Methods of minimizing errors

SONALI SHINDE PUNE

1. Methods of minimizing Errors 1) Calibration of apparatus : All apparatus like weights, flasks, burettes and pipettes should be calibrated. The appropriate corrections applied to the original measurements. In some case errors cannot be eliminated. Apply a correction for that effect. e.g An impurity in a weighed precipitate may be determined and its weight deducted.

2. 2) Running a blank determination It is carried out as a separate determination the sample being omitted, under exactly the same experimental conditions as employed in the actual analysis of sample. The object is to find out the effect of the impurities introduced through the reagents and vessels. 3) Running a control determination : Determination carried out as nearly as possible identical experimental conditions upon a quantity of a standard substance which contains the same weight of the constituent.

3. 4) Use of independent methods of analysis : Determination of strength of HCl both by titration with solution of a strong base and by precipitation and weighed as AgCl. If the results obtained by the two radically different methods are consistent. It is highly probable that the values are correct within small limits of error. 5) Running parallel determination : It is as a check on the result of a single determination and indicate only the precision of the analysis.

4. The values obtained should not less than three parts per thousand. If larger variation is there then it must be repeated until satisfactory concordance is obtained. Duplicate/ triplicate determination is suffice. A known amount of the constituent being is added to the sample which is then analyzed for the total amount of constituent present. The difference between the analytical results for samples with the recovery of the amount of added constituent. If the recovery is satisfactory our confidence in the accuracy of the procedure is enhanced.

5. 6. Standard addition: a known amount of the constituent being is added to the sample which is then analyzed for the total amount of constituent present. The difference between the analytical results for samples with and without added constituent gives the recovery of the amount of added constituent. If the recovery is satisfactory our confidence in the accuracy of the procedure is enhanced.

6. Definition of Accuracy Accuracy: How close you are to the actual value. it is the degree of agreement between the measured value and the true value. Calculated by the formula: % Error = (YV – AV ) X 100 / AV where YV is Your measured value and AV is the accepted value . An absolute true value is rarely known. so the term accuracy refers to how near the observed value is to true value.

7. Definition of Precision Precision is defied as the degree of agreement between replicate measurements of the same quantity. It is the repeatability of a result. So, the term precision refers to nearness between several measurements of the same quantity.

8. Accuracy and precision may be demon stared by shooting at a target. accuracy is represented by hitting the bulls eye ( the accepted value) Precision is represented by a tight grouping of shots( they are finely tuned)

9. For example : the measured density of water has become more accurate with improved experimental design, techniques and equipments. for example: if a student measured the room width at 8.46 m and the accepted value was 9.45 m what was their accuracy? Using the formula: % error= (YV- AV) X 100 / AV Where YV is the student’s measured value and AV is the accepted value.

10. since YV= 8.46M AV= 9.45m % error = (8.46- 9.45) X 100 /9.45 = -0.99 X100 /9.45 = -99/9.45 = -10.5% note that the meter unit cancels during the division and the unit is % . The (-) shows that YV was low. The student was off by almost 11% and must re-measure. percent error is used to estimate the accuracy of a surement.

11. Percent error will always positive what is the percent error if the measured density of titanium (Ti) 45g/cm3 and the accepted density of Ti is 4.50 g/cm3? Accepted error is =/- 5% value from -5% up to 5% are acceptable value less than -5% or greater than 5%must be remeasured

12. Significant Digits Generally, significant figures may be defined as—“ All digits* that are certain plus one which contains some uncertainty are said to be significant figures ”.  Examples: ( a )   Burette Reading:   Burettes are mostly graduated with the smallest graduation as   0.1 ml; hence, while taking the burette reading the figures 6.3 ml can be read off with ample certainty. However, the second place of the decimal is normally estimated by arbitrarily sub-dividing the smallest division into 10 equal parts. Consequently, the final burette reading

13. of 6.32 ml essentially contains three significant figures, of which two are certain, and one with some uncertainty.   ( b )  Measuring Weights:  In the two measured quantities : 4.7350 g and 4.0082 g the zero is a significant figure ; whereas, in the quantity 0.0065 kg the zeros are not significant figures. Thus, in the latter instance the zeros only serve to locate the decimal point and, therefore, may be eliminated completely by proper choice of units,  e.g. , 6.5 g. Moreover, the first two numbers do have five significant figures, whilst 0.0065 only has two significant figures.  

14. Rule 1: all non-zero numbers are significant e.g. 5489.213 ……. 7 significant digits. Rule 2: All zeros located between non-zero numbers are significant e.g. 0.08006 ………. 4 significant digits Rule 3: E.G 0.00004 ……… 1 significant digit Rule 4: Trailing zeros zeros that are located to the right of a value may or may not be significant e.g. 1000.0ml ………….4 significant digits e.g. 1000ml ………………1 significant digit.

15. Course outcomes : Pharmaceutical Analysis deals with the fundamentals of analytical chemistry and principles of electrochemical analysis of drugs. Objectives: Upon completion of the course a student shall be able to understand – the principles of volumetric and electrochemical analysis carry out various volumetric and electrochemical titrations Develop analytical skills

16. Thank you !!