Please look at the following below links which may help you in your analysis:

https://www.aapd.org/globalassets/media/publications/archives/ranly-06-02.pdf

http://pathresidents.com/usapathology/em-handbook/techniques-section/glutaraldehyde.html

https://link.springer.com/chapter/10.1007%2F978-1-4419-9244-4_2

Thanks

You have to calculate the volume of 25 % glutaraldehyde first for the making of 1 % glutaraldehyde using the formula X1 V1 = X2 V2. Where, X1 is the concentration or percentage of your 25 % glutaraldehyde, V1 is the volume of 25 % glutaraldehyde (which you have to identify for 5 %), X2 is percentage of working concentration of glutaraldehyde (such as 5 %) and V1 is the volume 5 % glutaraldehyde. After the identification of the volume of 25 % glutaraldehyde, 0.2 M phosphate buffer is made. For example, if you make 0.2 M 100 ml buffer and you need 20 ml of 25 % glutaraldehyde and 80 ml of 0.2 M phosphate buffer for the preparation of 5 %. In 80 ml of 0.2 M phosphate buffer, add all the components such as monobasic and dibasic phosphate compound required for 100 ml. Finally 5% glutaraldehyde solution is prepared.