Potassium ferrous oxalate crystal Kx[Fe(C2O4)y]? 3H2O can be used for photography and blue printing. The experimental process of preparing potassium ferrite crystal in laboratory is as follows.

(1) Ferrous ion is easily oxidized to iron ion by oxygen in the air, so it is necessary to add excessive iron powder to prevent ferrous ion from being oxidized, with 2Fe3++Fe = 3Fe2+;

So, the answer is: iron; Prevent ferrous ions from being oxidized by oxygen;

(2) The method of cleaning the sediment is to check whether it contains sulfate ions, take a small amount of the last washing liquid and add barium chloride solution. If a white precipitate appears, it means that the precipitate is not cleaned; If there is no white precipitate, it means that the precipitate has been cleaned;

So the answer is: take a small amount of the last washing liquid and add barium chloride solution. If white precipitate appears, it means that the precipitate is not cleaned; if not, it means that the precipitate is cleaned;

(3)① According to the analysis of experimental steps, the titration experiment is to determine the content of oxalate ion through the titration process of redox reaction, and it is necessary to put potassium permanganate solution in an acid burette for titration experiment, which is carried out at 75 ~ 80℃ and needs water bath heating to determine the temperature of water;

So the answer is: ad;

(2) The manganese sulfate added in the experimental step did not change after the titration reaction, which played the role of catalyst; The titration end point is judged by the color of potassium permanganate solution, and the last drop of potassium permanganate solution turns pale red and does not fade for half a minute, which proves that the reaction has reached the end point.

So the answer is: catalyst, the solution turns light red, and the inside fades within half a minute;

③ The quantitative relationship and calculation results of the reaction were obtained by experimental calculation, so as to determine the content of oxalate ions;

Therefore, the answer is: determine the content of C2O42-.