Sampling Techniques
- 2. Steps in doing analysis: 1. Deciding on an analytical method 2. Collecting and preparing bulk sample 3. Preparing the laboratory sample 4. Defining the samples to be analyze (replicate samples) 5. Preparing solutions of the sample 6. Elimination of interferences 7. Calibration, measurement and calculating results 8. Evaluating results and estimating reliability
- 3. Classification of analysis 1. Macro - more than 0.1 g of sample is available 2. Semi -micro- 10-100 mg of sample is available 3. Micro - 1-10 mg of sample is available 4. Ultra-micro – samples are in the order of micrograms
- 4. Divisions of quantitative analysis – the quantity of desired constituent is computed based on the volume of reagent needed for reaction with sample. – the quantity of desired constituent is computed based on the weight of precipitate obtained from sample. – the unknown is converted into a colored compound and the intensity of the color is compared with standard. such as turbidity, nephelometry, polarimetry, refractometry.
- 5. Volumetric methods – completion of the reaction is indicated by the change in color of the indicator used. - completion of the reaction is indicated by the electrical means. a) Potentiometric methods – changes of potential of a cell of which the solution being titrated is a component part. b) Coductometric method – changes in conductivity of the solution being titrated. c) Amperometric method – changes of amperage of an electric current passing through a solution during titration. a)Coulometric method – quantity of unknown is established for ampere-time measurements of an electrolytic cell where the solution to be titrated is added.
- 6. Gravimetric methods a)Analysis of sulfates by precipitation as BaSO4 • SO4 2-(aq) + BaCl2 + HCI BaSO4 (s) b) Analysis of iron content • Fe3+ + OH- Fe(OH)3 Fe2O3 • Analysis of Cu– pass electric current through a solution of Cu2+ to deposit Cu metal.
- 7. Gas-Volumetric methods – amount of desired constituents is computed based on the change in volume of the gaseous sample after selective absorption of the component. Pass through KOH Mixture of CO2 + O2 O2 Change in volume = volume of CO2 absorbed 2. – based on change in volume brought about by burning a mixture of gasses.
- 8. Sampling and Sampling Preparation 1. The sample is collected in the manner consistent with the goal of the analysis. 2. The sample should be handled so to prevent its contamination or alteration. 3. The sample is brought into the laboratory and prepared for the technique chosen.
- 9. A good sampling plan must be able to guarantee that the information needed to meet the objectives of the measurements being made is obtained. The design of such a plan must take into consideration of the following: a. What do you want to know? b. What do you need this information? c. What happens to the results? d. What actions will follow? 2. A sampling plan should identify what analytes (constituents of) the material are to be determined. The preferred method to be used is that one has been standardized and whose accuracy and precision can be verified. 3. It is important to identify the parts of a material and the corresponding portions that will make up the sample. 4. Sampling Plan
- 10. Different Approaches to sampling – refers to sampling without bias. Increments are taken in such a way that any portion of the bulk of the material has the same probability of being included when a sample is taken. – one of the most commonly employed sampling techniques; incremental samples are taken at preset intervals. – an extension of systematic sampling where each batch is further divided into groups (strata). – samples are taken at preset but random intervals to check for compliances to a specific parameter; also reduces the number and amount of samples taken. – deals with determining changes in analyte concentration over a specific time period or location: could be monitoring study or simply a report of an average value.
- 11. Preparation of a Laboratory Sample The gross sample is systematically mixed and reduce to obtain the laboratory sample. If the mixing is done by hand, the following methods can be used.
- 12. Liquids Reagents for Dissolving or Decomposing Samples – the concentrated form of the acid (12 M) is an excellent solvent for metal oxides and metals that are more easily oxidized than hydrogen. – hot concentrated nitric acid (~15 M) is able to dissolve all common metals except for aluminum and chromium (due to surface oxide formation). – hot concentrated sulfuric acid (~18 M) is able to effectively decompose and dissolve many samples due to a high boiling point of 3400C.This mixtures dehydrates and decomposes samples containing organic compounds. – hot concentrated perchloric acid (~9 M) is considered a potent oxidizing agent and is able to attack alloys and stainless steels that are resistant to other mineral acids. – rapid dissolution can be achieved by using a mixture of acids or by adding an oxidizing agent to a mineral acid e. g. aqua regia ( 3 parts concentrated HCI and 1 part concentrated HNO3) – this acid is primarily used in the decomposition of silicates and minerals where silica is not to be determined. It’s also occasionally added to other acids to act on difficult to dissolve steels
- 13. Types of errors in experimental measurements – errors which do not affect each determination in the same manner. These are due to inherent limitations of the equipment, to limitations of observation and to lack of care in making the measurement. These errors can be reduced by performing experiments replicate. Examples: variation in judgement of end points, variation in temperature, mistakes in observation of balance rest points. 2. - errors that affect each individual result of replicate determinations in the same manner. Examples: – weights that are incorrect in mass, balance arms of unequal lengths. - coprecipitation of impurities, slight solubility of precipitates and occurrence of side reactions.
- 14. Common weighing errors: . This can be eliminated by calibration. condensation water vapor on dry samples or precipitates may cause a weight gain so it is standard practice to weight all samples in a covered bottle. – object to be weight should be at room temperature. If object weight are still warm, weights are too light because convection currents set up in the balance tend to push the left pan up and retard the swings of the balance. – glass vessels acquire a charge of static electricity when they are wiped with a dry cloth. This may cause the swings of the balance to change in an allowed to stand until most of the surface moisture is evaporated. – can be corrected by applying the equation below: Wv = weight in a vacuum Wa = weight in air d0 = density of object dw = density of weights (approx. 8) dair = density of air (0.0012 g/mL) at ave. T ,P humidity
- 15. – an analysis of a material containing all the constituents of he given sample except the desired constituent. The numerical result obtained in the blank is used to correct the value obtained in the unknown. – an analysis of a material containing all the constituents of the given sample except that the material used is of known composition.
- 16. 1. Absolute error = True Value – Experimental Value 2. % Relative Error = True value – Expt’l value x 100 True value
- 17. 1. Range: highest value – lowest value 2. Average deviation (d) 3. Standard deviation (s) 4. Student’s t - to correct for uncertainties due to small population; to take into the fact that x is not the same as u.
- 18. • 4d rule 1. Compute the mean and average deviation of the “good” results. 2. Find the deviation of the suspected result (outlier) from the mean of the “good” ones. 3. Find the deviation of the suspected result from the mean of the “good” ones is at least 4 times the average deviation of the “good” results then the outlier is rejected.
- 19. • Q test 1. Calculate the range of the results 2. Find the difference between the suspected results and its nearest neighbor. 3. Divide the result in NO. 2 by the range in No. 1 to obtain Q. 4. If Qcomputed > Qtable then the result can be discarded.