Titring analysis.

titration analysisanalytical chemistry is a science that studies the measurement methods and principles of material composition. It is divided into qualitative analysis and quantitative analysis in two parts. The task of qualitative analysis is to determine the composition of matter, and the task of quantitative analysis is to further determine the relative content of each component on the basis of qualitative analysis.quantitative analysis can usually be divided into chemical analysis and instrument analysis. Chemical analysis is an analytical method based on the chemical properties of matter, which includes weight analysis and titring analysis. Instrument analysis is based on the physical or physical chemistry of matter, using special test instruments for analysis.titration analysis is not only very practical in the field of chemistry and chemical industry, but also widely used in medical and health work because of its simplicity, fastness and enough accuracy.the first section of titration analysis concept 1, the characteristics and methods of titration analysis titration analysis is a method of analysis achieved through "titration". During titration, a known and accurate solution is used called a standard solution, and a titration solution is called a sample solution. When the reaction between the standard solution and the measured composition is exactly the same, it is the theoretical end point of the reaction, which is determined by the easily observable changes that occur near the chemical metering point. If there is no such change in the reaction itself, an indicator must be used. The reaction endpoint indicated by the indicator is called the titring end point.titration analysis method is based on the standard solution and the measured composition between the
biochemical
reaction, there is a certain chemical measurement relationship between their quantities, using the concentration of the standard solution and the volume consumed to calculate the content of the substance under test. According to the different chemical reactions used in the analysis, the titration analysis is divided into
acid
titration method, redox titration method, with titration method and precipitation titration method.It is worth noting that not all reactions can be used for titration analysis, and that reactions suitable for titration analysis must meet the following criteria: first, the reaction must be carried out in a certain reaction manner, i.e. there must be a definite chemical measurement relationship; In addition, the response is required to be carried out more quickly. Slower reactions can sometimes be accelerated
by
heating or adding catalysts., titration analysis of the operating procedures(i) standard solution preparation1. The direct preparation methoda certain amount of the baseline material, dissolved and transferred to the
capacity bottle
, dilution of the fixed capacity. The exact concentration of the solution can be calculated according to the quantity of the solution and the volume of the solution.so-called baseline substances are those that can be used directly to make solutions. They must meet the following conditions:(1) substance shall be chemically consistent. If crystalline water is included, the content of crystalline water should also be consistent with the chemical type.(2)
reagents
high purity. The general content is more than 99.9%.(3) reagents should be stable. Such as not easy to absorb moisture and carbon dioxide in the air, not easy to be oxidized by air and so on.(4) has a relatively large molar mass to reduce the relative error caused by weighing.commonly used benchmark substances are potassium hydrochloride, grassic acid, sodium carbonate and so on. 2. Indirect preparation method is because most reagents do not meet the conditions of the baseline substance, nor can they be used directly to make standard solutions. At this time, they can be rationed to a solution approximated by the desired concentration, and then the concentration can be determined using the following method, which is called indirect preparation. (ii) calibration of the concentration of the standard solution refers to the process of determining the exact concentration of the solution by titration method. A certain amount of baseline concentration of the solution to be calibration can be used, or it can be titrified to each other with a standard solution with a known accurate concentration to compare the concentration of the solution to be measured. For example, to determine the concentration of hydrochloric acid solution, it may be called a certain amount of analysis of sodium carbonate (base substance) dissolved in water, and then with this hydrochloric acid solution for titration: 2HCL plus Na2CO3→2NaCL plus H2O plus CO2 when the reaction is complete, from the use of the volume of this hydrochloric acid solution and the quality of sodium carbonate, you can find the exact concentration of hydrochloric acid solution. (iii) the determination of the content of the substance under the concentration of the standard solution, the content determination of the measured substance can be treated. For example, a standardized hydrochloric acid solution can be used to determine the content of certain alkaline substances, and a standardized oxidant solution can be used to determine the relative content of certain reductive substances. calculation method of titration analysis (i) direction method the calculation in titration analysis mainly includes the calculation of standard solution concentration and the calculation of the content of the measured substance. The concentration of a standard solution is expressed by the concentration of a commonly used substance. The concentration of substance B can be found in the following type: cB-nB/Vb (7-1) or cBVB-mB/MB (7-2) above In the sub, mb is the mass of substance B (g), MB is the molar mass of substance B (gmol-1), VB is the volume of substance B solution (L), cB is the volume concentration of substance B (molL-1). For any titration reaction: aA-bB-dD-eE If aA and bB are selected as the basic units, there is a relationship as follows: n (aA) - n (bB) (7-3) the upper-level represents: when the chemical metering point is reached, the amount of aA and bB is equal, that is, the quantity rule of the same substance. This rule is one of the main basis for titration analysis calculations, and is based on formula (7-1) and formula (7-3): c(aA)V(A) s c (bB)V(B) in practice, A and B are often selected as basic units for convenience. (7-2) and (7-5) are two commonly used calculations in titration analysis, and their applications are described in examples below. example 1 accurately named the reference substance K2 Cr2 O7 2.4530g, dissolved and moved into a 500 ml capacity bottle, diluted with water to the scale. Find c (K2 Cr2 O7) and . : K2 Cr2 O7 has a molar mass of 294.2g.mol-1 case 2 called pure Na2 CO3 1.2738g, dissolved in water diluted to 250mL, taken the solution 25.00mL, titration with HCl solution, with methyl orange as an indicator. When the end point is reached, the exact concentration of the HCl solution is 24.80mL. solution: Titration reaction is Na2 CO3 s 2HCl→2NaCl s CO2 s H2 O by (7-5) because so the volume of the solution in the upper mode is in liters, if volume V is specified in milliliters, then the upper can be then in 25 The mass of Na2 CO3 in the .00mL Na2 CO3 solution is: so example 3 is called taking herbal acid 0.1670g dissolved in the appropriate amount of water, with 0.1000mol.L-1 NaOH standard solution titration, with 23.46mL, the quality percentage of H2 C2 O4.2H2 O in the sample. solution: The titration reaction is the mass percentage of herbal acid: (ii) the valid number and the calculation rule the effective number refers to the value that can actually be measured, in which only the last digit is a suspicious number, and the rest are reliable numbers. Its practical significance lies in the fact that the effective numbers reflect the accuracy of the measurement.
For example, measuring an object with a ruler with a minimum scale of 0.1cm is 11.23cm, it is clear that the first three digits of this value are accurate, and the last digit is not so reliable because it is estimated by the tester, the length of the object may be 11.24cm, or 11.22cm, and the measurement has an error of ±0.01cm.
we call the first three reliable digits of this number and the last suspicious number a valid number. This number is a four-digit valid number. in determining the number of valid digits, it is particularly important to note that the number "0" represents the actual measurement results, it is a valid number. For example, the analysis of an
balance
is called object mass of 7.1560g, titring
titer tube
reading is 20.05mL, both of which are valid numbers. "0" in 0.006g only serves as a positioning function, not a valid number. In calculations, you often encounter two kinds of factors and multiples in the chemical metering relationship, which are not measured, and their number of valid digits can be considered infinite, and in the case of pH, pK, and lgK, the number of valid numbers depends only on the number of digits in the fraction, because the integer part is only related to the 10 squares in that true number.
example, pH is a two-digit valid number, and integer part 13 is not a valid number. If it is expressed as a 7.1×10-14, it can be seen that the effect of 13 is only to determine that the mathematical significance of the 10-14 order of magnitude is the same as the "0" to determine the position of the scale. in titration analysis, the record of experimental data should retain only one suspicious number, and the calculation and data processing of the results should be carried out according to the calculation rules of valid numbers. addition and subtract operation, the effective number-to-value comparison is based on the value with the fewest number of digits after the number of points. For example, 0.0231, 24.57, and 1.16832 add up to the fewest numbers after the number of dots, so other values should also be taken. The result of the last two digits is 0.02 plus 24.57 plus 1.17 plus 25.76 which should be the least valid number in the multiplication and division operation. For example, 0.0231, 24.57 and 1.16832 are multiplied by three numbers, and 0.0231 has the fewest valid digits, only three digits, so the other numbers take only three digits. The result of the operation also retains three valid digits: 0.0231×24.6×1.17 x 0.665 in the counting operation, the number of bits taken should be the same as the number of valid digits of the real number. For example, if the true number of lg9.6 has two valid digits, the pair should be 0.98 and should not be 0.982 or 0.9823. For example, PH should be 1.52 when × 10-2mol L-1 is 3.0.0. the correct use of valid numeric rules to calculate, not only can reflect the credibility of the results, but also greatly simplify the calculation process. Four-digit valid numbers are commonly used in titring analysis. the error of the titration analysis (i) error and its source in the titration analysis, the accuracy of the analysis results is often expressed by the error. The error is the difference between the measured value and the true value, which reflects the degree of conformity between the analysis results and the real value. It is an important subject in quantitative analysis to reduce measurement error and improve the accuracy of analysis results. errors in titration analysis are systemic and accidental errors. 1. System errors system errors are caused by certain inevitable or frequent causes. The sources are method error, instrument, reagent error and operation error. The influence of system error on the analysis results is somewhat regular, and the size of the error is often close when repeated measurements are made, and they are repeated. Compared with the theoretical values, the experimental values are either higher or lower. System errors are often eliminated or overcome by methods of blank and controlled experiments. without samples, the analysis test according to the sample analysis steps and conditions is called a blank test, and the results are called blank values. By deducting the blank value from the sample measurement results, the system error caused by reagents, distilled water and experimental instruments can be eliminated. standard samples with known parts content and samples to be tested are analyzed and measured under the same conditions. Other measurements are corrected by the difference between the measured value of the standard sample and its true value, which is called a controlled test. addition, the instrument can be calibrated to eliminate instrument errors, through the development of correct operating procedures to overcome operating errors. 2. Occasional error error is caused by a series of small changes. For example, when weighing the same object, small changes in room temperature or humidity can cause accidental errors that make weighing results inconsistent. This error size is variable, positive and negative, often can not find a definite reason, because it is difficult to control, correct and determine it. However, the occasional error
the < b.