3 Errors in Analysis and Laboratory Safety

Dr. Varinder Kaur

Objectives: 

1. To know about the various types of errors encountered in the analysis and parameters used to represent these errors

2. To know the following about self generated questions.

  1.  What types of errors are encountered in analysis?
  2. How these errors affect the results?
  3. How various errors can be avoided?
  4. How analytical data is calculated to minimize the affect of errors?
  5. What is accuracy?
  6. What is precission?

3. To know about laboratory safety measures and following questions related to safety.

  1. What type of safety procedures can be practiced in a chemical laboratory?
  2. What type of precautions can be taken to protect eyes?
  3. What are the important measures for personal safety?
  4. How the hazardous materials are handled?
  5. What is the significance of various signs used on chemical bottles?

 

1. Description

In the previous module, we discussed about various analytical methods used for the measurement and/or analysis purposes. These methods involve optimization of several parameters using specific equipments operated by expert persons. In detail, sample preparation and analytical methods employ measurement of a particular property with the help of various equipments including glass apparatus like beaker, pipette, measuring cylinder, measuring flasks; auto-pipettes, weighing machines, weights, filtration units and instruments with their accessories like pH meter with electrodes, spectrophotometer with cuvettes, high performance liquid chromatography with columns etc. Secondly, various methods such as precipitation, washing, drying, filtration, extraction etc are commonly used in sample preparation procedures. Thirdly, man-power is also involved in the analytical experiments to perform the experiment and record the data or to operate the instruments involved in the analysis. In addition, calculative data such as amounts required for the experiment, yields, detection limits etc. is directly related to the purity of the chemicals and reagents. The involvement of all these parameters directly or indirectly (equipment, experimentation procedures and operational expertise) may alternate the final result by adding some errors. For instance, to prepare N/20 NaOH solution, following factors are responsible for the addition of errors at each step.

Procedure for the preparation
of N/20 NaOH solution		 Sources of errors involved
The weight of NaOH required
to prepare its N/20 solution is
calculated, weighed and
dissolved in distilled water in a
100 mL measuring flask. The
total volume is made upto the
mark by adding extra distilled
water.		 1. Purity of NaOH provided by the manufacturer
2. Accuracy of weighing machine used for weighing NaOH or the
physical balance and the weights used for weighing
3. Accuracy of the graduated apparatus used in the measurement
such as cylinder, measuring flask
4. Purity of the distilled water
5. Environmental parameters such as NaOH is hygroscopic so the
weight may vary.
6. Experimental hand of the person who is performing the
experiment
7. Judgment of the meniscus in the measuring flask to make it
100 mL

It is clear from the example that a single step may add large number of errors and deviate results from actual value. These deviations go on increasing with each step and may vary the results beyond acceptable limits. Therefore, it is important to discuss the errors, their origin, effect on the results and preventive measures to get error free results in detail. Before going into the details of errors, knowledge of terms ‘accuracy’ and ‘precision’ is very important.

Precision

Precision is the extent to which the results of a multiple repeat experiments agree with one another e.g. if an experiment is repeated five times and a same result is obtained, it is considered as a precise result. Therefore, precision can be used to check the reliability of a method. It may be defined as the “concordance between a series of measurement of the same quantity”. It gives information about the reproducibility of the measurements and reliability of the method. Precision is the degree to which several measurements provide results close to each other. It indicates the scatter in the data. Lesser the scatter higher is the precision. Good precision does not assure good accuracy. A systematic error does not affect precision of the method. In the analysis of unknown sample, precision is considered to know the exact value of the analyte in the sample. The poor precision in the result affects estimation of accuracy in the analysis.

Factors used to express precision

The precision of a method can be expressed in terms of three functions; 1) Standard deviation, 2) Coefficient of variation, 3) Variance

Standard deviation

Accuracy

Accuracy is the degree to which the results of an experiment agree with the true or accepted value. e.g. if the weight of a substance having known standard mass equal to 100 g is measured by a physical balance and the result shows it 79.5 g it means the result is not accurate. But if the result shows 99.9 g it means result is accurate.

The accuracy may be defined as the “nearness of a measurement to the standard or true value”. The highly accurate results are provided by accurate instruments. The accuracy of a result depends upon the instrument and the ability of an operator. A systematic error affects the accuracy of the measurement.

Example: The four different refrigerators were tested for accuracy and precision of temperature sensor by recording temperature at different times. The results are given below along with the accuracy and precision of temperature sensor. The refrigerator was kept at a constant temperature of 38.0 F.

 

Read Refrigerator 1 Refrigerator 2 Refrigerator 3 Refrigerator 4
1. 39.4 37.8 39.3 38.0
2. 38.1 38.3 39.2 38.0
3. 39.3 38.1 39.1 37.8
4. 37.5 38.0 39.0 38.1
5. 38.3 37.6 39.1 38.0
6. 39.1 38.2 39.3 37.9
7. 37.7 38.0 39.2 38.2
8. 37.1 38.0 39.1 38.0
9. 38.8 37.4 39.2 37.9
10. 39.0 38.3 39.2 38.0
Accuracy BAD GOOD BAD GOOD
Precision BAD BAD GOOD GOOD

Note : It is not necessary that good precision means good accuracy or vice versa.

Expression of accuracy

Accuracy can be expressed in terms of absolute error and relative error.

Absolute error

The absolute error of measurement is the difference between the measured value and the true value. It can be obtained by the following equation.

Where E is absolute error, xo is the measured value and xt is the true value If the measurement value is more than the true value, the sign for absolute error is positive. If the measurement value is less than the true value, sign for absolute error is negative.

Relative error

It is the absolute error divided by the true value. It may be expressed in percent or parts per million or parts per thousand depending upon the magnitude of the result. It can be obtained using following equation.

2. Classification of errors

The errors may be divided into categories; systematic and random errors. The main features of two types of errors are given below.

1.  Operational and personal errors:

These errors are associated with the expertise of the analyst and are added during experimentation. These errors depend upon the knowledge, experimental hand and experience of the person who is doing the experiment and interpreting the data. Examples:

1.  Weighing of a compound slightly more or less than the actual quantity.

2.  Prediction of the end point one drop after or before the appearance of accurate color.

3.  Weighing of hygroscopic materials on filter paper

3.  Overheating or insufficient cooling of contents

4.  Measurement of small volumes of reagents with inappropriate apparatus

5.  Inappropriate use of pipette

6.  Improper setting of baseline for recording data of a sample

2.  Instrumental and reagent errors

These errors are associated with the equipments and the quality of the reagents employed during the analytical procedure. In this type errors, instruments such as physical balance, electrodes for recording pH, conductance, potential etc., add errors to the measurement or analysis due to negligence in their operation such their cleanliness, calibration, improper washing procedures of electrodes, improper connections etc. Secondly, the quality of reagents used is equally important and use of low quality reagent and chemicals deviate the amounts and related data from the actual value. In addition, use of low quality apparatus adds impurities during experiment and act as additional source of errors. For instance, use of low quality glassware adds impurities such as metal ions due to the attack of reagents (acids or solvents) and dissolution of labels in solvents etc. These errors can be avoided by analyzing reference samples.

3. Errors of method

These errors are concerned with procedures followed during sampling and analysis due to incomplete reaction, loss of sample during washing, incomplete precipitation, volatility of the reactant or products involved etc. Like instrumental and reagent errors, use of reference samples help in the removal of errors related to method. Some of the examples related to errors of methods involved in a particular procedure are elaborated below.

  1. Precipitation: Incomplete precipitation, co-precipitation of impurities, loss of precipitates during washing, volatilization of precipitates during drying and transference from the filter paper for weighing etc.
  2. Titration: Incompleteness of reaction or equilibrium set up which prevents the completion of reaction, inappropriate reaction conditions such as pH, temperature etc which may also vary during the course of reaction, involvement of impurities with similar chemical properties in the reaction, interpretation of end point and improper shaking of the solution for maximum interaction between titrant and titrand .
  3. Liquid-liquid extraction: Choice of inappropriate solvent like partially miscible solvents, incomplete partitioning of analyte in the two phases, loss of analyte during repeated operations and separation etc.

Effect of systematic errors on analytical results

The effect of systematic errors on the final result varies on the basis of its nature. These may be either constant or proportional. The details of the errors are discussed below.

 

Constant errors Proportional errors
Magnitude  of  the  error  remains  constant
with the variation in size, so, it is termed as
constant error.		 Magnitude of proportional errors always shows
variation with the variation in the size of the
sample.
In such cases, the absolute error is constant
but relative error varies with the change in
the size of the sample.		 In  such  cases  absolute  errors  vary  with  the
sample  size  while  relative  error  is  always
constant with the variation in the size of sample.

LAB SAFETY

In this part, various safety measures are covered to prevent the accidents and mishaps usually encountered due to lack of knowledge for handling chemicals. The main purpose is to understand proper laboratory safety, to aware the student about possible risks or hazards involved with laboratory work so that a safe place can be created by following the proper safety guideline. The overall safety includes personal safety, chemical safety, radiation safety, gas safety, fire safety and steps to be followed in case of any accident.

Personal Safety

The individual working in a lab is responsible for his/her own safety as well as the safety of co-workers, lab staff, and maintenance of lab. It is the personal responsibility of worker to use personal protection equipment and follow the safety rules and regulations. Some of the general precautions which should be followed by a research personal while working in a chemical lab are given below.

S.No.     List of do’s and don’ts while working in a chemical lab
1   Eatables in the lab are strictly prohibited, so don’t eat or drink anything in the working area.
2 Don’t apply cosmetics in the lab.
3 Don’t wear contact lenses while working in the lab because the vapors or gases may cause eye damage.
4 Don’t store food/drinks in the refrigerators/freezers or cabinets used for the storage of chemicals.
5 Don’t put pens or pencils in the mouth while working in the lab.
6   Don’t wear very tight, extremely loose and short clothing in lab.
7  Don’t wear long necklaces, scarves and ties etc in the lab.
8   Always tie the long hair at the back to avoid their contact with chemicals and other toxic substances.
9   Don’t wear rings or other jewelery while working in the lab.
10 Don’t wash utensils in the sinks and drain boards of the lab.
11 Don’t consume ice made in the ice machine of lab.

Personal protective equipments

The personal protective equipments must be used for personal safety. These are described below.

 

Chemical safety

All the chemicals should be treated as per their nature, so special care must be taken during handling the reagents and chemicals. The materials safety data sheets (MSDS) concerned with the handling of each material should be available to each personnel. Therefore, a proper record and MSDS should be maintained in the lab. To work in a lab, research personnel must wear personal protective equipments like lab coat, gloves, lab shoes etc. After the completion of activity, gloves should be removed and hands must be washed properly. The personnel should remove protective equipment before leaving the lab. The personnel should be aware about the chemical hazard symbols given on the bottles.

Radiation safety

A radioactive material is an unstable substance which continuously produces radiations. These radiations are harmful and cause adverse effects to the living beings and environment. Therefore, the radioactive materials must be labeled and stored properly as per the guidelines. The materials must be handled carefully and their spillage or leakage should be prevented. In case of spillage, recommended adsorbents should be used and the area should be cleaned with soap and water.

Bibliography

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