Incremental Method VS Decrement Method in Sample Weighing
Sample weighing is an important part of the analysis experiment. The inaccurate weighing will lead to the failure of the whole experiment. Therefore, we should choose the right weighing method when weighing. Such as increment method and decrement method. When it comes to increment method and decrement method, what's the difference between them? How to choose?
The incremental method is also called direct weighing method, which is mainly used for weighing the non hygroscopic and other modified samples or reagents with a weighing range given by the sample to be tested. This method is similar to the designated mass weighing method, i.e. take the sample from the medicine spoon and put it on the clean and dry surface dish or sulfuric acid paper and other containers that have been tarnished. Take a certain amount of sample at a time, and the reading obtained is the sample mass. When transferring the sample, all the samples must be transferred to the container, and shall not be left on the weighing container.
In the direct weighing method, the following matters need to be noted: A when the sample to be tested is a sample with high grease or water content, the electro-optical paper and sulfuric acid paper shall not be used as containers to weigh the sample. B. the products are hygroscopic and should be weighed quickly in a crucible with a cover. C. if the temperature of the object to be weighed is high, the weighing result is generally less than the real value, so the dried or burned utensils must be cooled to room temperature in the dryer before weighing. It should be noted that there is no absolute non absorption of water in the dryer, but the humidity is only small. The same cooling time should be mastered, such as 45 minutes or 1 hour, and they will absorb a layer of water when exposed to the air, making the weight increase. Different space humidity, the amount of water adsorbed is also different, so the weighing speed is required to be fast.
In the analysis process, many reagents or samples to be tested are easy to be oxidized or reduced by O2, NO2, H2S, SO2, etc. in the air, some of the substances to be weighed are easy to work with CO2, NH3, etc. in the air, some of them are easy to be affected by water vapor in the air or the volatility of the sample itself, etc., causing quality changes, and the deterioration of reagents and samples is one of the important reasons for the errors in chemical analysis. For example, "sub" compounds in most inorganic reagents and compounds with strong reducibility such as hydroxylamine hydrochloride and ascorbic acid in organic reagents are easy to be oxidized by oxygen in the air, while strong alkali salts such as Koh, butanediol and sodium arsenate are easy to absorb CO2 in the air and then deteriorate. In addition, guanidine, hydration trap and other organic reagents can also absorb part of the above-mentioned various types of CO2 to be weighed and several parts of the same sample can be weighed by the reduction method.
The weighing method is as follows: put a certain amount of solid samples into the weighing bottle, for example, it is required to weigh two 0.4000-0.6000 g samples, take about 1.2000 g samples into the bottle, cover the bottle cap, put the weighing bottle on the balance plate, and weigh it. Take out the weighing bottle above the container (generally beaker or conical bottle), incline the weighing bottle, open the bottle cap, tap the upper edge of the bottle with the clamp cap, and gradually pour out the sample. When it is estimated that it is enough for 0.4000g, slowly erect the weighing bottle with a tap on one side, so that there is no sample left at the bottle mouth, and gently cover the bottle cap (all these should be done above the container to prevent the loss of the sample ）, put it back on the balance plate and record the difference of reading. If the weight is less than 0.4000g at one time, pour it again, but not too many times. If the poured sample exceeds the required value, it can not be put back with the help of the medicine spoon, only the weight can be discarded. Remove the sample according to the above method. The liquid sample can be weighed by the decrement method in a small drop bottle.
Advantages and disadvantages of increment method and decrement method
The advantage of incremental method is that it can be put in place at one time, and only incremental method can be used when specifying the weight, because the decrement method can not judge how much to pour out at one time. The disadvantage of incremental method is that there must be a clean container. If there is water in the container and it is not dry, then it cannot be put on the balance. You should know that the last position of the balance will change during the evaporation of water.
The mass difference of several groups of parallel measurement may be larger than that of reduction method, but it does not affect the experimental accuracy. Reduction method is relatively commonly used.
We also wrote this article about sample weighing: Analysis of the Reasons for the Inaccurate Weighing of Samples in the Experiment
. If you are interested, please click the link to browse. We also provide you laboratory crucibles in various styles. If you are interested in them, Please click the link below to enter the crucible product page.