.      Once all the parts are collected, the setup requires about half an hour.

6.      Each part of the experiment requires about 10 minutes including washing the beaker with distilled water between runs.

Part 1: setting and determination of the flow rate

1.      Put a 25 ml graduated cylinder underneath the end of the tubing. Turn on the air pump, and collect a certain volume (e.g. 20 ml) of the titrant in the cylinder. Measure the required time (t). Calculate the flow rate (F) as follows:

F= V(ml) / t (s)

2.      A flow rate of about 1-3 ml/min (0.0166-0.05 ml/sec) is appropriate Do not change the settings once you have measured the flow rate.

Part 2: determination of unknown HCl concentration (standardization of HCl)

1.      Fill the 1 l glass bottle with the unknown HCl solution (˜1 mol/l).

2. Pipette 5 ml of 1.0 mol/l Na₂CO₃ solution into a 125 ml glass beaker.
3. Add about 50 ml of distilled water.
4. Immerse the glass pH electrode in the solution.
5. Turn on the magnetic stirrer.
6. Set the PicoLog to monitor pH at a frequency of one sample every 2 seconds.
7. Simultaneously, start recording with DrDAQ and start the flow of titrant (just turn on the air pump).
8. Note that the initial pH is alkaline (sodium carbonate is a basic salt).
9. Observe how the pH falls slowly through the entire interval before the end point and how the pH changes abruptly over a very limited time around the end point.
10. Observe the advantage of the PicoLog auto scaling feature in this application.
11. Note that the curve shows two pH drops at equal time intervals (for equal volume added).
12. Measure the time (t) required for complete neutralization of the sodium carbonate (second end point).
13. Calculate the molarity (M) of HCl solution from the following expression:

M_(HCl) = [(M * V)_carbonate * 2] / [(t * F)]

This set up is almost the same as that provided with commercial automatic titrators, which have the integrated systems to: deliver the titrant, monitor the pH, plot the curves and detect the end point. Automatic titrators possess sophisticated mechanisms, which allow a variable flow rate for more precise end point location.

Part 3: determination of the concentration of sodium hydroxide solution

1.      Pipette 5 ml of the unknown sodium hydroxide solution into a 125 ml glass beaker.

2. Add about 50 ml of distilled water.
3. Use the same HCl used in the previous part.
4. Repeat as above.
5. Note that the starting pH is very high (strong alkali).
6. Only one large pH jump is observed.
7. Locate the time (t) of the end point (the steepest point in the curve that corresponds to pH 7 in this case). (The PicoLog cursor will help you to define the end point.)
8. Calculate the molarity (M) of NaOH solution from the following expression

M_(NaOH) = [M_(HCl) * (t) *F] / V_(NaOH)

Part 4: determination of the content of sodium bicarbonate in commercial baking powder

1.      Suspend a 5 g portion of baking powder in 100 ml of distilled water.

2. Shake well and pipette 50 ml aliquots into a 125 ml glass beaker.
3. Titrate as above using the same HCl solution.
4. Note that the initial pH of the bicarbonate solution is substantially lower (~7.2) than that of the carbonate solution described in part 2.
5. The % (w/w) of sodium bicarbonate is calculated from the following expression:

Sodium Bicarbonate % (w/w)= [(M_(HCl) * t * F * 84 * 2] * 100 / 5

Part 5: determination of acetic acid content in vinegar

1.      Fill the glass bottle with NaOH solution determined in part 3 to be used as titrant.

2. Calibrate the flow rate (F).
3. Pipette 10 ml of commercial vinegar into the 125 ml glass beaker.
4. Dilute with about 50 ml of distilled water.
5. Repeat as above.
6. Observe that the initial pH is in the acidic region. This is due to the presence of acetic acid in the vinegar.
7. Calculate the % concentration of acetic acid from the following experssion:

% (w/w) = [(M_(NaOH) * F * t * 60.05 * 10] / 1000

Part 6: comparison between the titration of acetic acid and HCl with NaOH:

1.      Pipette 10 ml aliquot of HCl solution used in parts 2-4 in a 125 ml beaker.

2. Dilute with about 50 ml of distilled water.
3. Repeat as in part 5.
4. Observe that the initial pH is in the acidic region and that only one pH jump occurs.

Questions and discussion of results:

Figure 2: titration curve of sodium carbonate against HCl

Figure 2 shows the titration curve of sodium carbonate with HCl. There are two abrupt pH changes in the curve. These correspond to the following successive reactions:

Na₂CO₃ + HCl -> NaHCO₃ + NaCl    (conversion of carbonate into bicarbonate)

NaHCO₃ + HCl -> CO₂ + H₂O + NaCl

The Calculated molarity of HCl in this experiment is 0.95 mol/l.

Figure 3: titration curve of NaOH against HCl

Figure 3 shows the titration curve of the reaction:

NaOH + HCl -> NaCl + H₂O

This reaction involves strong acid (HCl) and strong base (NaOH). You can notice how the pH changes from a very high to very low pH value. In such reactions, the pH at the equivalence point is 7. Move the cursor on the screen and see that the steepest trace occurs at pH 7.

The calculated molarity of NaOH in this experiment is 1.01 mol/l.

Figure 4: titration curve of baking powder against HCl

Figure 4 shows the pH changes during the titration of baking powder with HCl. In contrast to the carbonate experiment, we can see here only one abrupt pH step, which corresponds to the conversion of bicarbonate into carbon dioxide. It is interesting to note that Figure 4 is similar to the second portion only of Figure 2.

Figure 5: titration curve of Vinegar against NaOH

Figure 5 shows the titration curve of vinegar against sodium hydroxide. Note that the pH of the solution increases during the titration due to the addition of NaOH.

The Reaction involved is:

CH₃COOH + NaOH -> CH₃COO-Na+ + H₂O

The calculated concentration of acetic acid in vinegar in this experiment is 5.85 % (w/w)

Figure 6: titration curve of HCl against NaOH

Figure 6 is the opposite of Figure 3 where HCl is being titrated with NaOH. It is clear that the pH jump is larger in the case of titration of strong acids (e.g. HCl) than that in the titration of weak acids (e.g. acetic) with an alkali.

Questions:

1. Explain why the pH at equivalence point in Figures 3 and 6 is 7 whereas in Figure 5 is 8.86
2. Calculate the pKa of acetic acid from Figure 5.
3. Predict the titration curve if you titrate a mixture of 0.1 mol/l sodium carbonate and 0.1 mol/l sodium bicarbonate with HCl.
4. What would happen if you did not calibrate the flow rate?

Answers:

1. Figure 3 and 6 show titrations involving a strong acid and a strong base. The solution at the equivalence point contains their salt (NaCl) which is neutral, i.e., pH 7. Whereas, Figure 5 shows the titration of a weak acid (acetic) against strong base (NaOH). At this equivalence point, the solution contains their salt (sodium acetate) which is a basic salt the pH of which is > 7 , 8.86 in this case.
2. pKa of acetic acid can be calculated by determining the pH at half neutralization. At this point pH =pKa (theoretical value = 4.74, the experimental value 4.6).
3. The second step will be as twice as the first step.
4. You can still get the titration curve but you cannot tell the volume of the titrant required to reach the equivalence point, and of course cannot calculate the unknown concentration.