Sources and Types of errors

                  

                  Source and types of errors

Errors-

It is refers to the difference in the standard values and the true value(acceppted true result).

Sources of errors- 

⦁ Sometimes even standard weights have error.
⦁ Fluctuation of scale reading.
⦁ Dust particle in air may adhere to the balance.
⦁ Air could react with the substance being weighed.
⦁ Moisture from wet items evaporates while weighing.
⦁ Hot or cold items affecteda in weight.
⦁ Improper measurement quantity of chemical.
⦁ Wrong procedure are applied.
⦁ Vibration disturbances as produced by passing heavy vehicle which also affect weighing.
⦁ Delay in calibration and wrong in calculation.
⦁ Contaminated chemicals are used.

Types of error-

It can be brodly categorized into two categories.
1. Determinate errors (Systematic errors)
2. Indeterminate errors( Random errors)

1. Determinate errors (Systematic errors)- Determinate errors are those errors that are known and controllable errors these are known and avoidable.

Types of systematic errors-

i. Personal errors- These errors are completely due to the analyst's human error and have nothing to do with the prescribed procedure or methodology.

ii. Instrumenatl errors- Instruments need calibration and are not accurate. Eg- pH meter, single pan electric balance, UV spectrophotometer, potentiometer.

iii. Reagent errors- These errors are the results of the reagent itself, such as impurities inherent to th reagent. Eg- Platinum volatilization due to high temprature.  OR  Unwanted foreign substances introduced by reagents when they react with porcelain and glass appartus.

iv. Proportional errors- No matter how many samples are taken, the absolute error value remains constant regardless of that change.

v. Errors due to methodology-These  errors consist of a poor sample selection and incomplete reaction.

2. Indeterminate errors (Random errors)- Indetermined errors are represents random fluctuation in measuring devices that are not controllable. Eg- Students open and close lab doors cause changes in pressure reading. 

Sailent features of indeterminate errors-

i. Repeat mesurement Same variable several times and yielding correction to apply between                      replicates of the same variable is simply a coincidence.
ii. Sample handled improperly.
iii. Presence of bubbles in burettes.
iv. Temprature variation.
v. Noise as well as drift from an electronic circuit.

vi. Vibrations caused to building  by heavy vehicular-traffic.

Accuracy- 

These are measures of observational error. Accuracy is refers to how close a given set of (observation or reading) measurement is the true or accepted value.

Precision-

These are also measures of observational error. Precision of measurements system is the degree to which repeated measurements give the  same result. A measurement system can be accurate but not precise ,precise but not accurate ,neither or both.

Precision measurements can be of two types:

(1). Repeatibility(Test- Retest reliability)- The variation which arises in successive of the same variable taken under the same conditions ( Eg- Same observer,location,instrument and procedure) in a short period of time.

(2). Reproducibility - The variation which arises by extent to which a tool is capable of producing the same result when use repeatedly in the same process for measurement.

Significant Figures-

 The significant figures of a given number are those important digits, which convey the meaning according to its accuracy. Eg- 7.898 has four significant digits. These substantial figures provide precision to the numbers. They are also termed as significant digits.

Rules for significant figures-

I.    All non-zero digits are significant. Eg- 78982592 contains eight significant digits.
II.  Zero between two non-zero digits are significant. Eg- 3021 has four significant figures.
III. Leading zeroes are not significant. Eg- 0.12,0.0089 has  only two significant figures.
IV. Trailing zeros to the right of the decimal are significant. Eg- 25.00 there are four significant                    figures.
V. Trailing zeros in a whole number with the decimal shown are significant. Eg-248. indicates that the         trailing zero is significant there are three significant figures.
VI. Trailing zeros in whole number with no decimal shown are not significant. Eg- 248 indicates that           the zero is not significant, there are only two significant figures in this value.
VII. Exact numbers have an infinite number of significant figures. Eg- 1 Meter= 1.00 meters = 1.0000            meters = 1.000000000000000000000 meters ,etc.