间接碘量法测定铜合金中的铜（Determination of copper in copper alloy by indirect iodine method）

1、间接碘量法测定铜合金中的铜（Determination of copper in copper alloy by indirect iodine method）Answers to experimental thinking questionsHow can 1. Na2S2O3 solution be formulated, can Na2S2O3 be dissolved in distilled water before boiling? Why?Answer: Na2S2O3 is not a standard substance, therefore, it is not possi

2、ble to prepare standard solution directly. The prepared Na2S2O3 solution is unstableThis is because the microorganisms in the water, CO2, react under the action of O2 in the airIn addition, a small amount of Cu2+ or Fe3+ in water can also promote the decomposition of Na2S2O3 solutions2. to potassium

3、 dichromate calibration Na2S2O3 concentration, why add KI, why in the dark for 5 minutes, before titration why dilute starch, why close to the end of the accession?Answer: (1) the role of KI has two points: first, the role of K2Cr2O7 and quantitative generation of I2, in order to demarcate Na2S2O3;T

4、he other is the synthesis of I3- complex ion with I2 complex to prevent the volatilization of I2(2) when K2Cr2O7 and KI act, place 5 minutes in the dark for avoiding light, preventing I- from being oxidized by air, and facilitating the complete interaction of K2Cr2O7 and KI. Therefore, solutions sho

5、uld be appliedStore in an iodine bottle or cone (covered with a surface dish), place in the dark for a certain period of time, and then titrate when the reaction is complete(3) to keep the acidity at calibration, dilute the solution at 0.2-0.4mol. L-1 before titration(4) because the I2 can partially

6、 oxidize the starch and make the end point advance and not obvious, the starch indicator should be added at the near end point when the indirect dosage method is adopted, that is, the I2 has been greatly reducedThe solution is titrated and the solution is pale yellow when added to starchWhen measuri

7、ng copper by 3. iodine method, why must pH remain between 3.54 and its influence is too low or too high?Answer: when measuring copper with iodine, the indirect iodine method is used. It must be carried out in neutral or weak acid solution:In alkaline solution I2 and S2O32- will have the following re

8、action:S2O32- + 4I2= 2 SO42- + 8I- + 5H2OMoreover, I2 will react in alkaline solution to form HOI and IO3-, and Cu2+ may also undergo hydrolysis reactionIn the strong acid solution Na2S2O3 solution will decompose:H+ = S: S2O32- + 2The Fe, often containing copper alloy or copper ore in As, Sb and oth

9、er metals, the samples were dissolved in the solution, Fe3+, As (V), Sb (V) can be oxidized I-I2,.As interfere the determination of Cu2+ (V), Sb (V) oxidation ability decreased with the decrease in acidity, when pH3.5, the interference can be masked by F- the oxidation of I-.Fe3+.To sum up, when iod

10、ine is used to determine copper, why does pH have to be maintained between 3.54?:K2Cr2O7+ 6I- + 14H+= 2 Cr3+ + 3I2 + 7H2O1., the greater the acidity of the solution, the faster the reaction, but when the acidity is too large, I- is easily oxidized by O2 in the air, so the acidity is generally 0.2-0.

11、42. the action of K2Cr2O7 and KI is that the solution should be stored in an iodine bottle or a cone (covered with a table dish) and placed in the dark for a certain period of time before the reaction is complete and then titrated3., the KI solution should not contain KIO3 or I2, if the KI solution

12、is yellow, or after the solution acidification, add starch indicator blue, then should be in advance with Na2S2O3 solution drops to colorless after useAfter titration to the end, after more than 5 minutes,The solution appears blue again, which is caused by air oxidation of I-, which does not affect

13、the result of analysis. If the end of the titration is turned into blue quickly, the indication is indicatedIncomplete (referring to the reactions of K2Cr2O7 and KI) should be re calibrated by solution. The following points should be noted in the determination of the reaction: pay attention to the f

14、ollowing points:1., when dissolved samples, the H2O2 must be finished (according to practical experience, began to take small bubbles, and then take big bubbles, that H2O2 has been exhausted), otherwise the results can not be measured, this is a very critical step operation2., plus starch can not be

15、 too early, because the titration reaction produces a large number of CuI precipitation, if the starch and I2 premature formation of blue complex, a large number of I3- by CuI precipitation, adsorption terminal point is deeper gray, not good observation3., adding NH4SCN can not be premature, and aft

16、er adding to violently shaking, is conducive to precipitation, transformation and release of adsorption of I3-.1 what are the considerations in the process of dissolving copper samples? heating and cooling must never be done with plugs (why?) control the size of the flame, the solution can not be st

17、eamed dry (why? In case of carelessness, the solution is small in volume, but not dry. How do you handle it? the extra H2O2 must be broken up (how to judge that the excess H2O2 has been broken down?)2 if the hydrogen peroxide is not exhausted, what is the effect on the experimental results?If H2O2 i

18、s added to the KI, the following reactions will occur: H2O2 + 2I- + 2H+ = I2 + 2H2O, resulting in a high test result.3 why should I add 1:1 ammonia to the solution slightly muddy, and then drip 1:1 acetic acid to the solution to clarify and add 1mL?In a weak acid solution, Cu2+ reacts with an excess

19、 of KI to form a CuI precipitate, while the amount of iodine is precipitated quantitatively. Therefore, through the above operation (adding ammonia to 1:1 solution slightly turbid, adding acetic acid to 1:1 solution to clarify and add 1mL) HAc - NH4Ac buffer solution, the control solution of pH=3.5

20、4, the reaction can quantitatively.4 if the acidity is too low or too high, whats the effect on the result?If the acidity is small, Cu2+ hydrolysis, low results; slow reaction speed, end point out is not easy to judge. If the acidity is high, and because of the catalytic action of Cu2+, I- is easily

21、 oxidized to I2, so that the experimental results are on the high side5 how to adjust the solution to acidity?Regulation method: drip 1:1 ammonia to the solution slightly muddy, and then drip 1:1 acetic acid to the solution to clarify, and more 1mL.Because of the dissolution of copper alloy with HCl

22、 and H2O2, the solution is strongly acidic and requires 1:1 ammonia water to reduce the acidity of the solution.At the end of the sample solution, the solution is pale green (because of the presence of Cl-, the solution has both yellow CuCl42- coordination ions and a pale blue Cu (H2O) 42+ ion, and

23、the solution is a mixture of both green and green). Side drop with 1:1 ammonia water, and shake the conical bottle mixture solution. At first the solution was clarified and there was a white fog in the cone With the addition of ammonia water, the drop point appears dark blue (what substance)? Rotate

24、, disappear, slow down, and roll. Then, the color of the solution changed from green to pale blue. At this time, the method of 1 drops and more shake should be adopted until the rock slowly turns out to be cloudy with blue.Please note: it takes a long time to add ammonia to the precipitation. Please

25、 be patient. If the ammonia water is accelerated, the solution is dark blue, and no sky blue is cloudy The amount of ammonia added is related to the amount of 1:1, HCl, the heating time and the heating temperature.Side drop 1:1 HAc, spin the conical bottle, and need a drop more shake, tapered bottle

26、 turbidity gradually less, and finally in the roll of muddy turbidity disappeared. Because Cu (OH) 2 precipitation dissolution process also takes time, therefore, 1:1 HAc can not speed up.Add 1mL 1:1 acetate, with cylinder plus can also according to the dropper bottle, drops in how many drops of 1mL

27、, according to the number of drops plus.6 why should ammonium fluoride be added? What other impurities interfere with the determination of copper? and NH4F can make solid mask Fe3+, Fe3+ exists in FeF63-, so E (Fe3+/Fe2+) decreased, Fe3+ oxidation of I- to I2 reaction cannot occur, and because FeF63

28、- is colorless, no interference color change end point.As (V), Sb (V) impurities will interfere with the determination of copper, when pH3.5, As, Sb of the pentavalent oxidation can be reduced to be the oxidation of I-.7 why add excessive potassium iodide, whats the function of it?The reaction of 1

29、with I- produced by Cu2+ and CuI is reversible with I2, and the excess KI can make the reduction of Cu2+ to be complete. excess I- and I2 form I3-, increase the solubility of I2, and avoid volatilization of I2. From the above description, excess KI acts as a reducing agent and precipitant and forms

30、a multi - halide with I2.8 why should potassium iodide be titrated with sodium thiosulfate standard solution immediately after adding potassium iodide?Prevent volatilization of iodine and oxidation of I-.9 Why do you add starch solution near the finish line? When is the best time to add starch? beca

31、use starch adsorption I3-, so that I2 is not easy to release, affecting the experimental results, see sodium thiosulfate standard solution preparation and calibration of experimental steps in question 13. starch can neither be added prematurely nor too late, so as to avoid crossing the finish line.

32、When measuring copper, when the color of titration to solution is pale yellow, it is advisable to add starch.10 when adding potassium thiocyanate?After adding starch, continue to slow drop of sodium thiosulfate standard solution, when the blue light or just disappears with potassium thiocyanate, sev

33、ere shake flask spin blue reproduction. If you add late, the end is easy11 why should add potassium thiocyanate?CuI precipitation is easy to adsorb I2, which leads to low analysis result and insensitive end point. Therefore, at the end of the entry, the addition of KSCN transforms CuI into a less so

34、luble CuSCN, which basically does not adsorb I2 and makes the endpoint color sensitive12 why not add potassium thiocyanate early?Because: 6Cu2+ + 7SCN- = +4H2O + SO42- + 8H+ + 6CuSCN: CN-: 2Cu2+ + 4I- resulted in a reversible reaction: = CuI + I2Move to the left, thus reducing the precipitation of I2, and the result of determ