The use of deionised water would dilute the buffer thus changing the ionic strength, salt concentration, and overall buffer capacity. So, I would recommend the use of sodium citrate, or even better, a solution of sodium hydroxide (1M or 2M). The later changes to sodium citrate immediately and does not influence on the named parameters significantly.

Usually, 1-2M aq.HCl or NaOH solution is used to adjust a buffer's pH value.That wouldn't change the concentration of salt buffer and other parameters of the buffer system that are important to your injection.

For STZ induction you should use the pH 4.4 to 4.6.

Thank you Marko, Shouhoung & Gopinath for your response.

@ Marko: I agree with you & following sodium citrate for pH adjustment, However, can you throw some light on why they use deionized water for pH adjustment?

@ Gobinath: I standardized my protocol with pH 6.5 & it's highly robust in diabetes induction.

pH (pruisence de hydrogen.... french word) and conductivity are interlinked.......deionized water is used because it is expected not to contribute or vary conductivity of solution and thus pH.

For STZ expt. 6.5 pH is very suitable one than high acidic pH

Thank you Dr Chakravarthy, Do you mean the STZ stability in this buffer is ultimately linked with conductivity of solution ?

Dear Mr Vaibhav,

for pH adjustment we should always keep in mind that for stable pH always use weak acid and weak base.its best use sodium citrate in your case.

Regards

Amir

Dear Mr.Vaibhav

With STZ for better results freshly prepared solutions be used.However,

STZ is stable maximum upto pH 4. Conductivity and pH are linked

Pl. refer

1. STREPTOZOTOCIN MIXED ANOMERS Sigma No. S0130

www.qcbio.com/stz/s0130pis.pdfShare SOLUBILITY / SOLUTION STABILITY: Streptozotocin is soluble in water, the lower alcohols and in ketones. This product dissolves in water at 50 mg/mL to give a ...

Vaibhav, I believe that the most logical reason for the use of water is the stability of streptozocin, but this is arguable. As nitrosourea compound, STZ is sensitive to nucleophiles, and conjugate bases (e.g. citrate) are quite good nucleophiles. By adding just water, no change in concentration of nucleophile occurs (the kinetics of nucleophilic substitution remains the same, or even speeds down), and this might be the reason for it. I (partly) disagree with Amir that a weak acid or a weak base should always be used to obtain stable pH, as in the case of STZ, the promptness of work is crucial, and the use of aq. NaOH or HCl is the fastest way to change the pH.

Thank you Mr Amir & Dr Charkravarthy for your valuable suggestions.

@ Marko: Thanks for your reasonable answer for use of water. Agreed that STZ preparation should be quick to avoid its degradation but use of aq, NaOH or HCl has not been heard before in this critical preparation.

Which does not necessarily mean that it is wrong. aq. NaOH in citrate buffer produces sodium citrate. What is the difference between adding citric acid and NaOH separately or together? The only difference is that the increase in pH is higher (citrate is basic) so, the increase in the pH by the same molar quantity of NaOH vs. Na-acitrate is higher. And yes, you loose some buffer capacity, but I wouldn't consider it crucial.

Marko, your point is indeed valid.. & my opinion should not be taken in negative way, i just mentioned i've not tried to use aq. NaOH or HCl in this preparation.

Many thanks for your answers.